used, and the oxidation is much more rapid and intense, the skins being
packed in boxes or piled, and allowed to heat. The product obtained in
this way is much more viscous, and can only be recovered by scouring
with alkalis; and the product, recovered with acid, constitutes sod oil.
In many English factories, a modified method is now adopted, and a
product recovered by pressure, which scarcely differs from _moellon_.
An important peculiarity of dégras and sod oil is its ready
emulsification with water, which from its mode of preparation, it always
naturally contains, and which should be present in a good dégras to the
extent of not less than 20 per cent. Such a mixture, containing water,
is a sort of natural fat-liquor and is absorbed much more perfectly by
the skins than an oil alone. Sod oils, however, are frequently
“evaporated,” or deprived of water by heating above 100°, with the
object not only of effecting a fancied improvement, but of getting rid
more completely of the sulphuric acid which the water is apt to contain.
This makes them more homogeneous, and consequently much darker in
colour. It is not easy to neutralise the acid in an aqueous sod oil by
direct addition of alkali; possibly ammonia is best adapted for the
purpose; or a suggestion, I think due to Eitner, may be adopted, of
incorporating a small quantity of a suitable soap. In any case, very
complete mixture is required. If the sulphuric acid used in recovery has
been insufficient for complete neutralisation of the alkali, the dégras
or sod oil will naturally contain soaps, and sometimes also free alkali.
Free acid and free alkali are both injurious to leather, the former if
anything the more so, darkening the colour, and even rendering the
leather tender. When dégras is used in mixture with other fats, care
should be taken not to raise the temperature of the mixture so high as
to drive off the water, to which a good deal of its special efficacy is
due.
The chemical changes which take place during the chamoising process are
as yet incompletely understood. A large proportion of the glycerine is
dehydrated during the “heating,” forming acrolein (acrylic aldehyde), to
the action of which it is very possible that the actual conversion of
the skin into leather is due, while the fatty acids also undergo
oxidation. Dégras therefore always contains considerable quantities of
oxidised fatty acids, which are sometimes associated with nitrogenous
products from the skins, and which are soluble in alcohol, but insoluble
in petroleum ether. To these products Simand gave the name of
_Degrasbildner_ (dégras-former, Fr. _dégragène_), and it has been
considered a measure of the quality of the degras, but its exact value
and function is rather doubtful. According to Simand, a genuine dégras
should contain not less than 15 to 20 per cent. of the dégras-former as
estimated by his method, calculated on the dry oil, and a smaller
percentage is also present in the original fish oils. (For method of
estimation see L.I.L.B., p. 182).
As the process of dégras manufacture is obviously mainly one of
oxidation, many attempts have been made to produce it by direct
oxidation of fish oils, without the agency of skins, both by blowing air
through the oil, and by addition of oxidising agents such as nitric
acid. Eitner states that such oxidised oils are more liable to “spue”
than the original oils, as they already contain large quantities of
resinised products; but this is certainly not true of all artificial
dégras, some of which answers its purpose perfectly as a currying
material, though it is very probably justified in other cases. Of course
the methods of successful manufacturers are kept as profound secrets.
Dégras and sod oil, when deprived of water, are dark and viscous oils,
of high specific gravity (0·945-0·955), and therefore heavier than the
oils which have been employed in their manufacture.
WAXES, as has already been stated, differ in their chemical character
from true fats, in that their fatty acids, which are mostly of high
molecular weight, are combined, not with glycerine, but with alcohols,
also of high molecular weight and of wax-like consistency. Most waxes
are solid bodies of high melting point, but some oils, especially sperm
and bottlenose oils, are chemically liquid waxes; woolfat contains a
considerable proportion of waxes; and many marine oils, such for
instance as shark-liver oil (p. 366), contain waxes in smaller quantity
in mixture with true fatty oils.
_Sperm Oil_ (Fr. _Huile de cachalot_; Ger. _Spermacetioel_, _Walratoel_)
is obtained from the sperm whale, an inhabitant of the Antarctic seas.
“Arctic sperm” (Ger. _Doeglingthran_) is a very similar oil obtained
from the “Bottlenose whale.” These oils are very fluid, do not dry, and
are excellent lubricating oils for light machinery, and also good lamp
oils. They contain little if any glycerides, and about 40 per cent. of
unsaponifiable solid alcohols, which are soluble in ethyl-alcohol, and
must not be confused with ordinary unsaponifiable mineral oils, which
are frequently used as adulterants in mixture with fatty oils to adjust
gravity and the “saponification value.” Mineral oils are liquid, and
insoluble in alcohol. Sperm oil is the lightest of ordinary oils, its
gravity being only about 0·880 at 15° C. From its price it is
particularly liable to sophistication. It is used in leather manufacture
in the finishing of some fine leathers, and sometimes as a constituent
of fat-liquors. Spermaceti, a wax also obtained from the sperm whale, is
an occasional constituent of leather polishes.
_Beeswax_ (Fr. _Cire des abeilles_; Ger. _Bienenwachs_) is one of the
most important waxes for the leather-dresser. As is well known, it is
obtained from the honeycomb of the ordinary bee. It is a yellowish solid
body, fairly plastic when fresh, and of “waxy” feel. At low temperatures
it is brittle and of fine granular texture, and when pure is almost
tasteless. It is often bleached by repeated melting and exposure to
sunlight. As wax always contains a considerable amount of pollen it may
be identified when in admixture with other substances by means of the
microscope.
Beeswax is almost insoluble in cold alcohol, but boiling alcohol
dissolves out the contained cerotic acid, which crystallises from it on
cooling. Wax is saponified by alcoholic potash, but the resulting
myricyl alcohol (about 54 per cent.) is not capable of further
saponification.
Beeswax is frequently adulterated. Water and mineral matters (ochre,
gypsum, etc.) also flour, starch, tallow, stearic acid, Japan wax,
carnaüba wax, resin and paraffin-wax are among the substances most
commonly used in its sophistication.
The detection of these, and especially of the other waxes, is so
difficult that it will not be described here. The reader is, however,
referred to Benedikt and Lewkowitsch’s ‘Oils, Fats and Waxes,’ for
further information.
_Carnaüba Wax_ (Fr. _Cire de carnauba_; Ger. _Cearenwachs_,
_Carnaubawachs_) has come largely into use recently owing to the advent
of the coloured leather shoe. As it is a very hard wax it has become
very popular with boot polish makers, its low price being also in its
favour. Carnaüba wax is an exudation from the leaves of _Copernica
cerifera_, a palm indigenous to Brazil, and is, on this account, often
known as Brazilian wax. It is difficult to saponify, and with different
experimenters has yielded very varied results on analysis; it is
generally agreed, however, that it is a complicated mixture of several
of the higher alcohols and acids.
_Japan Wax_ is not a true wax, but a fat consisting of glycerides. It is
a pale yellow, hard, waxy substance obtained from the berries of a
sumach (_Rhus succedanea_, etc.). At ordinary temperatures its specific
gravity is exactly that of water, and it melts at 56° C. Any admixture
with other fats would lower the melting point, but japan wax is often
adulterated with 15 to 30 per cent. of water. It is chiefly valuable to
leather dressers as a substitute for beeswax on account of its lower
price.
VOLATILE OR ESSENTIAL OILS.
These oils are distinguished from those described in the previous
section in that they are capable of distillation without undergoing any
serious amount of decomposition. They occur to some considerable extent
in nature, but those of most importance to the leather trade are
produced by the decomposition of more complicated materials.
_Birch Oil_ is by far the most important of this class of oils so far as
the leather-dresser is concerned, since it is the substance which gives
to “Russian leather” its characteristic odour.
The oil is obtained by destructive distillation, and the process by
which the peasants conduct this is one of the rudest that can be
imagined. A cauldron is filled with dry birch-bark, closed, and heated
over a fire. The vapours which are evolved are carried, by means of a
pipe, to another vessel which is buried in the ground, and are there
condensed. The dark-brown liquid (birch-tar) is allowed to cool, and the
liquor which rises to the surface skimmed off. The tar is sometimes
distilled, and an oil is thus obtained which does not give the true
birch-oil scent very strongly though occasionally sold as a refined oil.
The true odorous substance is evidently of very high boiling point and
remains mainly in the tar.
The birch tar is almost entirely used for giving leathers a “Russian”
odour, for although it smells somewhat strongly of tarry products, the
oils causing this smell are far more volatile than the birch scent
itself, and therefore disappear on storing the leather a short time. Tar
obtained from various species of pine is sometimes substituted for birch
tar, but it may readily be distinguished from the latter by the odour
and the difference in the specific gravity. Birch tar has a specific
gravity of 0·925 to 0·945, whilst fir tar has one of 1·02 to 1·05; thus
the former floats on water while the latter sinks if it be entirely free
from enclosed air. Fir tar, too, gives up a yellow colouring matter to
water shaken up with it, while birch tar leaves the water colourless.
Birch tar has a distinctly acid reaction, and must not be kept in iron
vessels. (See p. 251).
The leaves and twigs of American black birch when distilled with water
or steam, yield an oil which is practically identical with that of
_Gaultheria procumbens_ (wintergreen), and consists almost entirely of
methyl salicylate. It is clarified, and to some extent decolorised, by
filtration through woollen blankets and redistillation. A ton of
brushwood is said to yield about four pounds of oil. This oil has quite
a different odour to that of the real Russian oil, and cannot be used in
the scenting of “Russia” leather. Sandalwood oil with a little black
birch or wintergreen oil is sometimes employed for scenting small fancy
articles and bears considerable resemblance to the true “Russia” leather
odour. Black birch, aniseed, sassafras and various other essential oils
are occasionally used in small quantities as preservatives, and to cover
disagreeable odour in blood-seasonings, cements and other products used
in the leather trade. The methods employed for their detection and
estimation do not, however, come within the scope of a work such as the
present one. Most essential oils have considerable power as antiseptics,
and in preventing mildew and the attacks of insects.
MINERAL OILS AND WAXES.
This class of bodies is totally different in chemical constitution from
the true oils and waxes, containing neither glycerides, fatty acids nor
alcohols, but consisting of carbon and hydrogen only, approximately in
the proportion of one atom of the former to two of the latter. They
occur in underground lakes, from which they are obtained by springs or
borings; or in shales, from which they are separated by distillation. It
is commonly supposed that they have been formed, at some remote period
of the earth’s history, by the decomposition of animal and vegetable
matters, at a high temperature and under great pressure.[167]
[167] Oils from wells or springs are technically called “petroleum
oils,” those from shale, “paraffin” oils, but chemically, there is no
definite distinction.
The mineral oils and waxes are largely capable of being distilled
without decomposition, but if heated to high temperatures, are readily
“cracked” or broken up into simpler and generally more volatile
compounds--a fact which is employed in the production of gas, and the
utilisation of some of the heavier products.
They differ greatly in their gravity and boiling-point, but not much in
their ultimate composition, consisting largely of saturated or nearly
saturated hydrocarbons (cp. p. 354), and hence are little liable to
oxidation, and acted on by few chemical reagents. From their
constitution they are of course unsaponifiable, and in this way can be
separated from fats and oils with which they have been mixed. (For
particulars of the method see L.I.L.B., p. 178.)
The heavier mineral oils are a good deal used in mixture with other oils
and fats, for stuffing leathers, those of a specific gravity of
0·880-0·900 being usually most suitable. They are quite incapable of
“spueing,” and are useful in lessening that tendency in other oils with
which they are mixed. They have not, however, the same affinity for the
leather fibre as some of the true oils, and are to a certain slight
extent volatile, and should generally be used in mixture, rather than
alone.
Most mineral oils, when held so that a strong light (daylight or
electric light rich in ultra-violet rays) falls upon them, show a green
or violet fluorescence or “bloom.” This is very persistent, even when
the oil is mixed with a large volume of other oils, and is often relied
upon as a means of detecting them when used as adulterants. The test is,
however, not infallible, since the effect is due to impurities which may
be removed by purification, or masked by the addition of such substances
as nitrobenzene or nitronaphthalene, and it also occurs in the
hydrocarbon products produced in the distillation by steam of animal
oils, and is occasionally seen to some extent even in oils which have
not undergone distillation.
_Vaseline and Vaseline Oil_ are the most viscous and densest of the
petroleum oil products. They probably differ from the solid paraffins in
chemical constitution, though their ultimate composition is almost the
same. They are often useful constituents of stuffing greases.
_Paraffin Wax_ consists of a mixture of hydrocarbons similar in chemical
constitution to the paraffin and petroleum oils, but of higher boiling
point, and solid at ordinary temperatures. Its hydrocarbons are mostly
saturated, and hence very stable bodies, and little liable to oxidation.
They are completely unsaponifiable, and unaffected by boiling with
alcoholic potash, and in most cases by boiling with strong sulphuric
acid, by which they may be separated from animal and vegetable waxes or
fats with which they have been mixed. They are quite incapable of
resinising by oxidation, or of causing “spueing” in leather. They are
soluble in petroleum spirit, carbon disulphide and most of the ordinary
solvents of fats, but insoluble in alcohol.
Paraffin wax separates from the liquid oils by crystallisation on
cooling, and the remaining liquid which adheres is removed by hydraulic
pressing, as in the case of tallow. The hardness and melting point vary
according to the extent to which the pressing has been carried, and the
temperature at which it has been done. The paraffins of higher melting
point are as a rule the more costly.
Pure paraffin wax is a white, more or less hard and brittle substance
which does not melt so easily as ordinary fats, and is on this account
used in stuffing certain kinds of leather, hardening the stuffing
grease, and making the leather feel less oily. When melted, paraffin wax
forms a thin liquid, more resembling an ordinary petroleum lamp oil than
the viscous vaselines and leather oils. On ignition it burns with a
bright somewhat smoky flame, and leaves no ash behind. It is found on
analysis when mixed with other waxes or oils in the “unsaponifiable
matter” (see L.I.L.B., p. 178).
_Ozokerit_ is a natural paraffin material used for the manufacture of
cerasin candles, which sometimes occurs in the vicinity of petroleum
springs, especially in Galicia. It is of pale yellow colour when pure,
and has then a melting point of about 70° C. Its chief impurities are
petroleum oils, water and clay. These are removed by melting the
ozokerit, decanting off the clear oil, and filtering it through fine
animal charcoal. If liquid oils are present the material is treated with
alkali or with strong sulphuric acid, and is pressed before filtering
through charcoal. The refined product is termed “cerasin,” and is of a
more waxy and less crystalline texture than ordinary paraffin wax.
_The Resin Oils_ are derived from resins, and _mainly_ from colophony or
common pine rosin, by destructive distillation. Their specific gravity
ranges from 0·96 to 0·99, but their chemical composition is very
imperfectly understood, and appears to be by no means constant. Like the
mineral oils they are “unsaponifiable,” but often contain small amounts
of soap-forming material (resin acids).
The detection and estimation of resin oils is often a matter of
considerable difficulty, but further particulars on this point will be
found in L.I.L.B., p. 180. From their cheapness, they are considerably
employed as adulterants of other oils, and their high gravity makes them
convenient to adjust the gravity of mineral oils when used for this
purpose, as the latter are usually lighter than the fatty oils. As
currying oils, they are not particularly suitable, though often employed
in stuffing picker bands, and other heavily greased leathers. They have
considerable antiseptic powers, and for this reason are useful in
leather greases, preventing heating, and checking mildews.
Resin itself is occasionally used as an addition to stuffing greases,
and is said to increase the waterproofness of the leather, and to give
it a drier feel. In mixture with about half its weight of paraffin wax,
and with a little grease if necessary to soften the mixture, it is often
used in waterproofing mixtures, which can be made to melt at 50° to 60°
C. Leather will bear immersion in the melted mixture without scalding if
thoroughly dried in a hot stove at a temperature of not less than 50° C.
before dipping. Any great increase of the proportion of paraffin wax
causes the rosin to separate. Rosin consists mainly of free acids which
easily combine with alkalies and alkaline carbonates in boiling. It is
hence largely used in the manufacture of soaps on account of its
cheapness, and to render them more soluble in water. The rosin acids are
not so strong as many of the fatty acids, and rosin soaps are therefore
somewhat strongly alkaline. Rosin soap, precipitated among the ground
paper pulp in the rag engine, by addition of alum or sulphate of
alumina, is largely used as a sizing for common papers.
CHAPTER XXIV.
_OIL TANNAGES, AND THE USE OF OILS AND FATS IN CURRYING._
The conversion of skin into leather by the agency of oils and fats is
probably one of the most primitive methods, and is used in different
ways suited to the skins and fats which are available, by savage races
in all quarters of the globe. In its simplest form, it consists merely
in oiling or greasing the wet skin, and kneading and stretching it as it
slowly loses moisture and absorbs the fat. Under these conditions, the
fibres become coated with a greasy layer, which prevents their adherence
after they are once separated by the mechanical treatment. At the same
time some chemical change takes place in the fibre itself, which has a
part in its conversion into leather varying in importance according to
the method and fat employed, and of which the chemistry will be best
discussed after some slight sketch has been given of the methods
themselves.
The most complete sort of oil-leather is that produced by “chamoising,”
or oil-dressing with marine oils, a process applied to the ordinary
“chamois” or “wash-leathers” (now made from the flesh-split or “lining”
of the sheep-skin), and to the manufacture of “buff-leather” for
military purposes. The process varies somewhat according to the
character of the leather, but the manufacture of the common wash-leather
may be taken as a type. For this purpose the sheep-splits are freed from
the loose and fatty middle layer (p. 51) by “frizing” with a sharp knife
on a beam similar to that used for fleshing (Fig. 30, p. 147), but much
more steeply inclined. The process is rather one of scraping than
cutting, and was originally adopted to remove the grain from the
deer-skins which were largely used for glove-leathers, since
oil-dressing does not easily penetrate a skin with the grain surface
intact. The fleshes are usually delimed by drenching, but removal of
fat is unimportant. After being well drained, they are “stocked” for
some time with sawdust till they become partially dry and porous, the
common “faller” stocks shown in Fig. 22, p. 116, being generally
employed. During the stocking, care must be taken that the goods are not
overheated by the friction produced. When the skins have become opaque
from the inclusion of air between the fibres, they are, according to the
Continental method, shaken out and oiled on the table, and after folding
into bundles, are put back in the stocks. In England, the oil is usually
added to them during the stocking, in small quantities, which become
rapidly and evenly distributed by the motion of the skins. In England,
cod oil is almost exclusively employed, but on the Continent, a
considerable proportion of seal and whale oils is used. As the goods are
apt to heat, not only from friction, but from the oxidation of the oils
employed, they are removed from the stocks at intervals, and allowed to
cool, usually hung on hooks exposed to the air. In France this exposure
to the air is much more considerable than in England, the skins being
hung for eight or twelve hours after each stocking. The drying rooms are
kept moderately warm, and a good deal of oxidation of the oil takes
place in them, which materially affects the character of the product,
and especially of the residual oil or dégras, which is afterwards
squeezed out of the skins and used for currying (p. 368). Great care is
required to prevent any parts of the skins becoming dry before they are
completely saturated by the oil, which causes hard and transparent
patches which the oil will not afterwards penetrate. After each exposure
to the air, the skins are oiled on the table and returned to the stocks.
The stocking has to be continued for many hours, even for wash-leather;
and as it proceeds, the skins lose the smell of limed skin, and acquire
a peculiar mustard-like odour from the volatile products of oxidation of
the oils. When the skins are completely saturated, they are, according
to the English method, packed in boxes, and allowed to heat
spontaneously by oxidation of the oils, during which great care is
required, especially at the outset, that the heat does not rise so high
as to destroy the skins. To prevent this, they are removed at intervals
from the boxes and spread on the floor to cool, and then re-packed, and
this treatment is continued until the oxidation is complete, and the
skins cease to heat. During the heating, large quantities of volatile
and very pungent products are given off, and especially acrolein
(acrylic aldehyde, from the dehydration of the glycerine), which is
excessively irritating to the eyes. The German method is not unlike the
English, but in France, the packing in boxes is omitted, and the
oxidation is completely effected in warm stoves in which the goods are
hung on hooks. The heating in this case is much more moderate, and the
oil less thickened, a result which may be partly due to the different
oils employed, and which leads to differences in the subsequent
treatment of the leather.
In the French process, the oily skins are dipped in hot water and wrung
or hydraulic pressed, the expressed oil constituting _moellon_ or
_dégras_ (p. 368), and the skins are afterwards washed in a hot soda or
potash solution, from which a further portion of an inferior _dégras_ is
recovered. In the old-fashioned English method, the oil became so
thickened that it could not be pressed out, and the whole was removed by
washing with soda or potash solution, from which it was recovered by the
use of acid, constituting “sod oil” (p. 369.) Now many English
manufacturers adopt a modified method, and remove a good deal of their
oil by pressure.
Buff leather, much used for military accoutrements, is made in a similar
manner to chamois, from ox or cow hides, the grain of which is frized
off. The bleaching, both of buff and chamois, is done by exposing to the
sun in a damp condition, the skins being watered as required with water
or fat-liquor, or the alkaline emulsion of _dégras_ obtained in washing
the skins. It may also be bleached by oxidising agents, such as
permanganate of potash or acidified sodium peroxide. If permanganate is
used, the leather is treated in a solution of perhaps 5 grm. per liter
till of a deep brown colour, and then in a solution of sulphurous or
oxalic acid till the colour is removed.
Messrs. J. and E. Pullman, of Godalming, make a species of buff leather,
which they style “Kaspine” leather, by treating limed and drenched hides
or skins in a drum with a very dilute solution of formaldehyde
(“formalin”) rendered alkaline with sodium carbonate (Eng. Pat. 2872,
1898). The change to leather takes place very rapidly, and the leather
is afterwards treated with soap solutions of fat-liquors, to feed and
soften it. It is almost indistinguishable from genuine buff leather,
except from the fact that it is white throughout, and needs no
bleaching. It is finding considerable application for military purposes.
A type of leathers which bear a close chemical relation to oil-leathers,
is that including “Crown,” “Helvetia,” and fat-tanned leathers. The
first leather of the sort was invented by a German cabinet-maker named
Klemm, by whom the secret was sold to Preller, who manufactured it in
Southwark, under the name of “Crown” leather. Klemm used flour,
ox-brains, butter, milk, and soft fat, which was made into a paste with
water, and spread on the limed, drenched and partially dried skins,
which were rolled into bundles, and drummed in slightly warmed drums for
some hours; taken out, again dried slightly, and coated with the
mixture, and again drummed. For thick hides the process was repeated a
third time, drumming in each operation for about eight hours. The
leather was used for laces, picker-bands, light belts, and other
purposes where great toughness and flexibility were required. It was
found by further experience (if indeed, it was not known to Klemm
himself) that the only really essential ingredients of the mixture were
the soft fats and flour; and even the latter could, for some sorts of
leather, be dispensed with. It was further ascertained that only the
gluten or albuminous part of the flour was absorbed by the leather, the
starch serving mainly to facilitate the emulsification of the fats. The
proportions used in the paste are about seven parts of flour, seven
parts of soft fat such as horse grease, two parts of tallow, four parts
of water, and a little salt or nitre to act as an antiseptic. Other
greases, such as mixtures of tallow and oil, can be substituted for the
horse grease, and pipe-clay or ochre may to some extent take the place
of the flour, while soap may also be added. The similarity of the
mixtures used to the tawing paste in calf- and glove-kid dressing (pp.
191, 196) is obvious, and Klemm had an earlier process in which the
operation just described was preceded by a slight alum tannage, and
which was almost identical in its detail with the methods now in use for
the production of so-called “raw-hide.” On the other hand it is nearly
allied to the production of “Riems,” or raw-hide straps in South Africa,
for which a long thong is cut spirally from a hide, and wound into a
sort of skein which is suspended from a crossbar, with a heavy weight at
its lower end, and oiled and twisted, with frequent changes of position,
until the water is dried out, and the thong is saturated with fat,
forming a very tough and durable leather. A similar material can be made
by fulling or otherwise working grease into a raw hide prepared for
tanning. Eitner examined samples of “Crown-leather” chemically, by
removing the gluten of the flour with an alkaline solution, and found
that an imperfectly chamoised leather remained, which when restuffed
with fat, was much less full, and carried a much smaller quantity of
grease than before.[168]
[168] Gerber, 1878, p. 2.
Various theories have been proposed to explain the reaction which takes
place in the production of oil-leathers. Knapp supposed that it was
merely a case in which the smallest fibrils of the hide were coated with
the products of the oxidation of oils, and so prevented from adhering
together, and protected from the action of water by the sort of
waterproof coating which was formed. This explanation is scarcely
feasible in the face of the fact that chamois leather can be treated
even with hot dilute solutions of the caustic alkalies without
destruction, while cotton fibres waterproofed by treatment with drying
oils have their coating entirely removed by treatment with alkalies.
Lietzmann supposed that the whole of the gelatinous fibres were removed
in the liming and subsequent treatment, and that the finished leather
consisted only of the skeleton of yellow or elastic fibre which exists
in the skin, and which is remarkable for its resistance to heat, acids
and alkalis. Unfortunately the proportion of these fibres does not
exceed about 6 per cent. of the total, so that they are quite
insufficient to account for the production of the leather. We now know,
however, that aldehydes, including the acryl-aldehyde, which is evolved
in the oil oxidation of chamoising (and which is covered by Messrs.
Pullman’s patent) are capable in themselves of converting gelatinous
substances into a material identical in its properties, and especially
in its power of resisting hot water and alkaline solutions, with the
fibre of chamois leather. In all cases where perfect chamoising is
produced, intense oxidation takes place, and oxidisable oils are used
which will evolve acrylic and other aldehydes. Where oils of little
drying power are employed, as in the case of Crown- and other
fat-leathers, only an imperfect chamoising occurs, and we are therefore
justified in attributing the special qualities of chamois leather to a
natural aldehyde tannage. On the other hand, there is no doubt that the
coating of the fibres with oxidised oil-products really occurs, and is
probably a powerful factor in the leathering of Crown-leather, and other
similar products which are not washed out with alkaline solutions. Knapp
proved by treating raw pelt which had been dehydrated with alcohol (p.
74) with a very dilute alcoholic solution of stearic acid, that a thin
coating of stearic acid on the fibres would confer great softness and
considerable resistance to water. Even where no stearic or other fatty
acid is purposely added to alcohol used for dehydrating pelt, traces are
present from the decomposition of the natural fat of the skin, and there
is little doubt that this is the cause why such alcohol-leathers are
much more difficult to wet back again to the state of pelt than would _a
priori_ be expected; and why hide-powder dehydrated in this way is
unsuitable for use in the hide-powder filter (p. 311) from its
non-absorption of water.
[Illustration: FIG. 85.--Scouring large Seal-Skins by Hand.]
It is not within the scope of the present volume to describe in detail
the processes used in currying, many of which are purely mechanical, and
of no theoretical interest, whatever their practical importance; and
with which the writer hopes to deal fully in a future book. The leather
is usually scoured with stone, brush and sleeker to free it from “bloom”
and loose tan (Fig. 85); or by machines such as Fig. 86; and is often
reduced in thickness by shaving by hand (Fig. 87), or by machine (Fig.
88). In place of shaving, hides and skins are frequently split into two
or more thicknesses. This is done by various machines, of which the
“bandknife” shown in Fig. 89 is the most important; the cutting tool
being a thin steel belt stretched like a bandsaw and sharpened on one
edge by an emery-wheel.
[Illustration: FIG. 86.--Scouring Machine.]
[Illustration: CURRYING SHOP, LEATHER INDUSTRIES DEPARTMENT, YORKSHIRE
COLLEGE.]
Something must be said here about the function of the oils and fats used
in currying, and their general method of application. It is obvious that
the possibility of coating the finest fibrils of leather with a fatty
layer is not restricted to raw hide, but is present, sometimes even in a
higher degree, in tanned or tawed leathers, in which the fibres are
already so far isolated as to make the access of the fat easy. Even the
possibility of an aldehyde-tannage is not excluded, where the fibre is
not already completely saturated with other tanning agents or where
these agents, from their nature, have not so firm a hold on the fibre as
to be incapable of being displaced by the action of aldehydes. It is
therefore obvious that we may apply some of the ideas which we have
formed with regard to oil-tannages to the action of fats upon tanned
leather. In the first place, it must be remembered that gelatinous
matters are as a rule insoluble in fats; and _vice versa_, that fats are
incapable of penetrating dry and solid gelatinous fibres. If the skin
becomes dry in the chamoising process, that part remains raw. It may
therefore be concluded that fats and oils have little power in
themselves of isolating the fibrils, and that this must be accomplished
by other agencies, since if they are still adhering together, the fats
cannot penetrate them. Hence the necessity of moisture, which keeps the
fibres soft and divisible; and with raw hide, the importance of powerful
mechanical treatment, which will work the minute globules of fat between
the fibrils. In the case of tanned leathers, the last condition is less
important, since the fibres are already isolated by the tannage, and
capillarity assists the penetration. Even in this case the distribution
of the fat is much assisted if it is already in a state of fine division
(emulsification), and if the surface-tension (p. 76) between it and
water is low, as is the case with dégras and other partially oxidised
oils. On this rather than on any special chemical affinity probably
depends the importance of the “dégras-former” and other products of
oxidation which are present in dégras; and the difference in penetrating
power of different oils. So long as oil remains in an undivided
condition, so long can it be squeezed out, and the leather will feel
and appear greasy; while, when it is thoroughly emulsified, and adherent
to the fibre, it can no longer be expelled by mechanical means. No doubt
the different power of different tannages to “carry grease” without
appearing greasy, is also related to the degree of isolation of the
fibrils, and their surface tension with regard to fats. We may judge
that the more readily an oil can be emulsified, the more freely and
completely it is likely to fix itself on the leather fibre.
[Illustration: FIG. 87.--Hand Shaving.]
[Illustration: FIG. 88.--Shaving Machine.]
It is a practically invariable rule that the leather-fibre must be wet
when it is stuffed. The surface-tension between the water and the fats
is less than that of either with regard to air; and therefore, as the
water dries out of the small interstices of the leather, the fat follows
it in, and gradually takes its place. Generally speaking, the amount of
water should be such that some exsudes in minute drops when the leather
is pinched, that is, that not only the minutest spaces between the
fibrils are filled, but even the larger ones between the fibre-bundles
to a considerable extent.
[Illustration: FIG. 89.--Band-Knife Splitting Machine.]
In “_hand-stuffing_,” the leather is now coated on the flesh side, or
occasionally on both sides with “dubbing,” which is a pasty mixture of
fats usually mainly composed of cod-oil and tallow, which is applied
rather thickly with a brush and smoothed down with the fleshy part of
the forearm. When such constituents are melted together, the harder fats
dissolve in the oils, and as the mixture cools, much of the hard fats
again crystallise out. To make a good dubbing, the cooling fats must be
stirred continuously till this has taken place, as otherwise the mixture
separates into little globular masses of crystals with liquid oil
between them, instead of forming a uniform body of salve-like
consistency. The proportions of the hard and soft constituents of the
dubbing should be adjusted to the season, and to the temperature at
which the drying of the stuffed leather is to take place, so that on the
one hand, the dubbing will not melt and run off, and on the other, that
it should not solidify more than is necessary, as only the liquid
solution which remains entangled among the crystals can be absorbed by
the leather. The solid crystalline fats remain on the surface, and are
scraped off by the sleeker in finishing, as “table-grease,” which is
generally re-melted and used over again. It does not answer, in
hand-stuffing, to carry this re-use too far, as the table-grease
contains only the harder parts of the fat, with a continually increasing
proportion of stearic acid, so that if a dubbing be made continuously of
table-grease and oil, in the end little but the latter will be absorbed
by the leather; while where fresh tallow is used, a portion of its
softer constituents remains dissolved in the oil. The principal function
of the harder fats is the mechanical one of retaining the oil on the
surface of the leather; and to a certain extent they may be replaced by
other solids, such as steatite (“French chalk”), or perhaps other pulpy
materials. The use of a portion of soft fat, such as bone-fat, or the
better sorts of glue-grease, is quite practicable, especially if mixed
with the harder table-grease.
The drying of hand-stuffed leather should be slow, to allow time for the
absorption of the grease; and the temperature should be so regulated as
to keep the dubbing in a soft but not liquid condition. In winter, if
the temperature of the outer air be raised sufficiently for this, the
drying will be too keen (cp. p. 426) and the water will be dried out
before the grease is properly absorbed. It is therefore best, in cold
weather, to maintain the ventilation mainly by circulating the air in
the room, with little admission from the outside, and in extreme cases
even artificial damping of the air may be advantageous. Sometimes the
tendency to mildew during slow and warm drying is very troublesome. This
may be prevented by the addition of antiseptics to the stuffing grease.
Carbolic acid and creasote are effective, but generally objectionable
from their smell; rosin oil has considerable antiseptic power, and
mineral oils also in a less degree. Probably α-naphthol would prove an
efficient remedy, as it has little odour, and its antiseptic properties
are very strong, but it has not been tried by the writer. (Cp. Chapter
V.)
[Illustration: FIG. 90.--Haley’s Injector Stuffing Drum.]
In _drum-stuffing_ the conditions differ materially from those of
hand-stuffing. The goods, in a damp condition, are placed in a drum
(Fig. 90), which has been heated by steam to as high a temperature as
the leather will safely stand. Cold damp leather may be stuffed in a
drum heated to 60° C. and the grease may be run in at the same
temperature. The grease should generally be melted and mixed at a
somewhat higher temperature. Sometimes steam is merely blown into the
drum before introducing the leather, to heat it to the required
temperature; sometimes a steam-coil is placed in the drum itself. A more
modern method, which is now largely used in the United States, is to
heat by hot air, which is circulated by a fan over an external steam
heater and through the drum. The drum is set in rotation, and the
stuffing grease in a melted condition is run in through the hollow axle,
or if this is not provided, it is introduced through the door, and the
rotation is maintained for twenty to thirty minutes. During the last few
minutes the door is frequently replaced by an open grating or cold air
is drawn through the drum by means of the fan, in order to cool the
goods, which are set out with the sleeker on the table while yet
somewhat warm, and dried under much the same conditions as have been
described with regard to hand-stuffed goods.
In drum-stuffing, the hardness of the grease is limited by its
melting-point, which must not be so high as to damage the leather, but
it may be soft as is desired. As the grease is forced by mechanical
means into the interior of the leather, there is no danger of its
running off, but the drying must take place at such a temperature as to
keep it at least in a partially soft condition, as the drumming only
forces it into the coarser spaces of the leather, and does not complete
its distribution on the fibre. By the use of exceedingly hard greases,
such as “stearin” (p. 359) and oleo-stearin (p. 356), sometimes with
additions of paraffin wax, it is possible to introduce immense
quantities of grease, and yet to obtain a leather which will board up to
a good colour. In America, it is not unusual to reckon 100 or even 115
lb. of greases to 100 lb. of leather weighed dry after scouring, or
estimated from its wet weight; and the whole of this is absorbed,
scarcely anything coming off in “setting.” The leather, as it comes from
the drum, is dark brown, but when bent sharply in “boarding” to form the
grain, after cooling and drying, the very hard and crystalline fats
crumble into white powder, and the leather takes a light and pretty
colour. Such leather would of course darken at once if it were held to
the fire, but would again brighten on cooling and breaking up with the
“board.” Some portion of liquid fats, such as dégras or fish oil, should
be contained in the stuffing grease, as the solid fats alone will not
penetrate to the heart of the fibres, but will leave the leather dry and
harsh.
By drum-stuffing, it is possible to incorporate solid matter with the
leather, and barytes (ground heavy-spar or barium sulphate) was formerly
much used for this purpose, but has now been nearly abandoned. Glucose
is still used as an adulterant of leather, but is not introduced in the
drum, but by painting the goods with syrup before stuffing. It not only
adds weight, and gives the leather a lighter colour than an equivalent
quantity of grease, but at the same time lessens its toughness, and
ought to be prohibited in England, as it already is in Germany. On the
detection of adulteration of leather, see L.I.L.B., p. 212.
Drum-stuffing is in this country mainly applied to shoe-leathers, but in
America, with the hot-air drum, is coming into increasing use for
harness, and even belting.
A method of stuffing is used in Germany for heavy belting and the like,
which appears at first glance to contradict the axiom that leather must
be stuffed wet. It is called _Einbrennen_ (to burn in), and consists in
first drying at a high temperature (50° C.), to ensure the absence of
all moisture, and then either pouring hot melted tallow over the leather
on a table, and holding it over a brazier, to allow the grease to sink
in, or dipping it completely in a bath of melted tallow. The exception
is only apparent, because, though the leather is at this stage
completely saturated with tallow, it is only after wetting and drumming
that it attains the flexibility due to true stuffing. Similar methods
are applicable to alumed leathers, and even to chrome-leather; and
so-called “waterproof” or “anhydrous” leather is made by immersing
thoroughly dried leather in a bath of 2 parts of resin and 1 of
paraffin, or some similar mixture. If the leather is not first
thoroughly dried, it is scalded and destroyed by the hot grease.
The most troublesome defect to which stuffed leathers are liable, is
known as “spueing,” and is of two kinds, of which the first and less
serious (perhaps more properly distinguished as “striking out”) consists
of a white efflorescence rather like incipient mould, which is easily
wiped off, but generally reappears. This is due to the crystallisation
of the harder fats, and especially of the free fatty acids, on the
surface of the leather, and is almost sure to occur in greater or less
degree when the hard fats such as tallow or stearine are combined with a
non-drying oil such as neatsfoot, or when soft fats are present in the
leather. It is sometimes combined with actual mildew, from which it is
rather difficult to distinguish, even under the microscope, and may even
be caused by fungoid plants, which not only mechanically expel the fats
by their growth, but probably promote their rancidity and the separation
of the crystalline fatty acids. It is at most only a defect of
appearance, and does not in any way injure the leather. It is constantly
present in calf-kid, from the neatsfoot oil used in finishing, and is in
this case rather liked by the buyers, who for some reason regard it as a
proof of quality. A very similar appearance may be caused by the use of
solutions of barium chloride, alum or other mineral salts, for weighting
or other purposes; but is persistent when the leather is held to the
fire, while the crystallised fatty acids at once melt and disappear. The
fatty acids are at once removed by a drop of benzene or petroleum
spirit; but unaffected by water, while with water-soluble salts the
reverse occurs.
The second form of spueing is of a much more troublesome character, and
makes its first appearance as minute spots or pimples of resinous
matter, raised above the surface of the leather, which if removed,
generally reappear, and which may become so bad as to form a sticky
resinous coating over the whole surface. The exsuded matter consists of
the oxidised products of oxidisable oils, but the cause of its
appearance is not always easy to explain. The currier generally
attributes it to adulterated oils, and it must be admitted that some
oils almost invariably produce it, but it appears occasionally when only
the purest and absolutely genuine cod-oil has been used. It can only be
produced from drying or semi-drying oils, which include all the ordinary
fish oils and most of the vegetable seed oils, but can never arise from
tallow or stearine, from mineral oils or vaseline, or from genuine
non-drying oils, such as tallow, neatsfoot, sperm, or mineral oils, nor,
probably, from rosin oil. It is favoured by causes which promote the
oxidation of oils, such as moist heat with limited access of air, and by
the presence of oxygen-carriers, such as iron-salts in blacks, and
possibly also by the presence of free acids. A large amount of free
fatty acid in the oils themselves is suspicious, not only because the
free acids oxidise more freely than the neutral fats, but because their
presence is an evidence of the tendency to rancidity and change in the
oil. It is also said to be caused by previous mildewing of the leather,
and certainly often occurs where the grain has been rendered porous by
bacterial action in the soaks, limes, or bates, probably from the
greater quantity of oil absorbed by these parts. While it is easy to say
which oils may possibly spue, there is no known chemical test which will
foretell whether a given sample is likely to do so under ordinary
conditions. Eitner[169] states that seal oil extracted at a low
temperature is very liable to spue, but that when heated for a
considerable time to a temperature of 250°-290° C. it darkens in colour
and loses the tendency. This is probably true of many other marine oils;
and may be one cause of the frequent trouble with modern oils, many of
which, especially the lighter coloured kinds, are extracted by steam at
a temperature below boiling point. It is very probable that one effect
of heating to a considerable temperature is to dehydrate and separate
albuminous or gelatinous matters which are present in the fresh oils,
and which probably increase their tendency to decomposition. Many of
these substances separate as “foots” from oils during long storing, and
such old oils are said to be less liable to spue than those of recent
manufacture.
[169] Gerber, 1880, p. 243.
If oxidisable oils are used upon leather, they “dry” upon the fibre, and
if a sufficiency of non-drying constituents are not present at the same
time, the leather will ultimately become hard, and may even crack from
hardening of the fibre. Mineral oils are not liable in this way to form
a hard coating on the fibre, but as they are slightly volatile, though
of very high boiling point, they may ultimately evaporate, and leave the
leather insufficiently nourished. From their low surface-tension, they
have great powers of capillary penetration, as is witnessed by the way
that lamp oils “creep” over the surface of the lamp, but they have less
affinity for water than the more oxidisable oils, and probably do not
combine so intimately with the leather-fibre. They are probably better
used in combination with other greases than alone. The admixture of
solid paraffin with stuffing greases has the tendency to make the
leather feel less greasy and drier than it otherwise would; and crude
turpentine and rosin are said to have a still greater effect in this
direction.
The water which is required for satisfactory stuffing may in some cases
be introduced into the stuffing grease as well as into the leather. The
effect of dégras is largely due to the water with which it is intimately
mixed, and when dégras or sod-oil is deprived of that which it naturally
contains, by heating it to too high a temperature, either before or
after its mixture in a stuffing grease, its efficacy is greatly
lessened.
_Fat-liquoring_ (pp. 217, 239) may be considered a special case of
stuffing, in which the oil is very perfectly emulsified with a large
quantity of water. In this way, very considerable quantities of oil may
be introduced into leather without giving it the least greasy feel.
Egg-yolk contains about 30 per cent. of an oil chemically very like
olive, but with a larger proportion of palmitin, and may be considered
as a very perfect natural fat-liquor, containing also some albumen which
serves as “nourishment” for the leather. If a means of emulsifying
olive, lard, or tallow oil (with the addition of a little palm oil) with
albuminous matter as perfectly as in the egg could be discovered, the
problem of an egg-yolk substitute would in all probability be solved.
Milk and cream are also natural fat-liquors.
CHAPTER XXV.
_DYES AND DYEING._
Before the discovery of artificial organic dyestuffs, the only colouring
materials known to industry were those of mineral and direct organic
origin; and on this account the dyeing of leather was formerly subject
to great difficulties and limitations.
The discovery of the means of artificially preparing an organic dyestuff
(mauve) by Perkin some forty-five years since, opened up a new field for
research, and since that time, the list of commercial dyes has so
increased that there is now scarcely a tint or shade which cannot be
accurately matched and reproduced by the coal-tar colours. These colours
are often spoken of as “aniline dyes” owing to the fact that many of
them, and especially the earlier ones, have been derived from aniline,
one of the products of coal-tar; but more recently, a considerable
number of important colours have been prepared from other constituents
of the tar, and it is therefore more correct to term the whole of the
dyes obtained, either directly or indirectly, from coal-tar, the
“coal-tar colours.”
The coal-tar colours are generally soluble in water, or mixtures of
water and alcohol, and the majority of them combine with the fibre of
the leather without the use of any mordant, so that in most cases it is
only necessary to apply a solution of the dye direct to the leather,
though their suitability for the purpose varies considerably. A few
which are only soluble in oils or hydrocarbons, are not suitable for
leather-dyeing, though they may sometimes be utilised in conjunction
with fats in currying; and there are also certain colours which are not
applied to the fibre ready formed, but are developed on it by subsequent
chemical treatment, and which have only been applied to a limited extent
to leather.
A number of the coal-tar dyes, which are produced in the crystalline
form, have a totally different colour when solid to that of their
solutions, and to the colour they produce when dyed. A well-known
instance of this is magenta or fuchsine, which forms glistening green
crystals, while in solution it is a brilliant red dye. The colours of
the crystals are usually complementary to those of the solution, thus
several blues have the appearance of metallic copper, and violets, such
as methyl-violet, are greenish-yellow, generally with a pronounced
metallic lustre. This peculiarity is the cause of the defect in dyeing
known as “bronzing,” in which the dye, when applied in too concentrated
a form, takes a surface-shimmer of its complementary colour.
The coal-tar colours are mostly either “acid” or “basic.” The former are
the salts of organic colour-acids with inorganic bases (generally
sodium) and are usually readily soluble in water, but frequently do not
fix themselves on the fibre till the colour-acid is set free by the
addition of some stronger acid to the bath, and in many cases the free
colour-acid is of different colour to its salts. The “basic” colours are
salts of colour-bases (organic bases of the nature of very complicated
ammonia-derivatives) with acids (mostly hydrochloric, sulphuric or
acetic). Most of those in commercial use are soluble in water, though a
few require the addition of alcohol. The colour-bases themselves are
usually insoluble in water, and therefore precipitated by alkalies, and
in some cases they are also colourless. The basic dyes have generally
greater intensity of colour than the acid dyes, but large classes of
them are very fugitive when exposed to light, and in strong solution
many others are very liable to “bronze,” a defect which is generally
less marked with the acid colours.[170]
[170] It has recently been shown by Lamb (see App. D, p. 498) that
many basic colours are much faster to light on leather than on
textiles.
As it is not obvious at first sight whether a given dye is acid or
basic, a reagent to distinguish them is useful. For this purpose a
solution of 1 part of tannic acid and 1 part of sodium acetate in 10
parts (by weight) of water is conveniently employed, which gives
coloured precipitates with basic dyes, but is not affected by acid ones.
The fact that basic dyes are precipitated by tannins influences their
use in leather dyeing, not only as regards their fixation on the
leather-fibre by the tannin which it contains, but as the cause of their
precipitation in the dye-bath if great care is not taken to avoid the
presence of tannins in a soluble form. The use of the sodium acetate is
to combine with the mineral acid of the colour-salt, which if left free
would prevent complete precipitation, substituting for it acetic acid,
which is much weaker, especially in presence of excess of sodium acetate
(cp. p. 81).
In using the terms “acid” and “basic” with regard to dyes, it is not to
be understood that the dyestuffs as employed are acid or alkaline in the
sense that vinegar is acid, and soda basic, but merely that the actual
colour-constituent of the salt is in the one case of an acid nature, and
set free by stronger acids, and in the other case is basic, and
liberated (and often precipitated) by stronger alkalies.
There are several general theories with regard to the fixation of
colours in dyeing organic fibres, and it is probable that no one of them
affords a complete explanation in all cases. One holds that the action
of dyeing is mechanical rather than chemical, the colour adhering to the
fibre by surface-attraction; another, that an actual chemical compound
is formed between the dye and the dyed material or one of its
constituents; and a third, the “solid solution” theory of Witt, is in a
sense intermediate, holding that the colouring matter is actually
dissolved in the dyed fibre. The idea of a solid solution, strange at
first, offers little difficulty on consideration. The colouring metallic
salts in tinted glasses exist obviously in solution in the melted glass,
and can hardly be said to change their condition in this respect when
the glass becomes solid. Gelatine, indiarubber, and perhaps all other
colloid bodies, absorb water or other liquids without losing their solid
form, and these liquids may fairly be said to be dissolved in the solid.
All animal and vegetable fibres are in this respect like gelatine, and
during the process of dyeing are swollen with water. It is quite easy to
dye a mass of gelatine throughout with most water-soluble dyestuffs.
(Compare on these points what is said in Chapter IX. on the physical
chemistry of hide-fibre.) The distinctions between solution and
molecular surface-attraction on the one hand, and certain forms of
chemical combination on the other, are not wide ones, and probably all
three theories are true in different cases, and shade off into each
other by imperceptible gradations. The subject of leather-dyeing is, in
fact, a very complicated one, since we are not dealing with a fibre of
uniform composition, but with one which has had its structure (both
chemical and physical) altered by the processes to which it has been
subjected during its conversion into leather.
Although, strictly speaking, the constitution of the gelatinous fibre of
the skin is unknown, we are quite justified in stating[171] that, like
the amido-acids which are important proximate products of its
decomposition, it contains both acid and basic groups, and is therefore
capable of attracting both bases and acids. It is well known, for
instance, that the neutral fibre is capable of withdrawing sulphuric
acid from a decinormal solution with such vigour that the residual
liquid is neutral to litmus paper; and it will also absorb caustic
alkalies with perhaps equal avidity.[172]
[171] Procter, Jour. Soc. Chem. Industry, 1900, p. 23.
[172] Cp. Chap. IX.
It is thus readily dyed by colouring matter of either basic or acid
character, and in many cases will even dissociate their salts, dyeing
the characteristic colour of the free dyestuff, but possibly at the same
time fixing the liberated base or acid with which the colouring matter
has been combined. Many tanning processes consist in a somewhat
analogous fixation of weak bases and acids, and it is, therefore, to be
anticipated that they will profoundly modify the colour-fixing
properties of the original fibre, as indeed proves to be the case.
Exactly what the result of a particular tanning process in this respect
will be is less easy to foresee.
In the ordinary vegetable tanning process, the tannins, which are of
acid nature, are freely fixed by the fibre. It is, therefore, not
surprising that vegetable-tanned leather most readily fixes the basic
colours, especially as these form insoluble compounds with the tannic
acids, so that it is quite probable that the dyeing is mainly effected
by the formation of tannin-colour-lakes on the fibre, rather than by
actual fixation of the colour-base in combination with the original
matter of the skin. It is noteworthy, however, that even fully tanned
skin has by no means lost its attractions for acid colouring matters,
many of which will dye it even without the presence of free acid, though
it is possible that the tannic acid performs the function of saturating
the alkaline base with which the colour acid has been combined.
It should be pointed out that while the substance of animal skin
consists practically of gelatinous fibres, it is covered on the outer
surface with a thin membrane of extreme tenuity, called the hyaline or
glassy layer (p. 50) which, in the living animal, separates the true
skin from the epidermis. This layer, the chemistry of which is quite
unknown, reacts to colouring matters differently from the gelatinous
fibres, and probably is less absorbent for basic colours, and more so
for the coloured anhydrides of the tannins, and perhaps for acid colours
generally, than is the true skin. As a result, it colours more darkly in
tanning, and less so in dyeing with basic colours, and as it is
extremely liable to damage in the preliminary operations of removing
hair and lime by the tanner, this irregularity of colouring is a serious
disadvantage which is most marked with the basic colours. Small
quantities of lime left in the skin are also probably important causes
of irregular dyeing.
_Mordants_ are chemicals used to enable the fibre to fix dyes for which
it would not otherwise have sufficient attraction, and hence are
generally substances which have affinity both for the fibre and the dye.
Thus cotton, which does not itself attract the basic colours, is
mordanted for them by a solution of tannin, which it attracts, and
which, in its turn, attracts and fixes the colours. In many cases,
however, the function of mordants is more complex, not merely fixing the
dyestuff, but often modifying, or even producing its colour. Thus tannin
dyes black on an iron mordant, though it is itself colourless. Such
mordants may be applied _after_ the colouring matter, where it has
sufficient attraction for the fibre to be taken up alone, but does not
produce the required colour. This process is often called “saddening,”
as the colour is generally darkened. A familiar instance is the use of
iron solutions to darken or blacken tannin or logwood. There is scarcely
any distinction in theory between mordants of this class and the
constituents of dyes which are successively applied to the leather in
order to produce the colouring matter on the fibre. Among these may be
mentioned several mineral salts which were formerly employed in leather
dyeing, though their use is now nearly obsolete. Iron salts are easily
fixed by leather, whether tanned or tawed, and in the former case
produce a dark colour by action of the tannin. On subsequent treatment
with a solution of potassium ferrocyanide, a deep blue is formed
(Prussian blue). If copper acetate or ammoniacal solution of copper
sulphate be substituted for the iron salt, a deep red-brown ferrocyanide
is produced. Yellows are sometimes dyed by first treating tanned
leathers with lead acetate, which is fixed by the tannin, and then with
potassium bichromate, by which yellow lead chromate is produced. A more
important use of lead is in the so-called “lead-bleach,” which is really
a white pigment-dyeing with lead sulphate. The tanned leather, after
washing, is first treated with a solution of lead acetate (usually
“brown sugar of lead” of about 4 grm. per liter), and subsequently with
a dilute sulphuric acid of about 30 grm. of concentrated acid per litre,
and then thoroughly washed to free it from acid. The process is often
used as a preparation for dyeing pale shades, as many of the aniline
dyes are easily fixed on the bleached leather, but is subject to the
disadvantage attendant on all pigments containing lead, of becoming
rapidly darkened by traces of sulphur or sulphuretted hydrogen, such as
are constantly contained in lighting gas, or arise from the putrefaction
of organic matters. The use of acid is also liable to cause early decay
of the leather.
A large proportion of the coal-tar colours contain amido-groups (NH₂
groups) which, when treated on the fibre with nitrous acid (or an
acidified solution of sodium nitrite), become “diazotised” (converted
into --N : N-- groups with elimination of OH₂). On further treating the
diazo-compound with solutions of amines or phenols, combination takes
place, and new azo-colours are formed in or on the fibre, often
remarkably fast to washing or rubbing. Since these qualities are less
important in leather than in textiles, and the process is moreover
somewhat delicate, and the nitrous acid is apt to injuriously affect the
leather, these processes have been little used in leather-dyeing, and
are only mentioned here for the sake of completeness.
The use of the natural polygenetic colours in dyeing leather of
vegetable tannage, which was once universal, is gradually disappearing,
except for the production of blacks. Leather cannot be very
satisfactorily mordanted for these colouring matters; but they have some
natural attraction for the leather itself, and are generally dyed first,
and their colours afterwards developed by metallic mordants such as
iron, chrome, tin salts, and alum, which act not only on the absorbed
dyestuff, but frequently on the tannin and colouring matters derived
from the tanning materials. For black-dyeing, the use of coal-tar
colours, either alone, or to deepen the colours produced by iron, is
gradually extending. Claus and Rée’s “Autho-black,” the “Corvolines” of
the Badische Co., and Casella’s “Naphthylamine Black,” “Aniline Grey,”
and “Naphthol Blue-black” may be mentioned as useful colours. As
coal-tar blacks are mostly dark violets rather than dead blacks, their
colour may be deepened by the admixture of suitable yellows or browns,
and this has already been done in one or two of the colours named. Apart
from the coal-tar colours, black dyeing is generally produced by the
action of iron (and chrome), either on the tannin of the leather itself
or on logwood. As the leather is frequently greasy, and the satisfactory
formation of a tannin- or logwood-lake can only take place in presence
of a base to absorb the liberated acid of the iron salt, the skins are
either brushed with, or plunged in, a logwood infusion, rendered
alkaline with soda or ammonia, or the tanned leather receives a
preliminary treatment with weak soda or ammonia solution. As such
solutions act powerfully on tanned leathers, rendering them harsh and
tender, great care must be taken to avoid excess. The effect of this
alkaline treatment is not only to assist the wetting of the greasy
surface, but to prevent too deep penetration of the dye, by causing
rapid precipitation of the colour-lake. In recent times, however,
leathers are sometimes demanded in which the colour goes right through,
and in this case it might be well to reverse the treatment, beginning
with a weak solution of a ferrous salt, perhaps with addition of sodium
acetate or potassium tartrate, and finishing with alkaline logwood, as
without alkali the full colour is not developed. The use of iron salts
is not very satisfactory in regard to the permanence of the leather; and
in this respect it is of great importance that they should not be used
in excess, and that any strong acids they contain should be saturated
with permanent bases, and if possible washed out. Leather-surfaces
blacked with iron almost invariably ultimately lose their colour,
becoming brown if tannins, and red if logwood has been employed, and at
the same time the leather surface usually becomes brittle or friable.
This is to a large extent due to the effect of iron oxides as
oxygen-carriers. Exposed to light, they become reduced to the ferrous
state, oxidising the organic matters with which they are combined, and
in the dark they re-oxidise, and the process is repeated. It is
therefore of the first importance that excess of the organic colouring
matter should be provided, and that the quantity of the iron should be
as small as possible, and in stable combination. These points are
greatly neglected in practice, especially where blacking is done by the
application of iron salts without logwood, when the evils mentioned are
intensified by the actual removal of part of the tannin of the leather,
and perhaps by the combination of ferric oxide with the skin-fibre
itself, forming a brittle iron-leather. Treatment with alkaline sumach-,
gambier- or logwood-solutions, both before and after the application of
the iron, would lessen the evil. Iron-logwood blacks are much less
permanent, and fade more rapidly under the influence of light and air
than iron-tannin blacks. The use of iron-blacks on curried leathers
seems considerably to increase the tendency to “spueing,” a defect due
to oxidation of the oils (see p. 390). Copper salts mordant logwood a
very dark blue, which is much more stable than the iron compound, and
hence are often used advantageously in mixture with iron salts. In
practice, iron blacks are generally oiled in finishing, and this renders
them more permanent, both by protecting the lake from air and by forming
iron soaps which are stable. The use of actual soaps in blacking and
finishing is not unknown, and probably deserves more attention. Hard
soaps of soda and stearic acid,[173] form an excellent finish where a
moderate glaze is required, the soap jelly being applied with a brush
very thinly, allowed to dry thoroughly, and polished with a flannel or
brush, or glassed. Many acid colours are soluble in such soap jellies,
which may thus be employed for staining. Similar but harder finishes,
and capable of being glazed to a high polish, are made by dissolving
shellac with dilute borax or ammonia solutions.[174] Both of these
finishes are useful in lessening the tendency of iron blacks to smut or
rub off, a failing which is due to the precipitation of loose iron-lakes
on the surface, instead of in combination with the fibre, and is
particularly obvious where “inks” or one-solution blacks are employed,
or where the mordant and the colouring matter solutions are allowed to
mix on the surface of the leather. Such “inks” are generally made with a
ferrous salt and logwood or tannin, together with some aniline black,
and the colour-lake should only be formed on oxidation. Chrome is not
much employed in blacks with vegetable tannages, as it only produces
blacks with logwood, the chrome compounds of tannins having no colouring
value; and bichromates used at all freely being very injurious to the
leather.
[173] 1 of caustic soda in 10-15 of water, boiled with 8 of stearic
acid till clear, cooled to 25° C. and diluted with 400-800 water, with
constant stirring, till the white jelly of suitable consistence is
obtained. Somewhat similar, but harder preparations may be made with
waxes, or fatty acids still higher than stearic.
[174] 5 parts of shellac digested warm with 100 water and 3 of ammonia
fort., or 1 of borax. If the solution is used as a “seasoning” for
glazing, the waxy matter which separates on standing should be mixed
by shaking before use. As a varnish, a stronger solution should be
used and the wax skimmed off.
In dyeing blacks on other than vegetable tannages, however, chrome
becomes of importance, as logwood is principally employed, though
sometimes in conjunction with tannin, and often with addition of
quercitron or fustic, to correct the bluish shade of the logwood-chrome
or logwood-iron lake. It must not be overlooked in practice, that if
ferrous salts are mixed with bichromate solutions, the latter are
reduced, and the iron is oxidised to the ferric state.
In alumed leathers the fixing power of the original hide-fibre is much
less affected than in vegetable tannages. Whatever may be the truth with
regard to the latter, there is little doubt that physical influences are
at least as important as chemical ones in the production of mineral
tannages. The amount of the tanning agent absorbed is greatly influenced
by the concentration of the solutions, and in ordinary alum tawing much
of the alumina may again be removed by free washing. In this case, the
sulphate of potash present takes no part in the operation, but the
alumina salt is absorbed apparently as a normal salt. Alum or alumina
sulphate alone is incapable of producing any satisfactory tannage
without the assistance of common salt, the quantity absorbed being
small, and the fibre becoming swollen by the action of the acid. In
presence of salt the absorption is greater, and the swelling is
prevented. The explanation of this is not to be found in the formation
of aluminium chloride, for though this undoubtedly takes place, it has
been shown that the action of aluminium chloride without salt is not
more satisfactory than that of alum. It has long been known that salt
prevents the swelling action of acids on skin, although it does not
lessen the absorption of acid; and the fact is capable of explanation on
modern osmotic theories (cp. p. 89). The skin so treated is found to be
converted into leather, but if the salt be washed out, the acid is
retained by the skin, which returns to the state of acid-swollen pelt.
It is probable, therefore, that although the acid and alumina are
absorbed in equivalent proportions to each other, they are really
dissociated, and attached to different groups in the gelatine molecule,
and that the effect of the salt is to allow the absorption of the acid
without swelling, and, osmotically, to increase the dissociating power
of the pelt. If, in place of a normal alumina salt, a basic salt is
employed, such as may be obtained by partial neutralisation of the
sulphuric acid with soda, satisfactory tannage may be accomplished
without salt, a basic compound is absorbed, and the leather is much less
affected by washing. In the analogous case of chrome tannage, this basic
compound may be still further deprived of its residual acid, by washing
the tanned skin with alkaline solutions, leaving a leather which is
extremely resistant even to hot water; and a somewhat similar result may
be obtained with alumina, though with more difficulty, as apparently a
very small excess of alkali destroys the qualities of the leather. (Cp.
p. 187.)
The results on dyeing are almost what might have been foreseen. While
ordinary alumed leather absorbs both acid and basic dyes readily, the
basic chrome leather has practically lost its affinity for the latter.
Both chrome and alumina leathers readily absorb vegetable tannins, thus
supporting the view that the acid-fixing groups of the gelatine molecule
are still unsaturated (tannins are capable of tanning pelt swollen with
sulphuric acid and apparently of expelling the acid). In the case of
chrome leather the effect of re-tanning with tannins is greatly to
lessen its stretch, and if carried too far, to destroy its toughness,
but it at once becomes capable of fixing basic dyestuffs. This property
is frequently made use of in dyeing, but the effect on the leather must
not be disregarded where softness and stretch are important, as in the
case of glove-leathers. Polygenetic dyes are, of course, fixed on alum-
or chrome-leathers by the alumina- or chrome-mordant, though apparently
the bases are not present in the most favourable condition for fixing
colours. Thus logwood extracted without alkali dyes tanned leather
yellow, alumed leather violet-blue, and chrome leather blackish-violet,
and some of the alizarine group dye very well on chrome as its
resistance to hot water allows much higher temperatures to be used than
with most other leathers. The tannin contained in dyewoods has the
effect of lessening the stretch of chrome leathers.
Something should perhaps be said on the dyeing of oil and aldehyde
leathers, but the subject has as yet been scarcely treated
scientifically, and our practical knowledge of the subject is
insufficient to justify theorising. (See, however, p. 496.)
Defects in the colour of the finished leather are due to a variety of
causes, but many are produced by want of cleanliness and system during
the dyeing itself. The greatest care is needed in this respect, and in
brush-dyeing a different brush should be used for each different colour,
as it is impossible to thoroughly remove all traces of dye by the
ordinary methods of cleansing.
Irregular and surface dyeing sometimes occurs owing to too rapid
fixation of the colours; while in other cases the affinity of the dye is
too small to allow of reasonable exhaustion of the bath. Addition of
salts of weak acids, such as potassium hydrogen tartrate (tartar), or of
those like sodium sulphate, which form hydric salts, lessen rapidity of
dyeing with acid colours; while acids generally increase it, and it is
also often increased by addition of common salt, which lessens the
solubility of the dye. Weak acids, such as acetic or formic, or acid
salts, such as sodium bisulphate, are generally to be preferred to
sulphuric acid as an addition to the dye-bath; and if the latter is
used, great care is desirable in its complete removal. There is no doubt
that the rapid decay of leather bookbindings and upholstery is largely
due to the careless use of sulphuric acid in “clearing” and dyeing the
leather;[175] and even if it is fully removed, it has saturated all
bases such as lime, which are naturally present in leathers in
combination with weak acids, and which would otherwise act as some
protection from the sulphuric acid evolved in burning coal gas.
[175] See Report of Committee of Society of Arts on Bookbinding
Leathers, 1901.
“Bronzing,” the dichroic effect produced by light reflected from the
surface of many colouring matters, complementary to that transmitted by
them and reflected by the surface of the dyed material, is not peculiar
to basic colours, but is generally more marked in them than in acid
ones. Basic colours, from their great affinity for tannins, and
consequent rapid dyeing, are apt to dye irregularly, and without
sufficiently penetrating the leather, and if the soluble tannin is not
wholly washed out of the skins previously to dyeing, it bleeds in the
dye-bath, and precipitates insoluble tannin-lakes, which waste colour
and adhere to the surface of the leather. The inconvenience of basic
colours due to their too rapid fixation may sometimes be lessened by
slight acidification of the dye-bath with a weak acid, such as acetic or
lactic. The acid may be still further “weakened” if desired, by the
addition of its neutral (sodium) salt. The precipitation of tannin-lakes
in the bath may be prevented by previous fixation of the tannin with
tartar emetic, titanium potassium oxalate or lactate, or some other
suitable metallic salt.
The fading of the colours of dyed goods by exposure to light is a defect
which has been much more investigated in the textile industries than in
leather manufacture, though in the latter case, and especially with
regard to bookbinding and furniture leathers, it is of even greater
importance. It is probable that no colours are actually unaffected by
strong sunlight, but in many cases the action is so slight that it may
practically be disregarded; some of the coal-tar colours, and especially
some of the alizarines, being practically permanent, while others, and
particularly the aniline colours belonging to the triphenylmethane
group, such as magenta, are so fugitive as to be practically bleached by
a week of strong sunlight. Chrysoidine and the eosins are also very bad
in this respect. The fastness of colours to light is a good deal
influenced by the material on which they are dyed, and but little has
yet been published of the results of direct experiments on leathers, but
Mr. M. C. Lamb has been for some time engaged in a research of this
nature,[176] and the subject is now receiving a good deal of attention
in other quarters. Experiments are easily made by exposing samples to
sunlight under glass or in a south window, a part of the leather being
covered with wood or thick brown paper for comparison. The results are
often complicated by the tendency of all leathers tanned with tannins of
the catechol group, and especially with turwar bark (p. 298), mimosa and
quebracho, to darken and redden in sunshine, or even by exposure to
diffused light. Pure sumach tannages are nearly free from this defect,
and are also much less easily destroyed by the action of gas fumes
(sulphuric acid), and the other injurious influences to which books and
furniture are often subjected.[177]
[176] See App. D., p. 488, 498, and Journ. Soc. Chem. Ind., 1902, pp.
156-158.
[177] Cp. Report of Society of Arts Committee on Bookbinding Leathers,
1901.
Want of fastness to friction or rubbing is a defect generally more
important in textiles than in leather, where it is often prevented by
glazings or other finishes applied to the surface; but in some cases,
and, especially in black leather, it is apt to be annoying. If suitable
colours are used, the defect is generally due to the precipitation of
loose colour on the surface, either by the too free use of mordants, or
the dyeing of basic colours on leathers which have not been sufficiently
freed from loose tannin. It is also often caused by “flaming” or the
application of colour mixed with the “seasoning” used in glazing, to
hide imperfections in the dyeing, or vary its colour. Colour applied in
this way is only mechanically fixed on the leather, and is easily
removed by moisture, staining articles with which it comes in contact.
A very similar defect may be caused by incomplete washing of the dyed
leather, which leaves loose dye from the dye-bath in the goods. To avoid
it in glove-leathers, where its occurrence would be particularly
annoying, the natural mordant colours are still largely in use, which
being precipitated on the fibre in an insoluble form by the mordant or
“striker” (generally a metallic salt) are little liable to come off.
Basic colours may be fixed by a subsequent treatment with tannin, or by
topping with certain acid colours such as picric acid. Some few colours,
and especially Martius or “Manchester” yellow (dinitronaphthol) are
volatile at a low temperature, and therefore liable to “mark off” or
stain any materials with which the dyed fabric, even in a dry state, is
placed in contact.
[Illustration: FIG. 91.--Dyeing in the Tray.]
The practical dyeing of leathers varies considerably according to
whether they are tanned with vegetable materials, chrome, alumina salts,
or chamoising. Vegetable-tanned leathers are dyed either by hand in the
“dye-tray,” or in the drum or paddle, the two latter methods being now
largely employed. The dye-tray is a shallow vat, about 10 inches deep,
and large enough for the goods to be laid flat in it. In the English
method, one or two dozen skins, or even more, are dyed at a time, being
turned over in the tray by hand, the undermost pair being drawn out and
placed on the top (Fig. 91). The method is convenient where only a small
number of skins are to be dyed to one particular shade, which is more
easily matched as the goods are always under observation, and it has the
further advantage that, if desired, the grain sides only of the skins
can be coloured, by “pairing” or “pleating” them before dyeing. For this
purpose two skins of equal size are laid together flesh to flesh
(pairing), or each skin is doubled down the back, flesh side in
(pleating), and pressed firmly together with a sleeker on the table,
when the skins adhere so closely that if carefully handled, no colour
penetrates between them during the dyeing, except a little round the
edges. This effects considerable economy of dye-stuff, as the fleshes
would absorb a good deal, and for some purposes, an undyed flesh is
preferred. In dyeing in the paddle or drum, the skins are merely placed
loose in the dye-liquor, so that the fleshes are dyed equally with the
grain sides. Paddle-dyeing has the advantage of effecting a considerable
saving of labour, as compared with the dye-tray, in which constant
handling, which often lasts an hour or more, is required. It also allows
of almost equal facility in examining the colour of the skins, which is
very important when dyeing to shade; but it is less economical in
dye-stuff, as not only the flesh sides are dyed but a much larger
volume of liquor is used, and as the dye-bath can never be entirely
exhausted, more dye is run away in the used liquor. Drum-dyeing is much
less expensive in this respect, as the volume of liquor may be very
small, and from the efficiency of the motion, the dyeing is very
thorough, and penetrates deeply into or through the skin, which in many
cases is advantageous, but it is difficult to dye to exact shade, since
the skins can only be examined by stopping and opening the drum. Most
dyes are more readily fixed at high temperatures, and in this respect
the drum has an advantage over all other methods, as once heated it
retains its heat with very little loss to the end of the operation,
while both in the paddle and the dye-tray the liquor is rapidly cooled,
and special methods of maintaining the temperature complicate the
apparatus, and require great care to avoid overheating. It is usually
best to work at the highest temperature which the goods will safely
bear, and this varies to some extent with the class of goods, chrome
tannages and chamois leather being peculiar in standing almost any
temperature short of boiling. With vegetable tanned leather 50° C. may
be taken as a maximum; but cold wet skins may safely be introduced
rapidly into a liquor heated to 60°, as they will cool it sufficiently.
The Continental method of dyeing in two trays may be mentioned here, as
it produces very rapid and even dyeing, with considerable economy of
dye-stuff, and the principle is capable of application to other methods
where a large number of skins have to be dyed to the same colour. As
generally carried out, two trays are employed, each about 4 feet long,
18 inches wide, and 10 inches or a foot deep, and these are usually made
with a sloping bottom, or propped up in such a way that the dye-liquor
all runs to the further side of the tray. A single pair of skins is
usually dyed at once (in about 6 liters (5 qt.) of liquor for sheep and
goat). To begin with, the first tray is filled with a very weak liquor,
and the second with one of about half strength. The goods are entered in
the first tray, turned a few times, and passed into the second; the
liquor in the first is run away, and it is re-filled with one of the
full strength, to which the goods are then transferred, and dyed to
shade. The second tray is much reduced in strength by the skins, and now
serves as the weak liquor for a fresh pair, which in its turn passes
into that from which the goods have been dyed out, and then into a new
liquor; each pair of goods thus passing through three baths, of which
the last is of full strength, and which quickly brings up a full and
even colour. In the ordinary English method, the goods must, for the
sake of economy of dye-stuff, be dyed out in a nearly exhausted bath,
which is a tedious operation, the last stage of dyeing often taking a
time far longer than that required to bring the goods nearly up to
shade, and even then failing to produce a good and full colour. This
evil may be lessened by adding the dye-stuff in several successive
portions, as the bath becomes exhausted, but cannot be altogether
avoided with a single tray, if any reasonable exhaustion of the bath is
to be attained. At first sight it seems a very slow process to dye the
goods in single pairs, but this is to a great extent compensated by the
rapidity with which they take on colour. In the Continental system, the
dyes, mostly of the coal-tar series, are used as strong solutions, and
each new dye-bath is made up by filling the tray with a definite volume
of hot water and adding a measured quantity of the dye-solution.
The re-use of partially exhausted dye-baths is generally limited to
cases where either single dyes, or mixtures of very equal affinity for
the leather are employed, since where dyes of unequal affinity are
employed, one is more rapidly removed than the other, and the shade of
the dye-bath is altered. Many dyes sold as single colours are really
mixtures,[178] and alter in shade if successive quantities of leather
are dyed in their solutions. Basic dyes are also apt to be precipitated
by traces of tannin washed out of the goods, and thus rendered unfit for
use a second time. This may be avoided by suitable preparation of the
goods (see p. 411).
[178] Such mixtures may often be detected by putting a drop of their
solution on blotting-paper, when the dyes form differently coloured
rings according to their more or less rapid fixation by the paper, or
by dusting the dry dye _very_ thinly on wet blotting-paper, when each
particle produces its separate spot.
Much of the success of practical leather-dyeing depends on proper
selection and preparation of the goods. Sound uninjured grain is a
matter of first importance; no satisfactory dyeing can be expected on
skins which through carelessness in soaks, limes, or bates, are tainted
by what is known as “weak grain,” caused by destruction or injury of the
delicate hyaline layer, which forms the natural glaze and outer surface
of the skin (p. 50). For such goods, “acid” are to be preferred to
“basic” dyes, the latter having an especial tendency to dye darker and
deeper where the grain is imperfect. Goods of different tannages and
colours should never be dyed together, as they are certain to produce
different shades in the same dye-bath. Tanned skins which have been
dried, especially if they have been in stock for some time, should be
thoroughly softened by soaking in tepid water and drumming, a
temperature of between 40° and 45° C. being most advantageous. Skins,
such as calf of mixed or bark tannage, must now be freed from all bloom
by scouring with brush and if necessary with slate or stone, but great
care is requisite to avoid injury to the grain. A little borax or other
weak alkaline solution assists in removing bloom. Fresh sumach-tanned
skins merely require setting out with a brass sleeker, but those which
have been long dried often dye more evenly and readily if they are
re-sumached.
Dark coloured tannages, such as Australian bazils, and East India sheep
and goat tanned with cassia bark, are always improved by sumaching, and
if for light colours, by first stripping a portion of the original tan
by drumming for a quarter of an hour with a weak (¹⁄₄ per cent.)
solution of soap powder or borax at a temperature of 30° to 35° C. and
then passing (after well washing in warm water, but with as little
exposure as possible to the air) through a weak sour of sulphuric acid
of 1-2 per cent. The acid should now be as thoroughly removed as
possible by washing in water, and the goods should be sumached. The
process, and especially the use of sulphuric acid, is always deleterious
to the skins, and is one of the causes of the early decay of coloured
bookbindings and furniture leathers. Lactic, formic, or acetic acid may
be substituted for sulphuric with safety, and the risk of injury from
sulphuric, which generally is only apparent after the lapse of a
considerable time, is a good deal lessened by adding to the sumach
liquor a small quantity of potassium tartrate, sodium acetate or
lactate, or some other salt of a weak organic acid, which is thus
substituted for the much more dangerous sulphuric. Except in cases of
absolute necessity for the production of light shades, the use of
sulphuric acid should not be resorted to, and then only for goods which
are not expected to possess great permanence. For light shades for
bookbinding and upholstery, good sumach-tanned leathers and organic
acids only should be employed. Alkaline treatment also demands great
caution, as excess of strong alkalies is very injurious to the leather.
Another objectionable method for the preparation of leather for very
light shades, is the use of the lead-bleach described on p. 399.
The sumaching is best done in a drum, at a temperature of about 40°.
Lamb advises that 1 to 2 lb. of sumach per dozen is sufficient for calf,
and recommends running in this liquor for two or three hours. The skins
are then rinsed in water to free them from adhering sumach, and set out
on a table with a brass sleeker, and are now ready for dyeing with
“acid” dye-stuffs. If “basic” dyes are used, thorough washing in several
tepid waters is necessary to free them from the loose tannin; and if
deep colours are to be dyed, it is better, instead of too much washing,
to fix the tannin, which then serves as a mordant for the colour. For
blues, blue-greens, or violets, this is done with a solution of “tartar
emetic” (antimony potassium tartrate, of 5 to 20 grm. per liter
according to the amount of tannin to be fixed, often with addition of
some common salt), which produces no alteration in the colour. For
browns, yellows, deep reds, or yellow-greens, it is advantageous to use
titanium-potassium lactate or oxalate (2 grm. per liter), which in
combination with the tannin produces a very permanent yellow coloration
on which the basic colours dye freely. In many cases the titanium salt
is best applied after dying with one of the dyewoods (Dreher).
The basic colours usually require simple solution in hot water before
adding to the dye-bath, and are used in quantities of 0·5 to 2·5 grm.
per liter of dye-bath, according to their colouring power, which varies
a good deal, and to the depth of shade required. The solutions should
not be boiled, and some colours are injured by too high a temperature.
Some colours dissolve incompletely, and require filtration through a
cotton cloth. As basic colours are precipitated by calcium carbonate, it
is important that “temporary” hard waters should be neutralised with
acetic or lactic acid till they faintly redden litmus; and in the case
of colours which, from their attraction for the leather fibre, dye too
rapidly, and consequently unevenly, better dyeing is often obtained by
the use of a small excess of acetic acid, which also increases the
solubility of the colour. Too much acid, however, will prevent the
proper exhaustion of the bath. Some few colours, now little used,
require to be dissolved in the first instance in a little methylated
spirit; and the addition of spirit will often assist dyeing and staining
where the leather is slightly greasy, though considerations of cost
generally prevent its use. Sodium sulphate is not unfrequently added to
dyeing baths to improve equality of dyeing; and with some of the cotton
dyes common salt is used to lessen their solubility and facilitate the
exhaustion of the dye-bath.
“Acid” colours usually dye better if acid is added to the bath, to
liberate their colour-acids, and for this purpose sulphuric acid is
generally used in weight about equal to that of the colour used. Its use
is, however, objectionable, in this case, for the same reasons as in
bleaching, since it is impossible by mere washing to remove it entirely
from the leather, which it ultimately rots when concentrated by exposure
to a dry atmosphere or high temperature; and it is better to use formic
or acetic acid to the extent of two or three times the weight of the
dye-stuff. Sodium acid sulphate may also be used, but is probably more
objectionable than an organic acid. Many acid colours, however, dye
quite satisfactorily from a neutral bath. The acid colours are used in
somewhat similar quantities to the basic, but are generally inferior in
colouring power, though they dye more evenly, especially on defective
grain, and are often more permanent to light.
Mention has already been made of the polygenetic or mordant dye-stuffs,
which are still used to some extent for dyeing glove-leathers, and of
which logwood is important in dyeing blacks. Fustic and Brazil-wood
(peach-wood) are not quite gone out of use among old-fashioned dyers,
even for dyeing moroccos and other coloured leathers of vegetable
tannage. Peach-wood, with a tin mordant (generally a so-called “tin
spirits” made by dissolving tin in mixtures of hydrochloric and nitric
acid) was formerly much used in dyeing cheap crimsons, but is now quite
displaced by the azo-scarlets. The acid tin-solutions were frequently
very injurious to the leather.
The wood-infusion, rendered slightly alkaline with soda, ammonia or,
formerly, with stale urine, is usually dyed first on the leather, and
followed by the mordant “striker”; ferrous or ferric solutions, and
potassium bichromate being used for dark colours, and tin salts, or
sometimes alum, for the brighter ones. The mordant is sometimes added
to the dye-bath towards the end of the operation, but is better used as
a separate bath, as it is apt to produce a precipitate of colour-lake on
the surface of the skin, which rubs off on friction. In some cases, and
especially in black dyeing, the strong infusion of dye-wood, and the
necessary “striker” are successively applied by brushing instead of in
the dye-tray.
Logwood and Brazil wood are both Cæsalpinias closely allied to
divi-divi. Logwood is _Cæsalpinia_ (see p. 287) _Campechianum_. Its
colouring matter is hæmatoxylin, a substance nearly allied to tannins,
and almost colourless; which on oxidation gives hæmatin, which dyes a
yellow-brown, only developing other colours by the aid of mordants.
Logwood chips are extracted by boiling or heating under pressure for
some time with water; and as hæmatin gives dark purplish-red compounds
with alkalies, soda or stale urine is frequently added under the
mistaken belief that it produces a better extraction, but really leads
to waste of colouring matter by oxidation. It is best to extract with
water alone, and add any necessary alkali to the infusion before use.
1-2 lb. of wood per gallon is frequently employed in making the
infusion, and as this proportion of water is quite insufficient to
properly extract the wood, the residue should be boiled with one or more
further quantities, which are employed in turn for extracting fresh
portions of wood. Logwood dyes best at high temperatures, and especially
in the case of chrome leather with which a temperature of 80° C. may be
safely used. The presence of a trace of a salt of lime is advantageous,
and with very soft waters a little lime water or chalk may be added to
the logwood liquor.
In blacking skins, the strong infusion is rendered slightly alkaline
with sodium carbonate or ammonia, and brushed undiluted on the leather.
If employed as a bath, a somewhat weaker infusion is used, and the
leather is frequently treated first in an alkaline bath, to which a
small quantity of potassium bichromate is often added. The object of the
alkali is not only to assist in the formation of the colour-lake, by
saturating the acid set free from the iron-salt used as a striker, and
thus to prevent the colour from penetrating the leather too deeply, but,
at the same time, to overcome the resistance to wetting caused by grease
or oil which the leather may contain. It must thus be used more freely
when stuffed leather is to be blacked, but excess should be carefully
avoided, as it easily renders the leather tender and brittle. The
potassium bichromate oxidises the hæmatoxylin, or the ferrous salt
subsequently applied, and forms a nearly black chrome-logwood lake.
The iron solution is generally either of ferrous sulphate of perhaps 5
per cent. strength, or commercial “iron-liquor,” which is a
“pyrolignite” or crude acetate of iron, containing catechol-derivatives
and other organic products from the distillation of wood, which act
advantageously, both as antiseptics, and in preventing the rapid
oxidation which occurs when pure ferrous acetate is used. Iron-liquor is
generally to be preferred to ferrous sulphate (“green vitriol”), as the
sulphuric acid of the latter, unless completely neutralised by the
alkali employed in preparation, acts in the end disastrously on the
leather. Commercial iron-liquor is often adulterated with ferrous
sulphate, which may be detected by its giving a precipitate with barium
chloride. Great care should be taken not to use iron in excess of the
logwood or tannin present, as it otherwise takes tannin from the leather
itself, making it hard and liable to crack, while any uncombined iron
acts as a carrier of oxygen, giving up its oxygen to the colouring
matter or tannin with which it is in contact, and again oxidising from
the air, and so causing “spueing” or oil-oxidation, and other evils.
Good blacks which are more permanent than those with logwood, may be
obtained by merely treating leather containing an excess of oak-bark
tannin or sumach, first with an alkaline solution (not at the most
stronger than 2¹⁄₂ per cent. of liquid ammonia, or 5 per cent. of soda
crystals), and then with iron-liquor. If it is not certain that the
leather contains excess of a suitable tannin, a tannin-solution must be
employed like the logwood infusion, or the leather must be sumached. The
addition of some sumach to logwood liquor is often advantageous, and a
blacker (i.e. less blue) black, especially on alumed leathers, is
obtained by using a proportion of fustic. Solutions made by boiling 10
per cent. of cutch with 5 per cent. of sodium carbonate give good blacks
with iron-liquor, and do not make the leather tender, and they can be
used in mixture with logwood. Many commercial logwood extracts contain
chestnut-wood extract as an adulterant.
Instead of dyeing in the bath, it is very common, especially for the
cheaper leathers such as linings, and coloured leathers of the commoner
sort, to apply the colour by brushing (commonly called “staining”). Many
colours, however, which dye well with time and warmth, are inapplicable
in this way, and only those should be used which have a strong
attraction for the leather, and hence go on well in the cold. If “acid”
colours are employed, it is essential to select those which can be used
in neutral solution, or at most with addition of some mild organic acid
such as formic or acetic, since, as the leather is not washed after
staining, the sulphuric acid would remain in it, and would ultimately
destroy it. Where leathers have a hard and repellent surface, the
addition of a little methylated spirit to the dye is often very useful.
The colours are used in solutions of from ¹⁄₄ to 1 per cent., which
should be quite clear and free from sediment. Difficultly soluble
colours must be used in weak solution, or the dye kept warm while in
use. Dye-solutions will not generally keep for any great length of time
without change.
Before staining, the leather must be carefully “set out,” or otherwise
made as smooth as possible, and the staining is generally done after
most of the other operations of currying or dressing have been
completed. Staining is best begun with the leather in a slightly damp or
“sammied” condition, and the colour is applied evenly with a softish
brush in two or three coats, the leather being slightly dried after
each. As a rule the more coats are applied, the more even is the work;
but to save cost of labour it is common on cheap goods to be content
with two, of which the first is given, preferably with a weaker
solution, to the dry leather. Where the leather is “weak-grained” it is
sometimes advantageous to size it first with a weak solution of
gelatine, gum tragacanth, or linseed mucilage, and similar solutions are
often used to fix the colour and give a higher gloss. The stearine-glaze
mentioned on p. 401 may also be used for this purpose, and a weak
solution of it is sometimes employed as a vehicle for the acid colours.
Acid yellows and browns may also be dissolved in the undiluted glaze
where only a pale colour is required, or to heighten the colour of
leather already stained. A list of suitable colours for staining is
given in the Appendix, p. 486.
It rarely happens in leather dyeing that the required colour can be
given by the application of a single dye, most of the shades now
required being produced by mixtures. It is, therefore, necessary to say
a few words on the theory of colour combinations.
White light is of course composed of a mixture of all the
spectrum-colours, and can be separated into them by the prism. It is
probable, however, that the eye is only capable of three distinct
colour-sensations, and that all the colours we perceive are represented
by the excitement of these in different proportions, the actual
colour-sensations being red, blue-green, and violet.[179] If we
interpose a piece of yellow glass between the eye and white light, the
violet and blue are absorbed, and the remaining red and green rays
combine to produce the sensation of yellow. If pure blue glass is used,
the red is absorbed, and we have blue as the result of the remaining
mixture of green and violet. Red glass absorbs the whole of the green,
and greenish-blue, allowing red and much of the violet to pass. Thus, if
we combine blue and yellow glass, only the green is allowed to pass, and
similarly with red and blue glass, green and blue is cut out, and only
the violet remains. Thus red, yellow, and blue are frequently called the
primary colours, and by combining all three in equal proportions all
colours are cut out, and black or grey results. The blue and violet
which are stopped by yellow glass are those colours which would produce
the sensation of violet-blue, and hence the latter is called the
“complementary colour” of yellow, and so on with the rest. It will be
noted that all the colours of coloured objects are produced by
absorption of a part of the light, and therefore coloured bodies are
always darker than white ones, and where a colour is mixed with its
complementary in suitable proportion, all colours are absorbed and black
or grey is produced.
[179] The subject of colour is too complicated to be adequately
treated here; and for fuller information, readers are referred to
Abney’s ‘Colour Measurement and Mixture,’ S.P.C.K., London, 1891. It
may, however, be pointed out that, while the true primary
colour-sensations are unquestionably red, blue-green and violet, and
by mixture of _light_ of these colours, all other colours, including
white, can be produced; the primary pigments or dyes are red, yellow,
and blue; the effect being produced in the former case by the addition
of colours, and in the latter by their subtraction.
Colours which are made by mixing two primary colours are generally
called “secondary”; while the duller tints made by the addition to these
of black, or of a complementary colour which produces black, are called
“tertiary.” Any primary colour is complementary to the secondary colour
produced by mixing the other two primaries and _vice versa_. The
following tabular arrangement shows at once the effect of colour mixing.
PRIMARY. SECONDARY. TERTIARY.
Red }
} Orange with Black. Brown.
Yellow}
} Green „ Olive, Sage.
Blue }
} Purple (Violet). „ Puce, Maroon.
Red }
Theoretically, _any_ colour may be obtained by mixture of the primaries,
and that this is possible to a great extent is shown in the success of
modern “three colour” printing, by which pictures are obtained in
natural colours by the use of three primaries only; but in practice few
colours are quite pure, and if two very different colours are mixed, it
is difficult to avoid the production of tertiaries. The most brilliant
colours are generally produced by dyeing with the nearest colour which
can be obtained to that required, and shading with another which is
near, but on the other side of the desired tint.
Thus if we want to produce bright shades in dyeing, we must avoid the
introduction of complementary colours. A bluish red mixed with a reddish
blue will produce a bright shade of violet, but if we mix an orange-red
with a greenish-blue, we introduce yellow into the mixture, and obtain a
dull maroon or puce according to the proportion of the other colours. In
a similar way, the introduction of a blue dye will dull a bright orange
to a brown, and a little of a yellow dye will dull a bright purple to a
maroon. This fact is frequently used in producing the quiet shades of
colour often required from the most brilliant dyes. If to a bright
orange we add black, or a blue dye which as its complementary produces
black, we convert it into a brown. If instead of blue we use green for
dulling, we give the brown a yellower shade, since the green produces
black at the expense of the _red_ of the orange. Violet similarly used
gives a redder brown, since it produces black by combination with the
_yellow_. This shading, if small in amount, is frequently done by direct
mixture of a suitable dye, but if considerable, it is generally better
to top one colour with another. Thus a blue, topped with a powerful
orange, will produce a Havanna brown. For dark colours, it is frequently
convenient to produce a dark ground with some cheap dye, such as logwood
and iron or chrome, and to top it with a bright shade of the colour
required. In this way cheap dark blues and greens can be easily
produced. For reds and browns, mixtures of logwood and Brazil-wood, or
Brazil-wood and fustic may be used, topped with coal-tar colours.
Tanning materials, such as quebracho and mangrove extracts, which give
browns with bichromate, are also employed on cheap goods. It is also
frequently wise to dye with a basic colour and top with an acid one, or
_vice versa_; as in many cases the one fixes and combines with the
other, and an increase of fastness is obtained.
Morocco and many other coloured leathers are finished by damping the
surface of the dried leather with a very dilute “seasoning” of water,
milk, and blood or albumen, allowing the leather to become quite or
nearly dry, and polishing by friction under a cylinder of agate, glass,
or wood in the glazing machine. Many leathers are also grained by
printing from engraved or electrotype rollers, or by “boarding,” or a
combination of the two. “Boarding” consists in pushing forward a fold in
the leather on a table with a flat board roughed underneath, or lined
with cork, in a way which is difficult to describe, but which in skilful
hands wrinkles or “grains” the skin in a regular pattern.
The colour of a dyed skin is much altered by finishing and especially by
glazing, which always darkens and enriches the colour. In dyeing to
pattern, it is useful to glaze a little bit of the rapidly dried skin by
friction with a smooth piece of hard wood for comparison, and a portion
of the pattern may also be wetted for comparison with the wet skin.
Colours which look full and even in the dye-bath, often go down in a
most disappointing manner on drying, though to some extent they regain
intensity on finishing.
In comparing the dyeing value of colours, the most practical way is to
make actual dyeing trials with equal or known quantities of the colours
and of water. Such trials may be made, either by “turning” the samples
in photographic porcelain trays, kept warm in a water-bath (a “dripping
tin” may be used for the purpose, the trays being supported a little
above the bottom on tin supports soldered to the tin), or the leather
may be hung from glass rods, by hooks of copper wire, in glass vessels
(square battery jars), also placed in a water-bath. The leather samples
should be of equal _surface_ in every case; for suspension, pieces of
“skiver” (sheep-grain) of 8 by 4 in. or 20 by 10 cm. are very
convenient. These may either be “pleated” or suspended by the two ends
grain side out, with a short glass rod to weight the fold, and keep them
flat. The weight of colour used for a sample 8 in. by 4 in. multiplied
by 54 times the area of a single skin in feet, will give approximately
the weight of colour needed per dozen; which is, however, a good deal
influenced by the mode of dyeing, and the quantity of water used.
In dyeing on the large scale, iron, zinc and even copper are to be
avoided, the latter acting very injuriously on many colours, and on the
whole wooden vessels are to be preferred. Though these become deeply
dyed, they become very hard, and if well washed with hot water, and
occasionally with dilute acid, they may be cleansed so as to give up no
colour in subsequent dyeing operations, though of course it is not
desirable, if it can be avoided, to use the same vessel for very
different colours. Zinc rapidly bleaches many colours, especially while
wet and slightly acid, and discharge-patterns may often be produced by
pressing the wet leather on perforated zinc plates.
CHAPTER XXVI.
_EVAPORATION, HEATING AND DRYING._
Questions of evaporation, whether for raising steam, or for the
concentration of tanning extracts and other solutions are of
considerable importance in the tanning industry, and as the same natural
laws which apply to these equally govern the drying of leather, it is
convenient to study the theory of the whole subject in one chapter,
rather than to divide it, and place each part in a different portion of
the book.
The modern conception of evaporation and vapour pressures has been
described on page 75, but it will be necessary to recapitulate a little.
It is a well-known fact that most liquids, if left exposed in an open
vessel, gradually disappear by evaporation into the air, even at
ordinary temperatures. If the vessel is heated sufficiently, the liquid
“boils”; that is, bubbles of vapour are formed in it, and escape, and
the evaporation is therefore much more rapid. To avoid complication, let
us first imagine a liquid sealed in a glass flask, which contains no
air, but which is only partially filled by the liquid. It has been
pointed out that the motion of heat by which the molecules of the liquid
are agitated, enables some of them to break away from the attraction by
which liquid particles are held together, and pass into the form of gas
or vapour, which will fill the empty part of the flask. This evaporation
will, however, soon reach a limit, since the vapour cannot escape from
the flask. The flying molecules of vapour produce pressure by striking
the walls of the flask, while a proportion of them will strike the
surface of the liquid, and again be caught and retained by its
attraction; and as the pressure rises, the number of these necessarily
increases till a point is reached when as many fall back and are
retained (or “condensed”), as those which evaporate, and the pressure
will then remain constant. The amount of the pressure will vary with
the nature of the liquid, and will be the greater the more volatile it
is, or, in other words, the less the power of its internal attraction.
It will also increase with rising temperature, which, by increasing the
velocity of motion of the molecules, renders their escape from the
liquid easier, and their recapture more difficult. It will not be at all
affected by the volume of vapour or the size of the flask, but so long
as any liquid is present, it will depend merely upon the nature of the
liquid, and the temperature. If the flask is large, more of the liquid
will evaporate till the same pressure is reached. If at the outset the
flask is not empty, but filled with air, it will make no difference to
the pressure or quantity of the _vapour_ in it, which will be added to
that of the air, whatever that may be. If the sealing of the flask is
broken so that it is open to the atmosphere, air and vapour will escape,
or air will pass in, till the total pressure is equal to the atmospheric
pressure outside, (about 15 lb. per square inch). As, however, the
vapour in the flask is always renewed by evaporation, so that the full
vapour-pressure of the liquid is maintained, the “partial” pressure (as
it is called) of the _air_ in the flask will be less than that of the
outer atmosphere by the amount of the vapour-pressure, which makes up
the difference. Once this balance is attained, evaporation will go on
very slowly in the flask, as it can only replace the small quantity of
vapour which escapes. If, however, the vapour is removed by blowing
fresh air into the flask, it will rapidly be replaced in the old
proportion by fresh evaporation. Thus goods in a close room will dry
only very slowly, even if the temperature is high, unless the moistened
air is replaced by dryer air from the outside by some effective system
of ventilation. In absence of this, evaporation only becomes rapid when
the temperature of the liquid is raised to its “boiling point,” that is,
when the vapour-pressure becomes slightly in excess of that of the
atmosphere, so that the freshly formed vapour can push out that already
in the flask or chamber into the outer air, and at the same time,
bubbles can be formed in the interior of the liquid by the escaping
vapour. As the vapour-pressure of a liquid rises continuously with
increasing temperature, and its boiling point is defined as that
temperature at which it is equal in pressure to the air (or vapour) in
contact with it, it is evident that the boiling point must entirely
depend on the pressure. Thus the boiling point of water in a boiler at
a pressure of 55 lb. per square inch above the atmosphere is 150°C., and
in a partial vacuum equal to 5·8 inches of barometric pressure, is only
60° C., a fact which is made use of in the concentration of extracts and
other liquids at a low temperature in the vacuum-pan. (Atmospheric
pressure is taken at 30 inches or 760 millimeters of the barometer or
14·7 lb. per inch, or 1·033 kilos per square centimeter.)
If a piece of iron is placed over a powerful gas-burner, it will go on
getting hotter till its temperature is nearly or quite equal to that of
the gas-flame. On the other hand, a pan of water, in the same condition,
once it has reached its boiling point, becomes no hotter till all the
water is evaporated. It is evident that the whole available heat or
energy of the gas-flame is consumed in converting the water into steam.
We might convert a proportion of this energy into mechanical work, by
using the steam in a steam engine; but even without this, work is
actually being done by the escaping steam in raising the weight of the
atmosphere, and in overcoming the attractive force which holds the
particles of water together in the liquid form. It is of course known to
everyone, that energy may change its form, as from heat to work, but
that it cannot be destroyed, diminished or increased; and therefore the
whole of the work performed in converting the water into steam is again
recovered as heat when the steam is condensed. In this connection a
clear distinction must be made between _quantity_ of heat, and
_temperature_, which in popular language are often confused. It is for
instance obvious that if we mix a pound of water at boiling temperature
with another pound at freezing point, the temperature is altered to 50°
C., but the total _quantity_ of heat is unchanged. It is equally clear
that no change in quantity of heat takes place when 1 lb. of mercury at
100° is mixed with 1 lb. of water at 0°, though in this case, owing to
the small capacity of mercury for heat, the common temperature would
only be raised to about 3°. We must therefore have some measure of heat
apart from the mere direct indications of the thermometer, and that most
generally used is the quantity of heat required to raise 1 kilo of water
1° C. (kilogram-calorie).[180] In England the heat required to raise 1
lb. of water 1° F. is also in use as a unit. The k.-calorie is equal to
3·97 (very approximately 4) lb. × F. units. For our purpose it may be
taken that 100 k.-calories of heat are required to raise 1 kilo or liter
of water from freezing to boiling temperature. If, however, the water is
actually frozen, we require 80 k-calories merely to melt the kilogram of
ice without perceptibly raising its temperature, and when the water is
raised to 100°, 536 calories of heat are still necessary merely to
convert it into steam at the same temperature. To melt 1 lb. of ice
requires 144 lb. × F. units, to raise it to boiling point 180 more, and
to evaporate it 965 additional. The quantity of heat required for actual
evaporation varies a little at different temperatures, being somewhat
larger at lower temperatures, but the total heat required to raise water
from the freezing point, and convert it into steam at _any_ pressure is
nearly constant, being 635 calories at atmospheric pressure, and only
about 650 calories, or 1180 lb. × F. units at 50 lb. per sq. inch. The
quantity of heat evolved by the combustion of 1 lb. of good coal is
13,000 to 15,000 lb. × F. units; or of 1 kilo, 7200 to 8300 k-calories,
but in raising steam in a good boiler coal will only evaporate 10 times
its weight of water at 100° (5360 calories or 9650 lb. × F. units), the
remaining heat being lost. 1 horse-power (33,000 foot-pounds per
minute)[181] in the best engines requires about 1¹⁄₂ lb. of coal or 15
lb. of steam per hour, but in those of worse construction may run up to
many times that amount. As, even theoretically, not 20 per cent. of the
total heat can be converted into mechanical work in a “perfect” engine
working at 75 lb. pressure, it is often economical to use waste steam
for heating or evaporation, and where this can be done profitably, the
additional cost of the mechanical power is very small.
[180] A gram-calorie of one-thousandth part of the above is also in
use for some scientific purposes, but the kilogram-calorie only is
used in the following pages.
[181] This is equal to 76·04 kilogrammeters per sec., but the metrical
horse-power is only taken at 75 kilogrammeters in France and Germany.
In evaporating liquids in the open pan 536 calories is required to
evaporate 1 kilo of water already raised to boiling temperature, and a
larger amount for salt-solutions, and it makes comparatively little
difference whether this is done at 100° or at a lower temperature.
Where, however, evaporation is done in _vacuo_, considerable economy can
be effected by what are known as multiple “effects,” in which the steam
from one vacuum-pan is employed to boil a second under a reduced
pressure, and consequently boiling at a lower temperature. This
principle can be practically applied to as many as five or six
successive “effects,” the weaker liquor being usually evaporated at the
highest temperature and lowest vacuum in the first “effect,” by the
exhaust steam of the engine used for the vacuum pumps, while the steam
from the first effect heats that of the next higher concentration, and
so on. In the Yaryan evaporator (p. 339), the boiling liquid is sprayed
through coil-tubes, thus exposing an enormous surface to evaporation,
and the whole concentration of any given portion of liquid takes place
as it passes through the apparatus, which does not, even in multiple
effects, occupy more than 4 or 5 minutes; and without the temperature of
the liquid ever rising above 60° or 70° C. In the case of liquids, like
sugar- and tannin-solutions which are liable to chemical change from
continued heating, the shortness of the time is a very great advantage.
The number of effects which it is desirable to use depends greatly on
the cost of fuel as compared to the largely increased cost of the
apparatus. 1 lb. of coal employed in raising steam will evaporate 8¹⁄₂
lb. in a single-effect Yaryan, 16 lb. in a double-effect, 23¹⁄₂ lb. in a
triple, 30¹⁄₂ lb. in a quadruple, and 37 lb. in a quintuple-effect
apparatus.
Where liquids are evaporated in the open air at temperatures below
boiling, it is advisable by some means to spread the liquid in a thin
film, so as to expose a large surface, which must be continuously
removed by agitation, so as to prevent the formation of a skin. A good
apparatus for this purpose is the Chenalier evaporator (Fig. 92), which
consists of steam-heated copper discs rotating in a trough containing
the liquid, which is taken up by buckets attached to the rims of the
discs, and poured over their heated surfaces. In other forms, the liquid
is allowed to trickle over steam-heated pipes or corrugated plates. Such
evaporators should be placed in a current of air so as to rapidly carry
off the vapour formed. Their use is very objectionable for liquids, like
tannin-liquors, which are injured by oxidation, and they are not nearly
so economical as vacuum-pans.
_The drying of leather_ depends on the same laws as the evaporation of
liquids, but demands special consideration from its very different
conditions of temperature and supply of heat. It is important to
remember that evaporation cannot go on unless the vapour-pressure of the
liquid to be evaporated is higher than that of the vapour in contact
with it, and that air-pressure does not prevent evaporation, so that if
we sweep away the stagnant vapour with dry air, evaporation will go on
as quickly as in vacuo, except that the liquid cannot boil. We must also
bear in mind that evaporation consumes quite as much heat at low
temperatures as in a steam boiler, and that this heat must generally
come from the surrounding air, the temperature of which it reduces.
[Illustration: FIG. 92.--Chenalier Evaporator and Glue Coolers.]
The rapidity of evaporation, and the quantity of moisture which can be
taken up by a given volume of air depends on the vapour-pressure, which
increases with temperature. The relation between the two, and the weight
of water in grams per cubic meter which can be dissolved in dry air is
given in the following table. (Grams per cubic meter is practically
equivalent to ounces per 1000 cubic feet. Vapour-pressure is given in
millimeters of mercury of the barometer, p. 422.)
VAPOUR PRESSURE OF WATER.
----------------+---+---+---+---+---+----+----+----+----+----+----
Temperature, °C |-10|- 5| 0 | 5 | 10| 15 | 20 | 25 | 30 | 35 | 40
„ °F | 14| 23| 32| 41| 50| 59 | 68 | 77 | 86 | 95 |104
Pressure, mm. |2·2|3·2|4·6|6·5|9·1|12·7|17·4|23·5|31·5|41·9|54·9
Grams per cb. m.|2·4|3·4|4·9|6·8|9·3|12·8|17·2|22·8|30·1|39·2| ..
----------------+---+---+---+---+---+----+----+----+----+----+----
Air is practically never dry, and in damp weather is frequently
saturated with moisture to the full extent corresponding to its
temperature. In England the average quantity of moisture contained in
the air throughout the year is 82 per cent. of the total possible, and
even in the driest summer weather it is never less than 58 per cent. So
long as the water is in the form of vapour, the air remains quite clear
and does not feel damp; in fogs, the air is not only saturated with
moisture, but contains small liquid particles floating in it. Of course
when the air is really saturated with moisture, it has no drying power
whatever.
As is evident from the table, the amount of water which can be dissolved
in a given volume of air rapidly increases with temperature. Air at 0°
C. is only capable of containing 4·9 grams per cubic meter, or not much
more than 20 per cent. of what it can contain at 25° C. It hence rapidly
increases in drying power as it is warmed, and consequently the air in a
warm well-ventilated drying room in winter is generally much drier, and
has greater capacity for absorbing moisture than the open air in the
driest summer weather. This is the principal cause of the tendency to
harsh and irregular drying by the use of artificial heat; and may be
remedied by a proper circulation of the air by a fan without too
frequent change with the colder air outside. On the other hand the use
of a little artificial heat in damp summer weather, when the air is
saturated with moisture, may be quite as necessary as in winter. The
amount of moisture in the air is most easily ascertained by a device
known as the “wet and dry bulb thermometers.” This consists of two
thermometers mounted on a board; one of which has the bulb covered with
muslin, and kept moist by a lamp-wick attached to it, and dipping in a
vessel of water. The temperature of the wet bulb is lowered by the heat
consumed in evaporation, and the difference of its temperature from that
of the dry bulb is proportionate to the drying power of the air. This
may be approximately calculated in grams per cubic meter by multiplying
the difference by 0·64 for Centigrade or 0·35 for Fahrenheit degrees;
and if deducted from the total capacity for moisture corresponding to
the temperature of the _wet_ bulb as given in table, p. 426, will give
the actual moisture in grams contained in a cubic meter of air; but for
practical purposes, all that is necessary is to find by experience the
temperature and difference between the wet and dry bulbs, which gives
the best result for the drying required, and to maintain it as nearly as
possible by regulation of the heating and ventilation. Cheap forms of
the instrument are made for use in cotton-mills, where it is necessary
to maintain a certain degree of moisture; or it may be improvised from
two chemical thermometers which agree well together. Distilled (rain or
steam) water should be used to moisten the bulb, or it will quickly
become coated with lime salts, and it should be placed in a draught, or
its indications will not be accurate.
It is of course obvious that not only the wet thermometer, but the wet
hides or skins are cooled by evaporation, and they, in their turn, cool
the air with which they are in contact, which not only becomes
moistened, but is lessened in its capacity for moisture by cooling, and
thus rapidly reaches a condition when it can absorb no more moisture. It
is thus necessary to maintain its temperature by artificial heat, or to
replace it constantly by fresh air from the outside, and which of these
expedients is most economical will depend on the temperature of the air
outside as compared with that which it is required to maintain. If the
outside air is sufficiently warm, and not saturated with moisture, it is
generally best to use it in large quantities without artificial heat,
wind usually supplying the necessary motive power for its circulation.
Wet goods from the pits may thus be dried to a “sammed” condition by any
air which is not saturated, and above freezing point; though the drying
will often be slow. For drying “off,” artificial heat is generally
necessary, since the attraction of the fibre for the last traces of
moisture is very considerable, and to remove it the drying power of the
air must be considerably higher than that required for the evaporation
of free water.[182] In drying stuffed leather a temperature must
generally be maintained sufficient to keep the fats employed in partial
fusion, and so permit their absorption by the leather, while at the same
time the drying must be gradual, or the water may be dried out before
the fats have time to take its place. This is generally best attained by
the use of artificial heat, and ventilation by circulating the air by a
fan without its too frequent renewal, especially in cold weather.
Frequently air which has been heated and used for drying off finished
goods, and so partially saturated with moisture, may be used with
advantage for wet goods, or for other purposes where a more gentle
drying is required. If the temperature is low outside, the amount of
heat consumed in heating cold air to the temperature required may be
very considerable. The weight of a cubic meter of air at 0° C. and
atmospheric pressure is 1·293 kilos, and its specific heat at constant
pressure is 0·2375 of that of water. Therefore to heat a cubic meter of
air at ordinary pressure and temperature 1° C. will require the same
amount of heat as that used to heat 0·3 kilo of water to the same
extent, or in other words 0·3 of a k.-calorie. If steam-heating is used,
1 kilo of good coal burnt under the boiler should heat about 1800 cubic
meters 10° C., or 1 lb. should heat 52,000 cubic feet 10° F., assuming
that the condensed water is not cooled below 100° C. These seem large
volumes, but if we reflect that a 48-inch Blackman fan may move 30,000
cubic feet per minute, we shall realise that the cost of coal in heating
air is not inconsiderable.
[182] Commercially-dry leather generally, if unstuffed, contains about
15 per cent. of residual moisture, which varies in amount with the
weather, and can be more or less completely removed by drying at high
temperatures. If leather has been over-dried, it only slowly regains
its weight on exposure to cold air. Commercial disputes not
unfrequently arise on the dryness of leather. In the opinion of the
writer, a customer can only claim that the leather should be
sufficiently dry not to lose weight when exposed to dry air at the
ordinary temperature and degree of dryness of a warehouse or factory,
and claims based on re-drying in hot drying rooms are distinctly
fraudulent.
We must now consider the heat consumed by the actual evaporation of the
water in the leather. The actual evaporation of water already raised to
100° C. consumes 536 k.-calories, but the evaporation of water which has
not previously been heated so far consumes more heat, and we may take
that required at ordinary temperatures as in round numbers 600
k.-calories per kilo, or 1080 lb. × F. units per lb. Disregarding small
fractions, this is equivalent to the cooling to the same temperature of
an equal weight of steam in the heating pipes, and this, as we have
seen, demands about ¹⁄₁₀ of its weight of coal for its production from
water already heated to 100° C.
The cooling takes place, in the first instance, in the leather, the
temperature of which is reduced like that of the wet-bulb thermometer;
and this in its turn cools the air in contact with it. Thus in
air-drying without artificial heat, the whole heat must be supplied by
the air and the loss reduces its capacity for moisture, greatly
increasing the volume required. This is not of much consequence in
open-air drying, since even a light wind will supply air in enormous
volume. A moderate breeze of ten miles an hour moves about 15 feet or
4¹⁄₂ meters per second. When, however, the air must be moved by fans,
the power required becomes important. The evaporation of 1 kilo of water
at summer temperature will cool about 2000 cubic meters, and that of 1
lb. 32,000 cubic feet of air 1° C.
In calculating the ventilating and heating power required in fitting up
drying rooms, it is usually necessary to ascertain that required under
the most unfavourable circumstances, and then add a liberal margin to
cover errors and accidents. As the calculations are, in consequence of
the many varying conditions, somewhat complex, it may be convenient to
give as examples the quantities of air and heat required to evaporate 1
kilo (2·205 lb.) of water under different ordinary conditions, and these
may serve as a basis of calculation of the drying power which must be
provided for different tanneries.
1. _Indifferent Open-Air Drying._--Air at 10° C. (50° F.), wet-bulb
thermometer 7° C. (44·3° F.), indicating a total capacity for moisture
of about 2 grm. per cubic meter; air not to be cooled beyond 7·75° C.
(46° F.), leaving a residual capacity for moisture of 0·5 grm. per cubic
meter. Each cubic meter will therefore take up 1·5 grm. of moisture, and
as 1 kilo contains 1000 grm. we have
1000
----
1·5
= 666 cubic meters per kilo required to absorb moisture; and
600
-----------
2·25° × 0·3
= 888 cubic meters reduced 2·25° to furnish the 600 cal. required for
evaporation. Total air used 1554 cubic meters or 54,900 cubic feet.
2. _Drying with Heat._--Outside-air at 10° saturated with moisture,
heated to 20° C. (68° F.) acquires a capacity for 7·9 grm. per cubic
meter. If we assume that a drying capacity of 2 grm. per meter is
required to complete the drying, we have an effective capacity of 5·9
grm.
1000
----
5·9
= 170 cubic meters or 6000 cubic feet, and to heat this 10° C. will
require 510 cal. Evaporation of 1 kilo will consume 600 cal. Total heat
1110 cal.
3. _Drying with Heat._--Outside-air at 10° as above, heated to 25° C.,
giving an effective capacity for moisture of 13·5 - 2·0 = 11·5 grm. per
cubic meter.
1000
----
11·5
= 87 cubic meters or 3070 cubic feet. To warm this 15° requires 391
cal.; and 600 cal. added for evaporation gives a total of 991 cal.
Comparing 2 and 3 we see that the higher temperature is more economical,
where it can be allowed, than the lower, both in air and heat, though
this is partly compensated by the greater loss of heat by cooling of the
building, etc., which it entails.