WHITES.
In the whole range of pigments there is no more important class than
those to be described in this chapter. Not only are the white pigments
largely employed for the sake of their distinctive colour, but they are
probably even more extensively applied as a basis of other pigments,
both as an ingredient in the composition of the other coloured paints
and for ground coats where the final coat is to be of a delicate shade.
They are among the cheapest and most permanent pigments, and possess as
a whole remarkably good covering powers.
BARYTA WHITE.--Barytes or sulphate of baryta, the most
important of the salts of barium, is found native in large quantities,
forming the species of mineral termed barites or barytes, and commonly
known as heavy-spar, on account of its weight (sp. gr. from 4·3 to
4·7). It is found in Derbyshire and Shropshire, and often occurs in
fine tabular crystals. The massive variety found in the mountain
limestone of the above counties is sometimes called “cawk”; it is more
frequently found in white or reddish-white masses. In Saxony it occurs
as the mineral _stangen-spath_, in a columnar form; and at Bologna, a
nodular variety is found, called Bologna stone, which is notable for
its phosphorescent powers when heated.
The pure salt may be prepared artificially for use as a pigment, by
adding dilute sulphuric acid to a solution of chloride of baryta,
when a white precipitate is formed; this is well washed and dried. It
is a heavy, white powder, insoluble in water and nearly insoluble in
all other menstrua. It may also be prepared by heating the native
mineral, grinding it to powder, and well washing it, first in dilute
sulphuric acid, in order to remove any traces of iron, and afterwards
in water; the white powder is afterwards thoroughly dried. This
process is employed at several works in the neighbourhood of Matlock
Bath, in Derbyshire, but much larger quantities could be produced in
different parts of the country if the demand for the article rendered
its production more profitable. The principal use of sulphate of baryta
is to adulterate white lead, and to form the pigment known as _blanc
fixe_, or permanent white. For these purposes, the native mineral,
ground and washed as described above, is commonly employed.
Improvements in machinery and in the process of treating natural
barytes have overcome many of the objections which formerly existed
to its utilisation, and considerable attention is now being given to
the localities in the United States where it is found. The mineral, in
order to be available for the uses to which it is put, must be fairly
free from quartz grains, the stain of iron rust, and other impurities.
If the barytes is stained to any extent, it is practically valueless,
as a good white colour is essential to its usefulness. Quartz grains or
other hard substances with which it is apt to be associated injure the
machinery in grinding. The purest barytes so far produced in America
comes from Missouri, though a very fair grade is now being mined in
considerable quantities in Virginia.
The returns from all producers of crude varieties show a product in the
United States, for 1889, of 21,640 short tons, valued at 106,313 dols.,
against 20,000 short tons in 1888, valued, approximately, at 110,000
dols. The product was limited to four States, as shown in the following
table:--
Short tons. Value.
Illinois 200 $1,300
Missouri 7,558 32,715
North Carolina 3,000 15,000
Virginia 10,702 57,298
------ --------
Total 21,460 $106,313
BLANC FIXE.--This name is given to baryta white when it has
been artificially prepared by adding sulphuric acid to a solution of
chloride of barium. (See p. 170.)
CHARLTON WHITE.--One of the names applied to a white pigment,
containing zinc oxide and sulphide, and described under zinc whites, p.
254.
CHINA CLAY.--This substance is also known as kaolin, porcelain
clay, and Cornish clay. It arises from the natural decomposition of
felspar in soft disintegrating granite, gneiss, and porphyry, the
rocks which are rich in soda-felspar yielding it most abundantly. The
main supplies of this country are derived from Cornwall and Devon; in
continental Europe, beds of good quality exist in France, Bavaria,
Saxony, Prussia, Bohemia, Bornholm island, and Hungary; in China, it is
very plentiful; and in the United States, it occurs in many localities.
The approximate composition of china clay may be stated as silica,
47·2; alumina, 39·1; water, 13·7 per cent. Often a little iron, lime,
and potash or soda are left in the prepared article by the imperfection
of the cleansing process. The most important characters are colour,
plasticity, and a capacity for hardening under the influence of heat.
The china-clay industry of Cornwall and Devon has been admirably
described by J. H. Collins, F.G.S., in a paper recently read before the
Society of Arts.
_Occurrence._--The natural clay rock is almost always covered with a
thick layer of stones, sand, or impure and discoloured clay, known as
“overburden.” This capping often much resembles glacial drift; but
it never contains any scratched or glaciated stones, or travelled
blocks. It varies in thickness from 3 feet to 40 feet, and must, of
course, be removed before the clay can be wrought. The clay rock,
being a decomposed granite, consists of china-clay, irregular crystals
of quartz, and flakes of mica, with sometimes a little schorl and
undecomposed felspar.
_Extraction and Preparation._--The following descriptions apply, with
more or less accuracy, to a majority of the larger works of the present
day, turning out from 2500 to 8000 tons of clay each, yearly. Two
somewhat different methods are employed, according to the situation of
the “bed” of clay in relation to the surface contour of the immediate
neighbourhood. The most general case is that in which the clay has
to be raised from a veritable pit, the bottom of which is lower than
the ground on all sides. The exact situation of the clay is first
determined by systematic “pitting,” to a depth of several fathoms, or
occasionally by boring. A shaft is then sunk either in the clay itself,
or, preferably, in the granite close to the clay. From the bottom of
this shaft, a level is driven out under that part of the clay which it
is intended to work first, and a “rise” is put up to the surface, which
should, by this time, be partially cleared of its overburden. A common
depth for such a shaft will be from ten to twelve fathoms. As soon as
the rise is completed to surface, a “button-hole” launder is placed
in it, and the remainder of the rise is again filled up with clay. In
the meantime, a column of pumps has been placed in the shaft, with an
engine to work them, unless water-power is obtainable.
For water, many works are almost entirely dependent upon that met with
in sinking the shaft and in driving levels; but, of course, this may
be, and is, eked out by catching the rain-water in reservoirs, and by
making use of such small streams as may happen to be available. A small
constant supply is sufficient even for a large work, as it is used over
and over again. The operation is begun by digging a small pit in the
clay, around the upper end of the button-hole launder, and running a
stream of water over the exposed clay, or “stope,” which is broken up
with picks. A very large quantity of sand is constantly disturbed, and
as constantly shovelled out of the way, while the water, holding the
clay and finer impurities in suspension, runs down the launder, along
the level, and into the bottom of the shaft, from whence it is pumped
up by the engine or water-wheel.
As the excavation becomes larger and deeper, more overburden is
removed, and the upper portions of the launder are taken away, until
at last the stopes reach the level, when the launder is, of course,
no longer required. At first, the sand is thrown out by one or two
“throws,” but very soon it becomes necessary to put in an inclined
road, for pulling up the sand in waggons; these are worked by a
horse-whim, or by winding gear attached to the engine or water-wheel.
As there are from three to eight tons of sand to each ton of clay,
its removal in the cheapest possible manner is a matter of great
importance. Any veins or lodes of stone, or discoloured portions of
clay, are raised from the “bottoms” in the same way as the sand. The
stream of water, holding in suspension clay, fine sand, and mica, is,
in well-arranged works, lifted at once high enough to allow of all
subsequent operations being carried out by the aid of gravity.
The stream is first led into one or two long channels, the sides of
which are built of rough stone. In these channels, called “drags,”
the current suffers a partial check, and the fine sand and rougher
particles of mica are deposited. From these drags, the stream passes on
into other channels, much resembling them, but of greater number, so
as to divide the stream still further. This second series of channels,
known as “micas,” are often built of wood, but sometimes of stone. They
differ in no essential respect from the drags, but are more carefully
constructed and better looked after, and, as the stream is greatly
divided and very gentle, the fine mica is deposited in them. The micas
are often about 11 inches wide, ten or a dozen in number, and 100 feet
or more long. Provision is made, by underground channels and plug
holes, for the periodical cleansing of the drags and micas. This may
have to be done twice a day, but generally only once.
The deposit in the drags is worthless at present, and is always thrown
away; but that from the micas is often saved, and sold as inferior or
“mica” clay. The refined stream of clay then passes on to the “pits,”
which are circular, 30 to 40 feet diameter and 7 to 10 feet deep. These
pits are built of rough masonry, and have an outlet at the bottom,
opposite the point at which the stream of clay-water is admitted. This
outlet is stopped by a gate or “hatch,” or by a plug, and is kept
closed until the pit is full of clay. In each outlet, however, is fixed
an upright launder some 4 inches square, provided with “pin-holes”
and wooden pins set close together. As the stream of clay enters on
one side, it is constantly depositing its burden, and the water is as
constantly drawn off nearly or quite clear from the pin holes, the pins
being put higher and higher as the clay rises in the pit. The effluent
water is conducted directly to small storage reservoirs, and thence
over the clay stopes, whence it does its work over again.
When the stream of clay-water enters the pits, it contains from 1½ to
3 per cent. of clay; and what is called a good washing stream will
carry about one ton of clay an hour. When the pit is full, the “hatch”
is drawn, and the clay is “landed” into the tank. The upper portion is
sufficiently fluid to run in of itself; but that near the bottom has to
be helped out by men using “shivers” of wood or iron, which resemble
large hoes; they are assisted by a small stream of water. The tanks are
commonly, but not always, rectangular, built of stone, and paved with
stone at bottom, often 60 feet by 30 feet by 6 feet or larger. Once in
the tank, the clay is left to settle, until it has the consistency of
cream cheese, the water being drawn off from time to time; it is then
ready to be trammed into the “dry.”
The “dry” is a large building erected in immediate proximity to the
tanks. It is always composed of two parts, the dry proper and the
“linhay.” The floor or “pan” of the dry is composed of fire-clay tiles
18 inches square, 5 or 6 inches thick at the fire end, and gradually
thinning off to 2 or 2½ inches at the stack end. The flues are built
of fire-brick, about 15 inches wide, 2 feet deep at the fire end, and
9 inches deep at the stack end. Each flue should be supplied with a
damper. The furnaces are built in and arched over with best fire-brick;
the fire bars run longitudinally, and are about 6 feet long. The grate
surface is about 2 feet 6 inches wide in front, and 4 feet 6 inches to
6 feet at back, according as each furnace supplies three or four flues.
The clay, brought in from the tanks in tram-waggons holding about half
a ton, is tipped on to the tiles, and spread in a layer from 9 inches
thick at the fire end to 6 inches thick at the stack end. The fire
end is loaded and cleared every day; the other end perhaps twice or
thrice a week, according to the length of the dry, thickness of tiles,
perfection of draught, &c. An average size for a first-class dry is
perhaps 15 feet wide and 120 feet long; but some have been constructed
considerably larger than this. The pan of the dry should be 6 or 8
feet above the linhay whenever possible, so as to afford storage space
for the dry clay, without expending labour in piling. The tiles should
be as porous as possible, for very much more water passes through
the tiles and into the flues than is driven upwards in the state of
steam. The temperature should never be allowed to rise so high that the
workmen cannot walk on the tiles, otherwise the clay may become baked
and damaged.
In cases where there are no means of artificial drying, as at some
old-fashioned works, the thick clay is at once transferred from the
original settling pit to shallow depressions in the ground, called
“pans.” Ten or twelve of these, each holding from 40 to 50 tons,
should be provided for each settling pit; they measure from 20 to 40
feet square, and 2 feet deep, and are enclosed by granite walls, the
interstices of which are rendered impervious by plugging with moss. The
clay, filling two-thirds of their depth, is here left exposed to the
sun and wind, by which it is partially deprived of its moisture.
In order to complete its desiccation, the clay is removed from the pans
after three or four months’ exposure. A number of parallel incisions
are made lengthwise in the clay, by means of a knife attached to a long
handle; the strips are next divided transversely, by men with spades,
who throw the blocks on to a board, upon which they are borne by women
and children to the sandy drying yard, where, in fine summer weather,
they soon become dry. They are then collected, and piled away in sheds,
under a number of thatched gates or “reeders,” or are placed in some
sheltered position where air can circulate around them without their
becoming wet from rain.
When required, the blocks are scraped by women armed with hoes, before
being despatched from the works. The transport is often effected in
small casks, holding about half a ton. A few years since, a machine for
drying china-clay was invented by a mechanical engineer named Leopoldo
Henrion, of Sampierdacena, near Genoa. It is said that, by its use, the
operation can be effected in a few hours, at a relatively small cost.
Collins was first led to adopt his arrangement in consequence of the
formation of the ground; but he is inclined to recommend it in most
cases if practicable. Very large quantities of stone are required in
the dry pits, tanks, &c. Very often this is got, in part or entirely,
in the process of excavating the pits, &c.; but if it cannot be so
obtained, a very serious expense will be incurred, in some instances
amounting to several thousand pounds. The total cost of the works may
even be doubled from this cause, if stone has to be fetched from a
distance of several miles.
Two modes of building with rough stone are adopted; they are known as
“lime building,” and “dry stone walling.” The first needs no special
remark, but the second is very ingenious and very effectual. The wall
is built up double, with a batter of about ¾ inch or 1 inch to the
foot. Moss is placed between the joints of the wall, and the space
between is filled in with sharp sand, the refuse of that or some other
clay works. A small stream of water is then made to flow over the sand,
which is well beaten in with rammers, or by treading with the feet.
This process is continued, a foot at a time, till the wall reaches the
required height, when it is either paved with rough stones set on edge,
or turfed. A wall properly built, in the manner just described, is
quite impervious to moisture, and will stand for fifty years or more.
It is, where the proper kind of sand is abundant, much cheaper than
lime walling, and is always preferred for the walls of pits and tanks.
Where the bed of clay is situated on a hill-side, with plenty of space
below, a tunnel is driven in from the hill-side or from the valley to
the required depth, and a rise is put up as before. This rise is then
divided off into two parts. In the smaller, a button-hole launder is
placed as before, and packed around with clay; but the larger is left
open. A stream of water, obtained by pumping or otherwise, is made to
run over the stope, and down the button-hole launder. It then flows
along a launder placed in the bottom of the level, until it makes its
exit in the valley. It may then be purified, settled, and dried exactly
as already described--the works being laid out at a lower level than
the adit; or, if the clear water is wanted to flow over the stope,
or it is, for any reason, necessary to place the pits and tanks at a
higher level than the stopes, the water is pumped up after partial or
complete purification.
The main difference in this mode of working is that instead of pulling
the sand and rubbish up over an incline, it may be tipped down the
pass into waggons, run out through the level, and tipped over the
hill-sides. In cases where waste water is abundant, it may even be
washed out at night, thus saving the expense of tramming. Of course,
when the workings have reached their full depth, the rise and the
launder are dispensed with, and the adit level communicates directly
with the “bottoms.” By this mode of working a considerable economy may
be effected, especially when it is not necessary to pump the clay water
for settling or repeating.
_Cost of Production._--Where the conditions of production vary so
greatly, there must necessarily be great differences of cost; but,
after having been at some pains to determine the cost under average
conditions, Collins thinks the following figures and statements may
be relied upon. A work capable of producing say 4000 tons of clay
yearly will cost from 2500_l._ to 5000_l._ To get the clay in the
linhay ready for the market will cost about 9_s._ a ton, of which about
2_s._ 6_d._ must be expended in fuel for pumping and drying, 1_s._ in
removing overburden, 1_s._ in removing sand, and 1_s._ for management
and office expenses, leaving 3_s._ 6_d._ as the net labour cost of
washing and drying a ton of clay. To the 9_s._ net cost of clay must
be added an average of 3_s._ for royalties, 4_s._ for transit and
placing on board ship, and 1_s._ for agencies, commission, bad debts,
and sundries, making the average actual cost amount to 17_s._ Some
favourably situated works can no doubt save 2_s._ or even 3_s._ on
this account; in others, the cost may amount to 20_s._ or even 22_s._
As to the selling price, this varies much more widely than the cost of
production, ranging from 14_s._ to 35_s._ _f.o.b._ Clays sold at the
lower rate are unremunerative.
_Nature and Utilisation of Waste Products._--Besides the clay proper,
there are certain waste or pseudo-waste substances produced in very
large quantities. These are as follows:--
Fine Mica.--This is deposited in the “micas”; a few years since it
was thrown away, or rather washed away, as is still the case in many
works. Sometimes, however, it is collected, dried in the manner of clay
proper, and sold to the makers of soft paper, paste-board, inferior
pottery, &c., at a low price.
Coarse Mica.--This is invariably washed away, or thrown away, there
being at present no demand for it. It, however, contains a very
beautiful material, which might be applied to many ornamental purposes.
Sand.--This consists of broken quartz crystals, mostly white or pale
brownish; when washed clean, it is the finest building sand known, as
the angles are all sharp. Mixed with one-eighth of Portland cement, it
forms a concrete as hard as stone.
Discoloured Clay.--This has to be dug out from among the good white
clay in many places. It has been successfully used in the manufacture
of white bricks for building purposes. In some instances, a quantity of
the sand already mentioned is mixed with the refuse clay, and produces
an excellent fire-brick. The same material is used in the manufacture
of the tiles used as a floor for drying the clay. The manufacture of
bricks and tiles from this debris is a growth, it is believed, of the
last twelve years.
Overburden.--The upper part of this consists of soil, or “meat earth”;
this is usually removed and carefully preserved. Underneath is a hard,
often stony or sandy layer, which, in districts where tin is worked,
often contains enough tin to pay for washing. With this stony or sandy
layer, is usually a considerable thickness of discoloured clay suitable
for brick-making.
Branches.--These are stony veins which run through the clay stopes in
various directions. Sometimes they are quite worthless; but in a few
instances they are veritable tin lodes, and contain enough tin to pay
for stamping and dressing. Thus at Carclaze, near St. Austell, each
1000 tons of clay yields something like one ton of oxide of tin, and
formerly the proportion was much greater. The proportions of these
waste materials, as compared with the fine clay procured, are thus
stated:--
For every 1 ton of fine clay there is removed--from 3 to 7 tons of
sand, average about 3½ tons; from 2 to 5 cwt. of coarse mica, average 3
cwt.; from 1 to 3 cwt. of fine mica, average 2 cwt.; from 0 to 1 cwt.
of stones, average ¼ cwt.
A cubic fathom of clay rock, of average quality, will yield about
2½ tons of fine clay; and about half a fathom of overburden must be
removed to get it.
_Suggested Improvements in Preparing._--Collins thinks that there is
still much room for improvement in the preparation of china clay, but
that such must be a growth of time and circumstances. At the present
time, about one ton of water has to be driven off from each ton of clay
in the “dry,” and this uses at least 2 cwt. of coals on an average, and
costs from 8_d._ to 10_d._ in labour. In a few modern drys, a small
economy in fuel has been effected, by lengthening the kiln; but in none
has it been brought so low as 1½ cwt. to the ton of clay.
Stocker, in 1862, suggested the use of filter beds, and also devised
a centrifugal dryer; but neither of these contrivances has come into
use, and the first would seem quite inapplicable on account of the
extreme fineness of the particles of clay, and the impermeability of
even a thin layer of that substance. Some economy might perhaps result
from the use of hydraulic filters of calico, such as are used in the
potteries for drying the slip; but it is very doubtful if any saving
would be effected, as the labour would be about the same, and, against
the 2_s._ a ton for fuel, would have to be placed the wear and tear of
the calico.
In washing the clay from the stope, some benefit might accrue from the
use of a jet of water under a pressure of from 50 to 100 lb. per square
inch, as in the so-called hydraulic mining. This could only be applied
to stopes of even quality, where very little picking out of inferior
portions was required; but it would supersede the services of the
“breakers” on the stope, and greatly lessen the labour of the washers.
It is but rarely that a natural head of water is obtainable equal to
the required pressure; but where machinery is used for pumping, the
additional cost of pumping, say 250 gals. a minute to a height of 150
feet in a standpipe, would be very slight, as the extra power required
is little more than that of one horse.
_Statistics._--From statistics obtained from many sources, it is
evident that the production has very largely increased from 1809 to
1874--2919 tons against 226,309. In 1810, Trethosa (one of the largest
works) produced 300 tons per annum, and employed thirteen persons, viz.
eight in removing burden and raising (breaking) clay (at per fathom),
three washing, two attending ponds and packing. In 1874, one of the
works near St. Austell produced 9000 tons, employing about thirty men.
Many works produced 6000 tons, employing twenty men. The quantity sent
annually from Cornwall must average at least 150,000 tons. It goes
not only to Staffordshire, but also largely to France, Belgium, and
other countries. The extensive clay works recently opened in several
departments of Northern France have done much to curtail the export of
Cornish clay to that country, and the large deposits of the island of
Bornholm have lately been worked upon to supply the needs of Denmark,
Sweden, and Germany; while similar utilisation of native clays has been
carried out in America. Nevertheless, the growth of home industries
which depend in a measure upon this article will, doubtless, counteract
the influence of decreasing exports.
_Artificial China Clay._--The principal supplies of china clay are
obtained, as has been described, through the agency of natural
decomposing influences in granite rocks. In one instance, however,
at Betleek, County Fermanagh, it is procured by calcining the red
orthoclase granite of the district. The felspar is whitened by the
process, and the iron becomes separated in a metallic state, and is
removed by magnets.
_Characters._--Being virtually a hydrated silicate of alumina, china
clay is a remarkably stable pigment. Not only is it unaffected by
prolonged exposure to strong light and impure air, but is insoluble in
water, weak acids, and alkalies. It is moreover very much lighter in
weight than any other white pigment, an advantage on the score of cost
when buying by weight. Its covering power in distemper work and as a
water colour is good; but the addition of oil reduces its capacity.
The best qualities are exceedingly fine in grain and pure in tint, but
inferior samples sometimes have their yellowness “corrected” by the
addition of a little ultramarine.
ENAMELLED WHITE.--Another name for the finest kinds of baryta
white, see p. 170.
ENGLISH WHITE.--A synonym for whiting, see p. 246.
GYPSUM.--This very common and abundant mineral is a hydrated
sulphate of lime, occurring in several forms, of which only the opaque
white variety is useful as a pigment.
The native mineral is quarried, dressed, ground, and levigated, in all
which operations there is nothing special to be noted.
Whether obtained in this way, or prepared artificially, or formed as a
bye product in other industries, gypsum affords a permanent and neutral
white pigment, mixing well with oil or water, and possessing a covering
power which ranks between white lead and zinc white. It has a bluish
tint, but less so than ordinary white lead.
KAOLIN.--One of the names applied to china clay, see p. 172.
LEAD WHITES, OR WHITE LEADS.--On the grounds of the quantity
in which it is produced and the extent to which it is applied, probably
no pigment can compare with white lead, including in that term the
various white pigments having lead as a basis.
In its commonest form white lead is lead carbonate. There are many ways
in which it is made commercially, all dependent upon certain chemical
reactions.
When a solution of normal plumbic acetate is attacked by carbonic
acid, no precipitate is produced. That normal solution is formed by
the action of acetic acid or hydric acetate upon oxide of lead. It
consists of a certain weight of lead to a certain weight of acid, which
converts it into the acetate. Carbonic acid has no power to separate
out from it the lead, and form carbonate of lead. But this acetate of
lead has the power of dissolving a considerable quantity of oxide of
lead in addition to that which was used in its first formation, and
when this additional quantity of oxide of lead is dissolved by the
acetate, a substance is formed which is termed a basic acetate, that
means an acetate which contains more of the base (the lead oxide) than
the normal acetate itself does. From such a solution we are able to
precipitate, by means of carbonic acid, a white substance, which white
substance is a carbonate of lead.
Thénard, a French chemist, proposed to make white lead in this way, but
it was found that although the colour was pure and good, yet the lead
had not sufficient body to satisfy the wishes of artists and painters.
White lead has been made for years past according to what is called
the Dutch method. Lead is cast into plates, and these plates, in some
factories, are rolled into coils. These coils then are immersed in
earthen pots; the pots are placed in a row, and a small quantity of
vinegar is put into each pot. On the top of one row of pots a board
is placed, and then other pots above, and so a stack is made. Between
the interstices of the pots is put spent tan, or some other substance
which by oxidation will evolve heat, and also carbonic acid gas. Now
the heat which is evolved in oxidation of the spent tan is useful in
volatilising the acid from the vinegar, and in the presence of this
acetate the oxygen of the air oxidises the lead. The oxide of lead is
dissolved by the acid, and the normal acetate of lead is formed. More
oxide is produced, and this is dissolved by the normal acetate, and
then you have basic acetate.
When substances containing carbon are oxidised, carbonic acid is the
product of the oxidation when the oxygen is in excess, as in this
particular case. Carbonic acid is then formed by the oxidation of the
spent tan. The carbonic acid then unites with the oxide of lead which
was dissolved in the normal acetate, and a thin film of lead carbonate
is formed. These thin films go on forming in succession, until at
last nearly the whole of the lead is converted into carbonate, which
retains the shape of the original lead. In some cases, gratings of
lead are used. When the lead is converted into carbonate, it is ground
in water and reduced to a fine powder, and is then made up into the
sort of pigments required, either with water or with oil. This is,
or rather was, an operation attended with considerable danger to the
workmen, who were subjected to what is termed lead-poisoning, to which,
unfortunately, many painters, from want of cleanly habits, are subject
now.
_Dutch Process._--In the words of Mr. Carter Bell, who has read a
most interesting paper on the subject before the Society of Chemical
Industry, the manufacture of white lead is a most ancient proceeding,
and has been pursued with but little variation in the mode of
manufacture for some hundreds of years. The Dutch seem to have been
the originators of this method of making white lead, which is now so
largely conducted in this and other countries.
In this process metallic lead is piled in stacks, and submitted to the
action of acetic acid, watery vapour, air, and carbonic acid for some
time, by which means the metallic lead becomes gradually converted into
white lead.
This method is called the “stack” or Dutch process.
The construction of a stack is a very simple and rude operation. Layers
of dung or tan, or a mixture of the two, are so arranged as to imbed
a large number of earthenware pots, each containing some acetic acid.
These pots are about 4 or 5 inches in diameter, and about 7 or 8 inches
high; a coil of lead is placed in each pot, and buckles or gratings of
lead supported on oaken bearers are laid across and on top of the pots;
boards are laid to cover the whole, and form a floor.
The stack is composed of a number of such layers of pots, bearers, and
buckles or gratings, raised one upon another.
A stack chamber is a brick enclosure 10 or 12 feet square, and 20 or
25 feet high; such a chamber will contain about 70 tons of lead when
stacked and piled. In a white lead factory several of these chambers
are built side by side, and when they are in full operation a set of
chambers will contain as much as 700 or 1000 tons of lead.
Only the purest kind of lead will be suitable for conversion in
this stack process of making white lead, the common varieties being
inadmissible. Messrs. Pontifex and Wood have furnished the following
analyses of lead used for white lead making.
Copper. Antimony. Iron. Spelter. Silver. Lead.
A 0·00700 0·00490 0·00200 0·00080 0·00100 99·98430
B 0·07580 0·00320 0·00220 0·00320 0·00200 99·91360
C 0·00340 0·00460 0·00120 0·00070 0·00350 99·98660
D 0·05260 0·00740 0·00150 0·00180 0·00400 99·93270
E 0·00940 0·00210 0·00160 0·00100 0·00075 99·98515
F 0·02360 0·00580 0·00210 0·00180 0·00100 99·96570
The presence of silver, copper and iron in the lead would damage the
colour of the white lead resulting, and other admixtures retard or
prevent the progress of conversion.
In olden times horse dung was the only imbedding material used in
the stack arrangement. This material when heated evolves gases which
seriously interfere with the colour of the resulting corrosion. Dung
has been almost superseded in this country by tanners’ refuse; in
Belgium dung is yet employed, and in some places a mixture of dung and
tan.
Where dung is used, the process of corrosion of the lead goes on more
quickly than when tan alone is employed, but the use of tan offers
great advantages, especially this one: that it does not give off gases
that damage the white lead.
The operations of charging and discharging these chambers are
principally the work of women, and are most laborious and fatiguing.
In emptying the chambers and stripping the stacks, the women are fully
exposed to the heated gases which are yielded by the decomposing tan,
and the heated and corroded lead. These gases, in themselves most
injurious to health, are not to be compared in this respect to the dust
which pervades the air and fills the chamber in which these women work.
When a stack is charged, the chamber containing it is enclosed. The tan
or dung within soon commences heating, and the heat soon causes the
acetic acid in the pots, and the water in the tan or dung, to rise in
vapour and penetrate the stack. Air is admitted to the stack through
openings left for that purpose, and carbonic acid is evolved from the
heated decomposing tan or dung, and this gas also penetrates the stack,
and the process of converting blue lead into the white lead gradually
proceeds, and the blue metal becomes corroded and incrusted with a
white crust or covering.
As to the exact chemical changes and combinations proceeding in the
working of a stack, differences of opinion exist, but we may fairly
conclude that the process resolves itself into this--first, the
formation of sub-acetate of lead, which, decomposed by the agency of
the carbonic acid gas, becomes reduced to the condition of normal
acetate by loss of a portion of its basic oxide of lead. The reduced
sub-acetate then again takes up an additional molecule of oxide of
lead, and is re-converted into its original subsalt state, to be again
attacked and reduced by the carbonic acid gas, and so on continually
during the working of the stack. It will be evident that the “nascent”
state of the various substances disengaged during the chemical changes
which are proceeding in the stack is an important factor in this
process, and must be taken into account in considering the philosophy
of the operation.
It will also be evident that the mode of proceeding in white lead
making by the stack process is most crude and clumsy, and a most
uncertain method, one governed by rule of thumb, and, by no element of
certainty or science. White lead makers, as a rule, know nothing of the
chemistry of their subject. This absence of chemical knowledge of the
subject, by those who are engaged in this manufacture, may explain the
curious circumstance that for hundreds of years this industry has been
pursued in the same old-fashioned and uncertain way, and the stack,
or Dutch process, still holds its ground and displays little or no
advance in knowledge or improvements in its method of proceeding, even
in the present age of precision in almost every branch of manufacture.
The uncertainty of the stack process is shown most clearly in this:
That stacks may work and some do not work. In the latter case all the
time and labour spent in forming the stacks, and all the acid they
contained, is lost.
No amount of foresight will avail to determine beforehand which stack
shall accomplish the conversion of its contained metallic lead, and
which will not.
The stacks are generally allowed to remain in operation, after they
are charged, three or four months; in this time it is presumed all
profitable action in the stack has ceased. The temperature of the
stack, which had risen gradually from the normal temperature to 100°
or 150° F., will have gradually fallen, and this falling temperature
is the indication that the corrosion of the lead in the stack has
terminated.
After the three months’ action of the stacks, they are stripped and
pulled to pieces. Some will be found to be done better than others, and
one part of the same stack will be done more perfectly than another.
The coating on the lead will also differ; some will be smooth, regular,
and equal in formation, some will be rough and blistered, and far from
uniform. The rough blistered casting is rejected as unfit for white
lead making: the workmen call it “dross.” The smooth laminated coating
is the one preserved for the after manufacture.
It is a curious fact connected with the consideration of the total
want of educated guidance in these matters that prevails, that in all
factories of this description some chambers are noted as always working
well, and others are equally well known to always do their work the
reverse of well. No one knows why! No one stays to seek the reason.
The factory way goes on filling and emptying these white lead chambers
whether the stacks be working well or no.
The incrustation that is most esteemed by the manufacturers of white
lead in this old-fashioned style is a hard, china-like material, formed
of thin deposits, layer upon layer, in a slow, continuous, regular way.
It is at once conceivable that in the rough-and-ready manner of stack
manufacture most irregular action must proceed.
It would be almost impossible for the contents of the pile or stack to
be submitted to the same action of the gases throughout. Some parts of
the stack and its contents will be under more favourable conditions
than others, hence the reason why, in practice, it is invariably
found that some stacks, and some parts of a stack, work better than
others. Under the microscope, this good crust of white lead, the proper
incrustation from which to prepare white lead, will be readily seen
to consist of very thin coatings or layers of white lead, which have
been slowly formed on the metallic lead and piled one upon another to
the thickness of an eighth or a quarter of an inch. This formation
constitutes the hard, china-like substance, which alone possesses the
chemical constitution and the properties to form good white lead paint.
White lead makers, recognising this peculiar incrustation as the only
one capable of fulfilling their desired purpose of making good white
lead paint, do not even recognise any other as of any service for that
purpose, be it good or bad. Such a material, if obtained, however good,
would be outside their experience and beyond their philosophy. After
the stacks have been stripped, the gratings or buckles with their
adhering coating of white lead are moistened with water and are passed
through crushing rollers to separate the unconverted lead; then the
crust which has been detached is ground under heavy edge runners with
water.
This detached crust of white lead will vary much in colour: it
will be white in some parts, yellowish or greyish in others. These
discolorations arise from various causes, but they are principally
caused by the contact of the moist wood and tan. The white lead is now
a rough crushed material, very hard, and requiring to be ground to the
finest powder. It contains, also, small fragments of blue lead which
have passed the crushing rolls, and a quantity of acetate of lead.
The presence of acetate of lead is always found in larger or smaller
quantities, which vary with every operation, and which invariably
accompany white lead produced in the stack.
To remove discolorations--to separate the fragments of metal and to
dissolve out the acetate salt--much water and washing are employed. The
material is ground with water under the heavy edge runner stones, it
then proceeds to a series of horizontal mills, each succeeding mill set
closer than its fellow, and is further and further ground to fineness
with water. From these mills it runs as a milky liquid to a series of
settling tanks, where it is allowed to subside, and the clear fluid is
run off to waste, or into tanks to be used over again. This waste water
will now contain the colouring matter removed from the incrustation,
and the principal portion of the acetate of lead which the incrustation
previously contained, and any other soluble matters removed from the
washed and ground material. The small fragments of lead which passed
the crushing roll and edge runner mills will have been previously
removed by subsidence in water.
The white lead deposited in the tanks is in some factories ladled out
into skips and agitated by a “dolly,” which further enables the heavy
powder to get free from the water in which it is entangled. The moist
powder is next placed on trays or dishes, and is conveyed to the stove
or drying chamber. Women always perform this work.
The drying or stoving room is a large enclosed space heated by a
“cockle” arrangement; rough scaffoldings are erected within this
chamber, on which women mount to stow the trays on shelves fitted
for the purpose. The trays and their contents remain in the heated
atmosphere of this chamber for two or three weeks, by which time they
become dry and ready for removal, to be packed in lumps for certain
markets, or ground to dry powder and packed in barrels for others.
Women are employed to fill and also to empty the drying or stoving
chamber, and during this work they are fully exposed to its
contaminating atmosphere. Hot and dry, and charged with fine dusty
particles of white lead, it becomes a dangerous trap, and contaminates
the blood of those engaged with its deadly poison. It is in this part
of the manufacture that the principal damage to health occurs. This is
the most laborious work; heat makes it very fatiguing, the atmosphere
within this chamber being always much above the exterior air.
Recent Government regulations have sought to curtail these and other
evils in this manufacture. Women engaged in these stoves are ordered to
wear overclothing, headdress and respirators. The general experience
of their practice, notwithstanding Government regulations, is this;
that they cannot work in them with ease and convenience, and more often
wear the respirator around their necks than in front of nose and mouth.
The excessive mortality in women who work in these stacks and stoving
houses scarcely requires assertion. Few, even of those who employ them,
know the extent of the deadly operation. Recently, medical men have
made public that cases are within their knowledge of children born
already contaminated with lead poison. Woman labour should surely be
restricted by Government enactments in all such deadly occupations.
We may sum up the whole matter as regards white lead making by the
stack or Dutch method in a few brief words: It is a most tedious
and uncertain operation; it is a most dangerous occupation for all
concerned; it is founded upon no true principles of any kind; and of
science its whole course is ignorant. White lead making is ruled by a
“happy-go-lucky” philosophy. The representatives of this manufacture
are completely ignorant of the scientific details relating to it, and
hence we may not be surprised to find amongst them an enormous amount
of ignorance and prejudice.
Good white lead will not differ materially in its composition by
whatever process it may be made, but it may differ seriously in its
physical character, and in its fitness to produce a substance adapted
to the uses to which white lead paint is applied. Good white lead is
a compound which contains hydrate and carbonate of the metal, in the
proportions either of one molecule of hydrate of lead combined with two
of carbonate, or is made up of one molecule of hydrate with three of
carbonate of lead.
If we consider the first compound roughly
PbH_{2}O_{2},2Pb CO_{3}
white lead will be made up of one part of hydrate and two parts of
carbonate of lead.
The second compound roughly estimated
Pb H_{2}O_{2},3Pb CO_{3}
will be one part of hydrate, combined with three parts of carbonate
or lead. The latter will be in the proportion of 75 per cent, of
carbonate and 25 per cent, of hydrate of lead, and this represents the
composition which has been assigned to good white lead by those most
acquainted with the subject. The amount of hydrate contained in white
lead should never exceed the proportion above named of 25 per cent.,
nor should its amount be much below the 25 per cent.
The hydrate contained in the substance serves to unite with the oil in
the paint; it forms therewith a drying white and elastic varnish which
embraces and holds the particles of white carbonate and prevents their
subsidence and separation in the paint. There is a chemical action of a
much more intimate character between the components of good white lead
when mixed with oil which neither of the constituents of this compound
can alone produce.
For instance, hydrate of lead and linseed oil produce a varnish-like
substance, semi-transparent and of no covering capability.
Carbonate of lead and linseed oil produce a compound which is opaque,
but has no body or covering power, and in which the white solid
carbonate is held in feeble mechanical suspension.
Neither of them constitutes a paint, but when together as white lead
they are mixed with oil, combination takes place, and serviceable paint
of good body and covering power and enduring quality is produced. Good
white lead is a dense, perfectly amorphous powder of perfect whiteness,
possessed of great body and covering power when combined with oil. When
mixed with linseed oil and used as paint it rapidly dries in the air
and assumes a varnish-like, glossy, hard surface, and is capable when
once dry of resisting the action of air and water for any length of
time. It does not weep when laid on a surface with a brush, that is,
the oil does not separate from the solid material of the paint.
Attempts have been made to produce white lead quickly and cheaply by
precipitating processes, but in all such methods the resulting compound
is deficient in certain special qualities absolutely necessary to white
lead proper and to its uses. The precipitated white lead is always of
a crystalline structure, and crystalline lead can never furnish a good
body paint--no amount of pulverising and grinding of this crystalline
material will correct this defect in its nature, and deprive it of its
crystalline form.
“Once a crystal always a crystal” has an especial application to
this point of our philosophy. Pulverising a crystal will not alter
its structure, but simply reduces the size of the crystals. Crystals
of white lead are unable to effect the necessary combination with the
oil and form the true varnish which white amorphous lead so readily
produces. Paint made with the precipitated white lead lacks body and
covering power, and this because of the absence of this chemical union
with the oil.
The manufacture of white lead by process of precipitation, even were
the resulting preparation suitable, does not correct the evils of the
present method by Dutch or stack process of making white lead.
A solution of lead may be precipitated in a few minutes, but it cannot
be made so quickly. The white lead, after its precipitation, has to be
filtered or separated, washed and dried, and ground to powder, which
processes cannot occupy less time than a few weeks for completion.
Precipitated white lead has been made in France and Germany for some
years, and it is now manufactured in those countries. It is now made in
England by one patent process, but the product lacks certain qualities,
and is consequently still open to the objections already noted.
Substitutes for white lead of a non-poisonous nature, or of such a
nature as not to produce such deadly effects in their preparation or
use as white lead does, have been proposed; their introduction has
not, however, been a great success. A mixture of sulphate, sulphide
and oxide of zinc is a patent white made by subliming galena in an
oxidising furnace or hearth. This compound lacks body.
All of these so-called substitutes are very inferior to white lead,
not only as to quality but as to cost. They cannot compete with white
lead. A committee of enquiry on these substitutes for white lead,
reporting the result of their enquiry and examination, stated that they
found that these were mostly prepared with varnishes before they were
sold for use, and that in most instances they were mixed with a large
quantity of driers, and that the drier invariably was a compound of
lead.
The principal consumption of white lead is for paint; to produce this
paint it is ground with oil in varying proportions, about 8 to 15 per
cent. This produces the ordinary white lead in oil, and is worth from
19_l._ to 20_l._ a ton, but often more than this amount.
Dry powdered white lead is chiefly made for and used by grinders
and mixers, who combine with it a variety of other cheaper
materials--chalk, clay, sulphate of lime, and sulphate of baryta, but
principal use is made of chalk and barytes. These are mixed with the
white lead, and then the mixture is ground with oil and formed into
paint, sold under various names according to quality: thus--guaranteed
white lead, firsts, seconds, thirds, and fourths, the proportion of
white lead diminishing, and that of the adulterant increasing, as we
descend from the pure material. Guaranteed and best white lead is
not pure, and does not mean pure white lead. Pure white lead can be
purchased at some makers, but its price, if pure, can never fall below
19_l._ or 20_l._ per ton.
To sophisticate white lead, and produce the various inferiors named,
dry powdered white lead is needed as a starting point, and for this
purpose principally arises the necessity for its production. If ground
in oil the adulterants cannot be properly incorporated with it. Dry
white lead is used for nothing else that could ever give rise to any
great demand for it. We have already observed that the production of
this dry and powdered white lead is the most dangerous proceeding
connected with this industry. Grinding in oil is unattended with any
important consequence to the health and comfort of those employed. A
serious drawback to the “stack” production, the china-like incrustation
to which reference has already been made, is that it requires crushing,
grinding, washing, and drying, and a second course of dry grinding
after it is dried--the most objectionable step in its preparation.
Could the corrosion of the blue lead be effected in such a way as to
prevent any discoloration of the material by the tan and wood--could
the corrosion be so produced as to be easily separated from the buckle
or grating on which it has formed--could this separation be so effected
as to prevent the breaking up of the lead skeleton, and the presence
of small pieces of metal in the detached crust of white lead, two
principal reasons for washing and drying are removed.
There is yet another consideration, that is, the presence of acetate
of lead, always found in varying quantities in the incrustation
produced, and remaining at the close of the operation and conversion.
To remove this, careful washing, and after-stoving and drying must be
accomplished. The amount of this salt present is found to differ with
each operation, and in various portions of the same make.
The washing out of the acetate is never perfect, and it involves a
large amount of labour.
Opinions differ as to the effect of this acetate if allowed to remain
in the product. White lead makers on the “stack” principle aver that
it should and must be washed out, lest it should damage the qualities
of the paint. This is questionable, and not one can produce practical
evidence of its being the cause of any damage if still contained
in white lead. Facts seem to deny its harmfulness in this respect,
inasmuch as the best prepared samples, those washed and dried from the
most careful makers, will be found upon analysis to contain more or
less of acetate of lead.
A large proportion of this salt in white lead may not be beneficial
for many reasons, but a small percentage can do no harm; nay, for many
purposes it may be good.
There is no substance used for driers for white lead that is more
esteemed than this acetate of lead, commonly known as “sugar of lead.”
A small amount of this salt present in white lead would communicate
drying properties, and this alone is what it could do.
Granting that we can discover a method of producing white lead of
amorphous character, of good density, free from all discoloration, free
from all particles of metallic lead, and free from all but a small
percentage of acetate of lead, then washing will not be needed.
Stoving and drying become unnecessary. The work of women, their deadly
occupation, so burdensome to the operatives and to all with whom they
are concerned, is done away with.
_Condy’s Process._--An improvement in the manufacture of white lead
was patented by Condy, of Battersea, in 1881, which, though giving
perfectly satisfactory results when carefully conducted, necessitated
special precautions, and led to his substituting in practice the
following additions and modifications, which are of great consequence
in rendering the process more certain in the quality of its product,
and more valuable as a commercial manufacture on the practical scale,
by virtue of its offering greatly increased facility and economy in
production.
The results of numerous and repeated experiments on the larger scale
induced Condy to qualify the recommendation contained in his first
patent, viz. that of employing a solution of tribasic acetate of lead
and bicarbonate of soda in proper proportion to precipitate nearly the
whole of the lead, and further stating that he preferred to employ
“a slight excess of tribasic lead salt rather than find carbonate of
soda in excess.” Though, when carefully conducted, if the greatest
nicety is observed, a satisfactory result is obtained; in practice, the
least variation from the exact composition of the two substances is
attended with the drawback that the white lead is liable to a slight
uncertainty of tint after it is ground in oil, whereas by the process
hereinafter described a positive and reliable result can be obtained,
as the white lead produced will be of a uniform white colour, and not
liable to turn when ground in oil. Though the earlier process was in
itself complete for the manufacture of white lead from oxide of lead,
it afterwards occurred to Condy that a great object would be attained
in rendering the process more valuable and more practical, if a method
were devised, worked out, and described for the manufacture of tribasic
acetate of lead to be made entirely from metallic lead by the action of
acetic acid or neutral acetate of lead on metallic lead, and not to be
dependent on the employment in any way of previously manufactured oxide
of lead.
This portion of Condy’s invention relating to the manufacture of
tribasic acetate of lead may be described as follows: he melts, and,
after skimming carefully, feathers the metallic lead by dropping it
into water; he places this granulated lead in wooden vessels or vats
previously fitted with perforated false bottoms under which are fixed
taps for drawing the liquor off into other vats or tanks placed on a
lower level. Having filled with granulated lead the vessels fitted with
the false bottoms, he fills up the interstices with a dilute acetic
acid composed of one part, by weight, of acid (specific gravity 1·045
at 60° F.) and 12½ parts of water, and after allowing the dilute acid
to stand for two hours, draws it off through the taps into the lower
tanks. This allows access of atmospheric air to the lead, which has the
effect of heating the lead so that oxidation takes place.
After a time (about three or four hours), this oxidation begins to
slacken, when he pumps up a second time the acid solution from the
lower vat on to the granulated lead, and allows them to stand in
contact for one hour; he then again draws off the liquid into the lower
tank, and again exposes the metallic lead to atmospheric oxidation,
allowing three or four hours for the latter operation; and if the
solution of lead has not already attained the specific gravity of
1·040, at 60° F., he again repasses the liquor over the metallic lead
partially oxidised, until it has attained that specific gravity, when
he places the dissolved lead with fresh granulated lead and recommences
the manufacture in the same way.
This operation succeeds much better on the large scale than on a
laboratory scale. In vats containing upwards of one ton of lead, the
result is all that can be desired, and can be obtained by passing the
liquor from twice to three times over the metallic lead partially
oxidised. A little practice enables the operator so to control the
process that he can obtain the solution of the desired specific gravity
with perfect ease.
This plan of manufacturing tribasic acetate of lead possesses the
advantage of producing that substance wholly or nearly wholly free from
the impurities contained in metallic lead, such as copper and silver,
which are not taken up, or soluble, in the presence of metallic lead.
In consequence of the circumstance that foreign matter is left almost
untouched, it is practicable to make white lead of a fine quality from
old lead such as lead piping, roofing, and worn out lead generally,
which can thus be utilised to greater advantage than in any other way.
Having obtained this solution of basic acetate of lead of the specific
gravity of 1·040 at 60° F., Condy proceeds as follows:--To the solution
produced by each 60 lb. of acid and 750 lb. of water previously
pumped up into another vat or tank, he adds bicarbonate of soda in
the proportion of 30 lb. for each 60 lb. by weight of acid originally
employed, and agitates the mixture. This will generally precipitate
all the white lead, but it is necessary to test the filtrate to
ascertain the exact point when all the lead is thrown down. Sufficient
bicarbonate of soda should be added to do this completely, and it would
be better to use bicarbonate of soda in excess rather than leave any
lead unprecipitated, as by this means greater certainty is obtained
in securing on the large and practical scale a white lead capable of
standing the effect of light and grinding in oil without changing. The
white lead after precipitation can be washed, pressed, and dried in the
usual way.
The following variation may be made from the method described of making
tribasic acetate of lead, thus:--To each 60 lb. of acid and 750 lb. of
water may be added sufficient of the tribasic solution to make neutral
acetate of lead, with which to recommence the manufacture of tribasic
acetate of lead by the process described. Vague reference has been made
in works on technical chemistry to the possibility of using metallic
lead in the manufacture of sugar of lead, but such references have been
practically worthless, as they contain no information of a practical
nature even for the manufacture of neutral acetate of lead, and no
process at all has ever been described for the manufacture of basic
acetate of lead from metallic lead acted on by acetic acid or acetate
of lead.
_Gardner’s Process._--The conditions observed and fulfilled in the
arrangements adopted by Prof. E. V. Gardner, are founded upon a study
of the nature, properties and behaviour of the substances concerned,
under certain methods of treatment.
There are several oxides of lead which may be formed under special
conditions.--(1) Pb_{2}O, and the same oxide combined with water
Pb_{2}H_{2}O_{2}; (2) PbO, and the same oxide combined with water
PbH_{2}O_{2}. In the hydrated form these oxides combine readily with
carbonic acid, but they do not combine readily with carbonic acid when
dehydrated. The hydrates are most readily formed at about 120°-130° F.,
and are decomposed after they are formed if heated to 212° F.
These oxides and their hydrates combine with acetic acid to form
acetate and sub-acetate of lead, and with nitric acid to form nitrate
and sub-nitrate of lead.
Lead, submitted to the action of air, watery vapour, and acetic or
nitric acid, or a mixture of these acids, with air or oxygen, with
proper precautions, forms sub-acetate or sub-nitrate of lead, and this
sub-acetate or sub-nitrate of lead readily absorbs carbonic acid and
forms carbonate and sub-carbonate of lead.
Sufficient acetic or nitric acid, or a mixture of these and air or
oxygen, and watery vapour, must be constantly supplied to form the
subsalts of lead, to carry on the operation of converting blue lead
into white lead; but an excess or an insufficiency of these agents will
in either case prevent the formation of sub-acetate or sub-nitrate, and
consequently of the sub-carbonates.
Thus--too little air, acetic acid and aqueous vapour prevents the
formation of the hydrated sub-acetate and consequently sub-carbonate
of lead. Too much acetic acid and aqueous vapour, and too little air
forms an acetate on the surface of the lead, which, by the excess of
water, dissolves and wastes, and washes the lead; it also varnishes
the surface of the lead with a coating of acetate, and checks, if it
does not completely prevent, the formation of sub-acetate of lead, and
consequently the formation of carbonate and sub-carbonate of lead.
Similar rules hold good in the case of nitric acid and the formation of
nitrate and sub-nitrate.
The process of forming the hydrated sub-acetate or sub-nitrate of
lead is most energetic at a temperature of 120°-130° F. In a lower
temperature, a much longer time is occupied in carrying out the process
of conversion; while at a higher temperature the delicate sub-acetate
or sub-nitrate of lead first formed suffers loss of water, until, at
212° F., it is completely dehydrated. At a temperature about 135° F.,
the power of forming carbonate and sub-carbonates is lessened, and at
212° F. is considerably diminished. The carbonate itself is dehydrated
at 212° F., and is decomposed at a higher temperature.
To obtain a white lead of excellent quality for its various uses, it
is necessary to produce a substance which possesses sufficient body to
cover surfaces to which it may be applied as paint, and it must possess
sufficient base to combine with the oil of the paint to form a vehicle
or varnish to retain and hold the body on the surface. This is found to
be the case with sub-carbonate of lead, or especially with a compound
constituted of two or three equivalents of carbonate of lead, with one
equivalent of hydrated oxide of that metal.
Lead, to be attacked by chemical agents, should possess a clean
surface. If the surface of the lead is _chemically_ clean, so much the
better. The surface of the lead should be extended as much as possible,
and exposed to the action of the gases and vapours at a certain heat
for its conversion into white lead.
If the most favourable conditions are sought, the temperature should
be between 120° and 130° F.; and to carry on the chemical action
most satisfactorily, the lead during its conversion should present a
granular coating--that is, the coating formed by the chemical agents
should be granular and not smooth and continuous in character. It
should be porous, and not of a continuous varnish-like or vitreous
character.
To rapidly carry on the chemical action, the chemical agents should
be well diffused and commingled throughout each other, and the blue
lead should be so exposed as to be open to their attack on all parts
of its surface equally, and all should be at, and kept at, a proper
temperature and a proper degree of humidity. Too dry a heat prevents
the process of conversion. Too moist an atmosphere wastes the materials
and arrests their action. The conditions, therefore, which are most
favourable will be a certain humid atmosphere of well diffused and
commingled vapours or gases, acting on metallic lead exposed to them
under the physical conditions described, and at a temperature between
110° and 135° F.
These favourable conditions can be further augmented by certain
electrical arrangements in connection with them.
Again, in the sources from which the carbonic acid is derived, and
in the arrangement of the apparatus for applying the carbonic acid,
favourable and unfavourable conditions can be imported. Thus, by using
paraffin, petroleum, benzine, or light oil of paraffin or petroleum,
or similar carbonaceous substances free from sulphur, or mixtures of
such carbonaceous substances either alone or mixed with air or other
oxidising substance, comparatively pure carbonic acid may be furnished
without at the same time producing any objectionable compounds.
As a result of his researches and experiments, Prof. Gardner has
proposed (Eng. Pat. 1882, No. 731) certain improvements in the method
of converting blue lead into white lead, in the apparatus employed,
and in the method of making and applying the carbonic acid used in the
process. Some of these improvements are applicable to the open stack or
chamber process, while others relate to closed chambers.
Preferably, for the conversion of blue lead into white lead, Prof.
Gardner adopts a closed, but not air-tight, chamber. This chamber is
ventilated or relieved so as to enable the incoming gases and vapours
to enter it without hindrance, and to escape by means of an exit valve
and pipe, or an exit shaft, connected with the chamber, communicating
with the exterior air, and regulated by a valve or damper. The gases
and vapours within the chamber can find their way out by this exit
shaft only, by slight pressure on expansion from within the chamber;
thus the interior of the chamber is preserved from disturbance, by
preventing the formation of currents within its atmosphere, and yet a
perfect circulation of the gases and vapours is kept up.
To warm the interior of this chamber, which is constructed of any
material that will resist the action of the vapours and gases, and
which is not too absorbent, Prof. Gardner makes the bottom of the
chamber of such a shape as to form a heating vessel to hold water
or steam, this water or steam, or both, being kept at the required
temperature by means of a steam coil. Matters are so arranged that the
contents of this coil are protected from any excess of pressure, and
consequently the temperature seldom or never exceeds about 212° F.,
unless for any special reason it is desired to raise it to a higher
point. Sometimes the sides are constructed similarly to the bottom.
The materials of which these chambers are constructed must be capable
of resisting the heated and acidified vapours within them. Cast or
wrought iron faced with glass, slate, tiles, pewter, or glazed bricks;
tinned copper, tinned brass, or pewter; timber, whether green or after
carbonising by heat or sulphuric acid; all are more or less suitable.
Means of observing the progress of the conversion, and means of lifting
the contents in or out, must also be provided, as well as thermometers
to indicate the temperature prevailing inside.
The gases and vapours which effect the conversion of the blue lead
into white lead are generated outside the chamber just described, and
are conducted into it by means of pipes, being first raised to such a
temperature in excess of that which should exist inside the converting
chamber as to allow for the cooling effect arising from the friction
and loss of heat in passing through the various pipes and distributors
attached to the converting chamber.
Owing to this extra heating, the gases and vapours are expanded,
diffused, and commingled, and do not rob the interior of the converting
chamber of any heat on entering it, so that the heat inside the
converting chamber is kept constant, and can operate to further expand
and diffuse the gases and vapours in contact with the blue lead inside
the chamber. In practice it is found that the temperature of the
converting chamber cannot be suitably controlled if any portion of the
gases or vapours be generated within that chamber.
The blue lead is arranged in the converting chamber in trays, or on
shelves or frames, so as to allow it to be completely surrounded and
attacked by the vapours or gases, and thereby be converted into white
lead, at the same time preventing the formation of direct currents
or eddies. Framed supports resembling a dinner waggon serve well for
holding the lead, and may easily be arranged to lift bodily in and out
of the chamber with their burden of blue or white lead. The surface on
which the blue lead is directly supported is made of graphitic carbon,
hard coke, platinum, or carbonised or platinised material, such as is
used for plates in electric batteries, or of other material standing
in a similar electrical relationship to lead, or capable of generating
with it an electric current.
In applying this development of electrical energy to the ordinary
“stack,” as for instance the Dutch process, the pots containing
the acetous liquid and blue lead are made of, or lined with, such
electrical carbon, or contain a portion of it in suspension, by which
the same effect is realised. Advantage may also be derived from
furnishing a supplementary supply of carbonic acid to the stack beyond
that due to the decomposing dung, &c.; as well as from injecting
a current of air or oxygen at suitable temperatures, and from the
admission of steam in a coil throughout the stack.
The generation or production of the acetic or nitric acid vapours is
attained by heat in a vessel so arranged that its contents are kept
at a certain temperature for vaporising, by having the boiling acid
solution at about 1·003 sp. gr., procured by mixing water with vinegar
or acetic or nitric acids.
The supply pipes conveying the air or oxygen and the carbonic acid, or
either of them, can easily be made to emit their contents close to the
boiling acid solution, whereby the vapours arising from the latter are
mixed and intermingled with them, and all pass together through the
pipes and distributors into the converting chamber or stack.
In preparing the carbonic acid it is necessary to observe certain
precautions, especially that it shall be pure and that no carbonic
oxide shall gain admission to the chamber. If the ordinary chalk and
carbon method be employed, the furnace must be provided at the uptake
with a series of air inlets, so as to ensure that the gases passing
from the furnace to the delivery tube shall be most thoroughly oxidised.
The purification of the carbonic acid may be effected in the usual way
by passing it through a vessel containing water; or, if requisite, it
may be cleansed, expanded, and heated in one operation by passing it
through or over hot water, or over a hot aqueous solution of carbonate
of soda, all of which, however, incur considerable cost in plant and
manipulation.
Owing to these difficulties in purifying carbonic acid, Prof. Gardner
prefers to prepare a practically pure carbonic acid in the first place,
and this he does by allowing petroleum, benzine, paraffin, or other
hydrocarbon or carbonaceous liquid to gradually fall into a retort
containing chalk or other suitable carbonate at a high temperature,
whereby relatively pure carbonic acid gas is generated; and while still
in a highly heated state it encounters a stream of air or oxygen,
ensuring its complete combustion before entering the converting chamber.
Another method of procuring fairly pure carbonic acid is by the
combustion and oxidation of any of the liquid hydrocarbons mentioned
above, in suitable lamps; and when carbonic acid gas of exceptional
purity is required, it may be obtained by heating bicarbonates to a
sufficient degree to drive off one molecule of carbonic acid, reducing
the bicarbonate to carbonate, from which it can be reproduced by
treatment with carbonic acid gas obtained from cheaper sources.
The _modus operandi_ adopted by Prof. Gardner is as follows: The
lead is granulated and prepared for conversion in one operation,
thus--an iron or a slate slab about 2-3 inches thick is placed in a
tank containing acetic or nitric acid solutions rising about 3 inches
above the upper surface of the slab. The lead is melted at low red
heat and poured from a height of 4 to 6 feet into the acid solution,
through which it falls till it encounters the slab, and thereupon
passes away into the surrounding solution, being thus converted into
a spongy condition. In this condition the lead is spread on frames or
trays, which are then lifted bodily into their places in the converting
chambers. The latter is closed and heated to 120° F. for 3-4 hours, or
until the whole contents have assumed a uniform temperature. Thereupon
the acid vapours and air are admitted and distributed, taking care that
the temperature is not thereby reduced below 110° F. nor increased
above 125° F., steam being temporarily shut off if necessary. This
treatment is continued for 24 hours.
The admission of aqueous vapour should be so regulated that while a
dry atmosphere is avoided, yet there is no appreciable condensation of
moisture in the chamber. While ensuring this condition, the temperature
may reach as high as 130° F. for a second period of 24 hours, but must
not overstep the limits of 120° F. minimum and 135° F. maximum.
After 48 hours’ treatment, the supply of duly warmed carbonic acid
gas is admitted for a period of two hours, without discontinuing the
introduction of acid vapours, air and steam; and this addition of
carbonating gas is repeated for two hours at a time with intervals
of four hours during which it is cut off. When, after four or five
days, efflorescence or exfoliation appears on the lead, the supply
of carbonic gas is increased to two hours in every four, or four in
every six; and the admission of acid vapours, air and steam may also
be augmented so long as the temperature is not allowed to exceed
130°-135°F. The whole operation is completed in seven to fourteen days.
Of this process, Mr. Carter Bell, in his paper before referred to,
speaks in the highest terms. No washing or drying is necessary. No
women are engaged in the manufacture, and but few men. The white lead
thus produced by the aid of electricity is deposited in a peculiar
state of disintegration, it is perfectly amorphous and non-crystalline,
of the purest quality; its density is 5·8. When ground in oil and made
into paint, it possesses great body and a covering power inferior to no
other paint, if not superior to them all.
Painters who have used the paint, practical men, and amongst these
we may observe coach painters, have pronounced its excellence and
superiority to the best ordinary white lead paint.
By this electrical process of manufacture, not only is the time
consumed in the making and in the preparation of this material greatly
shortened, but the cost of preparation is reduced, and added to this
is the important fact, the vital factor in our consideration. The
labour of women is unnecessary. No lives are sacrificed to its working
requirements.
Prof. E. V. Gardner, who has been for some years occupying his
attention with the subject of white lead making, with a view especially
to remedying its existing evils, has invented his electrical chamber
process of manufacture, and an entirely new course of after treatment.
He has for the last seven or eight years been more or less occupied
perfecting his conception, and accommodating it to practical and
commercial claims.
Chamber processes are not new, there have been several patents
enrolled for making white lead in closed chambers, but none has proved
commercially convenient or practically successful in its adaptation,
and none has survived to the present time.
In Germany, white lead is made in chambers at the present day. The lead
in gratings or sheets is supported on wooden rods, saddle fashion, the
chamber is then filled, and its contents are submitted to currents
of acetic acid vapour, air, steam, and carbonic acid gas; the time
needful for conversion is six or seven weeks. The after steps in the
separation of the incrustation, and its preparation for trade purposes
are much the same as in the “stack” product preparation. It is washed,
stoved, dried, and ground. The white lead made on the German plan does
not differ in any material degree, save it be in price, from the best
English commodity. We may assume that the washing and drying in Germany
consumes a like period of time to that process in English works, viz.
two or three weeks. We then see that the German plan of making white
lead cannot be perfected in less than eight or ten weeks’ time from the
commencement of the corroding action in the chamber. To compare these
facts and the efficiency of the different plans described:--
The time required to complete the corrosion in the stack is at least 14
or 16 weeks.
The Gardner’s electric process requires for the same purpose only 14
days.
As to point of time, the German plan excels the stack, and can be
carried out in one-half the time required in the stack method of
conversion.
The Gardner’s electrical method excels the German, and can be perfected
in one-third the time needed for the German chamber operation, and
one-sixth the time required the stack.
These figures open out a most important matter when we regard the
capital invested and lying dormant in stack lead works.
It is well known that we have in electricity a most powerful agent
by which to effect the chemical combination of various substances on
the one hand, or on the other, by its means to break up and disrupt
a chemical compound. Professor Gardner’s main principle of action in
his new process is founded upon these facts, and he takes advantage
of electrical power to cause the combination of the lead with the
necessary elements to build up white lead in his chambers. He either
employs electrical discharges to energise and render active in their
chemical affinities the various materials engaged, or he so disposes
of them as to form an electric or galvanic combination in the chamber.
In the latter arrangement the chamber and its contents represent a
gas battery on an extensive scale. In practice he prefers the latter
plan; it is more simple, more manageable in the hands of the ordinary
workmen. The original plan was to have graphite or graphitic carbon
plates or supports for the buckles or gratings of lead within the
converting chamber. These carbon plates and the lead to be converted,
were so placed as to form a collection of galvanic “couples” or
“pairs,” and in this condition were submitted to the gases entering
the chamber from without. It will be understood that various modes of
arrangement would occur without departing from the principle concerned.
In practice this method answered very well, but presently a
difficulty arose; not only were the graphitic carbon or graphitic
plates expensive, but they were easily broken, and became friable
in use. Carbon plates of an especial kind were manufactured to meet
these failures and remedy these defects, not without success, but
still open to objection. In looking round for some substitute to
replace the carbon, two points were to be kept in view, to seek some
electro-negative to lead like the carbon, and some electro-conductive
like it, and some material that would bear rough handling such as
workmen give, and be practically convenient in its adaptation to a
working chamber.
Gold or platinum would excel in these particulars, but there are
considerations which debar their use. Pure tin presented itself, and
tin was tried with great success. Tin would at first be thought,
on account of its close electrical relationship to lead, far from
favourable to the purpose. Graphite or carbon would appear far more
suitable. Practice pronounced the tin to act as efficiently as carbon.
This may at first seem contradictory and strange, but if we consider
that while the carbon is certainly more highly electro-negative to
lead than tin, yet the tin is the more conductive, and offers the less
resistance to the electric current of the two; in this manner the tin
compensates by its conductive power all it may lack, as compared with
carbon, in electrical energy when coupled up with lead.
Tin plate is now used as the electric-negative element in the chamber
of the Gardner plan. Tin plate means pure tin. Ingot tin is rolled out
into plates, the bottom of the chamber is covered with this pure tin
plate, so are the bearers and the shelves, or supports which are to
hold the lead during its conversion. Tin pipes and tin fittings, which
resist the action of acetic acid, are also used to conduct the gases to
the chamber to carry on the converting operation, and to preserve the
product from any source of discoloration.