SOAP-MAKING.
_Classification of Soaps--Direct Combination of Fatty Acids
with Alkali--Cold Process Soaps--Saponification under Increased
or Diminished Pressure--Soft Soap--Marine Soap--Hydrated Soaps,
Smooth and Marbled--Pasting or Saponification--Graining
Out--Boiling on Strength--Fitting--Curd Soaps--Curd
Mottled--Blue and Grey Mottled Soaps--Milling Base--Yellow
Household Soaps--Resting of Pans and Settling of
Soap--Utilisation of Nigres--Transparent Soaps--Saponifying
Mineral Oil--Electrical Production of Soap._
Soaps are generally divided into two classes and designated "hard," and
"soft," the former being the soda salts, and the latter potash salts, of
the fatty acids contained in the material used.
According to their methods of manufacture, soaps may, however, be more
conveniently classified, thus:--
(A) Direct combination of fatty acids with alkali.
(B) Treatment of fat with definite amount of alkali and no separation of
waste lye.
(C) Treatment of fat with indefinite amount of alkali and no separation
of waste lye.
(D) Treatment of fat with indefinite amount of alkali and separation of
waste lye.
(A) _Direct Combination of Fatty Acids with Alkali._--This method
consists in the complete saturation of fatty acids with alkali, and
permits of the use of the deglycerised products mentioned in chapter
ii., section 2, and of carbonated alkalies or caustic soda or potash.
Fatty acids are readily saponified with caustic soda or caustic potash
of all strengths.
The saponification by means of carbonated alkali may be performed in an
open vat containing a steam coil, or in a pan provided with a removable
agitator.
It is usual to take soda ash (58°), amounting to 19 per cent. of the
weight of fatty acids to be saponified, and dissolve it in water by the
aid of steam until the density of the solution is 53° Tw. (30° B.); then
bring to the boil, and, whilst boiling, add the molten fatty acids
slowly, but not continuously.
Combination takes place immediately with evolution of carbonic acid gas,
which causes the contents of the vat or pan to swell, and frequently to
boil over. The use of the agitator, or the cessation of the flow of
fatty acids, will sometimes tend to prevent the boiling over. It is
imperative that the steam should not be checked but boiling continued
as vigorously as possible until all the alkali has been absorbed and the
gas driven off.
The use of air to replace steam in expelling the carbonic acid gas has
been patented (Fr. Pat. 333,974, 1903).
A better method of procedure, however, is to commence with a solution of
64° Tw. (35° B.) density, made from half the requisite soda ash (9-1/2
per cent.), and when this amount of alkali has all been taken up by the
fatty acids (which have been added gradually and with continuous
boiling), the remaining quantity of soda ash is added in a dry state,
being sprinkled over each further addition of fatty acid.
This allows the process to be more easily controlled and boiling over is
avoided.
It is essential that the boiling by steam should be well maintained
throughout the process until all carbonic acid gas has been thoroughly
expelled; when that point is reached, the steam may be lessened and the
contents of the vat or pan gently boiled "on strength" with a little
caustic lye until it ceases to absorb caustic alkali, the soap being
finished in the manner described under (D).
It is extremely difficult to prevent discoloration of fatty acids, hence
the products of saponification in this way do not compare favourably in
appearance with those produced from the original neutral oil or fat.
(B) _Treatment of Fat with Definite Amount of Alkali and no Separation
of Waste Lye._--Cold-process soap is a type of this class, and its
method of production is based upon the characteristic property which the
glycerides of the lower fatty acids (members of the cocoa-nut-oil class)
possess of readily combining with a strong caustic soda solution at a
low temperature, and evolving sufficient heat to complete the
saponification.
Sometimes tallow, lard, cotton-seed oil, palm oil and even castor oil
are used in admixture with cocoa-nut oil. The process for such soap is
the same as when cocoa-nut oil is employed alone, with the slight
alteration in temperature necessary to render the fats liquid, and the
amount of caustic lye required will be less. Soaps made of these blends
closely resemble, in appearance, milled toilet soaps. In such mixtures
the glycerides of the lower fatty acids commence the saponification, and
by means of the heat generated induce the other materials, which alone
would saponify with difficulty or only with the application of heat, to
follow suit.
It is necessary to use high grade materials; the oils and fats should be
free from excess of acidity, to which many of the defects of
cold-process soaps may be traced. Owing to the rapidity with which free
acidity is neutralised by caustic soda, granules of soap are formed,
which in the presence of strong caustic lye are "grained out" and
difficult to remove without increasing the heat; the soap will thus tend
to become thick and gritty and sometimes discoloured.
The caustic lye should be made from the purest caustic soda, containing
as little carbonate as possible; the water used for dissolving or
diluting the caustic soda should be soft (_i.e._, free from calcium and
magnesium salts), and all the materials carefully freed from particles
of dirt and fibre by straining.
The temperature, which, of course, must vary with the season, should be
as low as is consistent with fluidity, and for cocoa-nut oil alone may
be 75° F. (24° C.), but in mixtures containing tallow 100° to 120° F.
(38° to 49° C.).
The process is generally carried out as follows:--
The fluid cocoa-nut oil is stirred in a suitable vessel with half its
weight of 71.4° Tw. (38° B.) caustic soda lye at the same temperature,
and, when thoroughly mixed, the pan is covered and allowed to rest. It
is imperative that the oils and fats and caustic lye should be
intimately incorporated or emulsified. The agitating may be done
mechanically, there being several machines specially constructed for the
purpose. In one of the latest designs the caustic lye is delivered
through a pipe which rotates with the stirring gear, and the whole is
driven by means of a motor.
The agitation being complete, chemical action takes place with the
generation of heat, and finally results in the saponification of the
fats.
At first the contents of the pan are thin, but in a few hours they
become a solid mass. As the process advances the edges of the soap
become more transparent, and when the transparency has extended to the
whole mass, the soap is ready, after perfuming, to be framed and
crutched.
The admixture of a little caustic potash with the caustic soda greatly
improves the appearance of the resultant product, which is smoother and
milder.
The glycerine liberated during the saponification is retained in the
soap.
Although it is possible, with care, to produce neutral soaps of good
appearance and firm touch by this method, cold-process soaps are very
liable to contain both free alkali and unsaponified fat, and have now
fallen considerably into disrepute.
_Saponification under Increased or Diminished Pressure._--Soaps made by
boiling fats and oils, under pressure and _in vacuo_, with the exact
quantity of caustic soda necessary for complete combination, belong also
to this class. Amongst the many attempts which have at various times
been made to shorten the process of soap-making may be mentioned
Haywood's Patent No. 759, 1901, and Jourdan's French Patent No. 339,154,
1903.
In the former, saponification is carried out in a steam-jacketed vacuum
chamber provided with an elaborate arrangement of stirrers; in the other
process fat is allowed to fall in a thin stream into the amount of lye
required for saponification, previously placed in the saponification
vessel, which is provided with stirring gear.
When the quantities have been added, steam is admitted and
saponification proceeds.
(C) _Treatment of Fat with Indefinite Amount of Alkali and no Separation
of Waste Lye._--_Soft soap_ is representative of this class. The
vegetable fluid oils (linseed, olive, cotton-seed, maize) are for the
most part used in making this soap, though occasionally bone fats and
tallow are employed. Rosin is sometimes added, the proportion ranging,
according to the grade of soap required, from 5 to 15 per cent. of the
fatty matter.
The Soft Soap Manufacturers' Convention of Holland stipulate that the
materials used in soft-soap making must not contain more than 5 per
cent. rosin; it is also interesting to note that a patent has been
granted (Eng. Pat. 17,278, 1900) for the manufacture of soft soap from
material containing 50 per cent. rosin.
Fish or marine animal oils--whale, seal, etc., once largely used as raw
material for soft soap, have been superseded by vegetable oils.
The materials must be varied according to the season; during hot
weather, more body with a less tendency to separate is given by the
introduction of oils and fats richer in stearine; these materials also
induce "figging".
The most important material, however, is the caustic potash lye which
should average 40° Tw. (24° B.), _i.e._, if a weak solution is used to
commence saponification, a stronger lye must be afterwards employed to
avoid excess of water in the soap, and these average 40° Tw. (24° B.).
The potash lye must contain carbonates, which help to give transparency
to the resultant soap. If the lye is somewhat deficient in carbonates,
they may be added in the form of a solution of refined pearl ash
(potassium carbonate).
Caustic soda lye is sometimes admixed, to the extent of one-fourth, with
potash lye to keep the soap firmer during hot weather, but it requires
great care, as a slight excess of soda gives soft soap a bad appearance
and a tendency to separate.
The process is commenced by running fatty matter and weak potash lyes
into the pan or copper, and boiling together, whilst the introduction of
oil and potash lye is continued.
The saponification commences when an emulsion forms, and the lye is then
run in more quickly to prevent the mass thickening.
Having added sufficient "strength" for complete saponification, the
boiling is continued until the soap becomes clear.
The condition of the soap is judged by observing the behaviour of a
small sample taken from the pan and dropped on glass or iron. If the
soap is satisfactory it will set firm, have a short texture and slightly
opaque edge, and be quite clear when held towards the light. If the
cooled sample draws out in threads, there is an excess of water present.
If an opaque edge appears and vanishes, the soap requires more lye. If
the sample is turbid and somewhat white, the soap is too alkaline and
needs more oil.
The glycerine liberated during saponification is contained in the soap
and no doubt plays a part in the production of transparency.
_Hydrated soaps_, both smooth and marbled, are included in this
classification, but are _soda_ soaps. Soap made from cocoa-nut oil and
palm-kernel oil will admit of the incorporation of large quantities of a
solution of either salt, carbonate of soda, or silicate of soda, without
separation, and will retain its firmness. These materials are,
therefore, particularly adapted for the manufacture of marine soaps,
which often contain as much as 80 per cent. of water, and, being soluble
in brine, are capable of use in sea-water. For the same reason,
cocoa-nut oil enters largely into the constitution of hydrated soaps,
but the desired yield or grade of soap allows of a variation in the
choice of materials. Whilst marine soap, for example, is usually made
from cocoa-nut oil or palm-kernel oil only, a charge of 2/3 cocoa-nut
oil and 1/3 tallow, or even 2/3 tallow and 1/3 cocoa-nut oil, will
produce a paste which can carry the solutions of silicate, carbonate,
and salt without separation, and yield a smooth, firm soap.
The fatty materials, carefully strained and freed from particles of dirt
and fibre, are boiled with weak caustic soda lye until combination has
taken place. Saponification being complete, the solution of salt is
added, then the carbonate of soda solution, and finally the silicate of
soda solution, after which the soap is boiled. When thoroughly mixed,
steam is shut off, and the soap is ready for framing.
The marbled hydrated soap is made from cocoa-nut oil or a mixture of
palm-kernel oil and cocoa-nut oil with the aid of caustic soda lye
32-1/2° Tw. (20° B.). As soon as saponification is complete, the brine
and carbonate of soda solution are added, and the pan allowed to rest.
The soap is then carefully tasted as to its suitability for marbling by
taking samples and mixing with the colouring solution (ultramarine mixed
with water or silicate of soda solution). If the sample becomes blue
throughout, the soap is too alkaline; if the colour is precipitated, the
soap is deficient in alkali. The right point has been reached when the
marbling is distributed evenly. Having thus ascertained the condition of
the pan, and corrected it if necessary, the colour, mixed in water or in
silicate of soda solution, is added and the soap framed.
(D) _Treatment of Fat with Indefinite Amount of Alkali and Separation of
Waste Lye._--This is the most general method of soap-making. The various
operations are:--
(_a_) Pasting or saponification.
(_b_) Graining out or separation.
(_c_) Boiling on strength.
And in the case of milling soap base and household soaps,
(_d_) Fitting.
(_a_) _Pasting or Saponification._--The melted fats and oils are
introduced into the soap-pan and boiled by means of open steam with a
caustic soda lye 14° to 23.5° Tw. (10° to 15° B.). Whether the fatty
matters and alkali are run into the pan simultaneously or separately is
immaterial, provided the alkali is not added in sufficient excess to
retard the union.
The commencement of the saponification is denoted by the formation of an
emulsion. Sometimes it is difficult to start the saponification; the
presence of soap will often assist this by emulsifying the fat and thus
bringing it into intimate contact with the caustic soda solution.
When the action has started, caustic soda lye of a greater density, 29°
to 33° Tw. (18° to 20° B.), is frequently added, with continued boiling,
in small quantities as long as it is being absorbed, which is
ascertained by taking out samples from time to time and examining them.
There should be no greasiness in the sample, but when pressed between
finger and thumb it must be firm and dry.
Boiling is continued until the faint caustic taste on applying the
cooled sample to the tongue is permanent, when it is ready for "graining
out". The pasty mass now consists of the soda salts of the fat (as
imperfect soap, probably containing emulsified diglycerides and
monoglycerides), together with water, in which is dissolved the
glycerine formed by the union of the liberated glyceryl radicle from the
fat with the hydroxyl radicle of the caustic soda, and any slight excess
of caustic soda and carbonates. The object of the next operation is to
separate this water (spent lye) from the soap.
(_b_) _Graining Out or Separation._--This is brought about by the use of
common salt, in a dry form or in solution as brine, or by caustic soda
lye. Whilst the soap is boiling, the salt is spread uniformly over its
surface, or brine 40° Tw. (24° B.) is run in, and the whole well boiled
together. The soap must be thoroughly boiled after each addition of
salt, and care taken that too large a quantity is not added at once.
As the soap is gradually thrown out of solution, it loses its smooth
transparent appearance, and becomes opaque and granular.
When a sample, taken out on a wooden trowel, consists of distinct grains
of soap and a liquid portion, which will easily separate, sufficient
salt or brine has been added; the boiling is stopped and the spent lye
allowed to settle out, whilst the soap remains on the surface as a more
or less thick mass.
The separated spent lye consists of a solution of common salt,
glycerine, and alkaline salts; the preparation of crude glycerine
therefrom is considered in chapter ix.
The degree of separation of water (spent lye) depends upon the amount of
precipitant used. The aim is to obtain a maximum amount of spent lye
separated by the use of a minimum quantity of salt.
The amount of salt required for "graining out" varies with the raw
material used. A tallow soap is the most easily grained, more salt is
required for cotton-seed oil soap, whereas soaps made from cocoa-nut and
palm-kernel oils require very large amounts of salt to grain out
thoroughly. Owing to the solubility in weak brine of these latter soaps,
it is preferable to grain them with caustic soda lye. This is effected
by adding, during boiling, sufficient caustic lye (32-1/2° Tw., 20° B.)
to produce the separation of the granules of soap. The whole is allowed
to rest; the separated half-spent lye is withdrawn and may be used for
the pasting of fresh materials.
After the removal of the settled lye, the grained mass is boiled with
sufficient water to dissolve the grain and make it smooth ("close" it),
and is now ready for the next operation of "boiling on strength".
(_c_) _Boiling on Strength or Clear Boiling._--This is the most
important operation and is often termed "making the soap". The object is
to harden the soap and to ensure complete saponification.
Caustic soda lye (32-1/2° Tw., 20° B.) is gradually added until the soap
is again opened or grained, and boiling continued by the use of the dry
steam coil. As soon as the caustic soda lye is absorbed, another portion
is slowly added, and this is continued until the caustic soda or
"strength" remains permanent and the soapy mass, refusing to absorb
more, is thrown out of solution and grained. The granular mass will boil
steadily, and the boiling should be prolonged, as the last traces of
neutral oil are difficult to completely saturate with alkali. Thorough
saponification takes place gradually, and the operation cannot be
hurried; special care has to be bestowed upon this operation to effect
the complete combination of fat and alkali.
After resting for several hours, half-spent lye settles to the bottom of
the pan. In the case of yellow soaps or milling bases the settled lye is
removed to a suitable receptacle and reserved for use in the
saponification of other material, and the soap is then ready for the
final operation of "fitting".
(_d_) _Fitting._--If the operations just described have been properly
performed, the fitting should present no difficulty. The soap is boiled
with open steam, and water added until the desired degree of closing is
attained. As the water is thoroughly intermixed throughout the mass the
thick paste gradually becomes reduced to a smooth, thin consistence.
Samples are tested from time to time as to their behaviour in dropping
off a hot trowel held sideways; the thin layer should drop off in two or
three flakes and leave the surface of the trowel clean and dry. The soap
is then in a condition to allow the impurities to gravitate. According
to the required soap, the fit may be "coarse" ("open") when the flakes
drop off the trowel readily, or "fine" ("close") when the flakes only
leave the trowel with difficulty.
If the dilution with water has been allowed to proceed too far, and too
fine a fit is produced, which would be denoted by the layer of soap not
leaving the trowel, a little caustic lye or brine may be very carefully
added and the whole well boiled until the desired condition is obtained.
A good pressure of steam is now applied to the pan, causing the contents
to swell as high as possible, this greatly facilitating the settling of
impurities; steam is then turned off, the pan covered, and the boil
allowed to rest for several days.
The art of fitting consists in leaving the contents of the pan in such a
condition that, on standing, all the impurities precipitate, and the
settled soap, containing the correct amount of water, is clear and
bright.
The above is a general practical outline of the ordinary soap-boiling
process. It may be modified or slightly altered according to the fancy
of the individual soap-maker or the particular material it is desired to
use. Fats and oils not only vary in the amount of alkali they absorb
during saponification, but also differ in the strength of the alkali
they require. Tallow and palm oil require lye of a density of 15° to 18°
Tw. (10° to 12° B.), but cocoa-nut oil alone would not saponify unless
the lye was more concentrated, 33° to 42° Tw. (20° to 25° B.).
Cotton-seed oil requires weak lyes for saponification, and, being
difficult to saponify alone even with prolonged boiling, is generally
mixed with animal fat.
When fats are mixed together, however, their varying alkali requirements
become modified, and once the saponification is begun with weak lye,
other materials are induced to take up alkali of a strength with which
alone they would not combine.
It is considered the best procedure to commence the pasting or
saponification with weak lye.
In order to economise tank space, it is the general practice to store
strong caustic lye (60° to 70° Tw., 33° to 37° B.) and to dilute it as
it is being added to the soap-pan by the simultaneous addition of water.
Many manufacturers give all their soap a "brine wash" to remove the last
traces of glycerine and free the soap from carbonates. This operation
takes place prior to "fitting"; sufficient water is added to the boiling
soap to "close" it and then brine is run in to "grain" it.
After resting, the liquor is withdrawn.
Having described the necessary operations in general, we will now
consider their application to the preparation of various kinds of hard
soap.
_Curd Soaps._--Tallow is largely used in the manufacture of white curd
soaps, but cocoa-nut oil sometimes enters into their composition.
The first three operations above described, _viz._, pasting, graining
out, and boiling on strength, are proceeded with; the clear boiling by
means of a closed steam coil is continued until the "head" is boiled out
and the soap is free from froth. A sample taken and cooled should be
hard. Boiling is then stopped, and, after covering, the pan is allowed
to rest for eight to ten hours, when the soap is ready for filling into
frames, where it is crutched until perfectly smooth.
_Curd mottled_ is usually made from melted kitchen stuff and bone
grease.
Its preparation is substantially the same as for curd soap, but the
clear boiling is not carried so far. The art of curd mottled soap-making
lies in the boiling. If boiled too long the mottling will not form
properly, and, on the other hand, insufficient boiling will cause the
soap to contain an excess of entangled lye. Having boiled it to its
correct concentration the pan is allowed to rest about two hours, after
which the soap is ready for framing, which should be done expeditiously
and the frames covered up.
Some lye, containing the impurities from the fats used, remains in the
interstices of the curd, unable to sink, and as the soap cools it is
enclosed and forms the mottling. The mottling may, therefore, be
considered as a crystallisation of the soap, in which the impurity forms
the colour.
_Blue and Grey Mottled Soaps._--These are silicated or liquored soaps in
which the natural mottling, due to the impure materials used in the
early days of soap-making, is imitated by artificial mottling, and are,
consequently, entirely different to curd mottled soaps.
The materials employed in making mottled soap comprise bleached palm
oil, tallow, bone fat, cocoa-nut oil, palm-kernel oil, cotton-seed oil,
and, in some instances, rosin.
The choice of a charge will naturally depend upon the cost; the property
of absorbing a large amount of liquor, which is characteristic of soaps
made from cocoa-nut oil and palm-kernel oil, is taken advantage of, as
are also the physical properties of the various fats and oils, with a
view to the crystallisation of the resultant soap and the development of
the mottle. The fat is saponified, grained and boiled on strength, as
previously described. After withdrawing the half-spent lye, the soap is
just closed by boiling with water, and is then ready for the silicate or
other saline additions.
Soap intended to be liquored with silicate of soda should be distinctly
strong in free alkali; the crystalline nature of the soap is increased
thereby, and the mottled effect intensified. Some makers, however, fit
the soap coarsely and allow a nigre to deposit; then, after removing the
nigre, or transferring the settled soap to another copper, containing
scraps of mottled soap, get the soap into a condition for mottling, and
add the silicate of soda solution. To every 1 cwt. of soap, 28 lb. of
silicate of soda solution, 32-1/2° Tw. (20° B.) is added, whilst
boiling; the strength of the silicate solution, however, will depend
upon the proportion of cocoa-nut oil and palm-kernel oil present in the
charge. Many soap-makers use 20° Tw. (13° B.) (cold) silicate solution,
whilst others prefer 140° Tw. (59.5° B.), with the gradual addition of
water to the soap, kept boiling, until the product is in the correct
mottling condition, and others, again, use bleach liquor, soda crystals,
pearl ash, and salt, together with silicate solution.
Considerable skill and experience is necessary to discern when the soap
acquires the correct mottling state. It should drop off the spatula in
large thick flakes, take considerable time to set, and the surface
should not be glossy.
When this mottling condition has been obtained, the colouring matter,
which would be ultramarine for the blue mottled and manganese dioxide
for the grey mottled soap (3-4 lb. ultramarine or 1-3 lb. manganese
dioxide being sufficient for 1 ton of soap), is mixed with a little
water and added to the boiling soap--the boiling is continued until all
is thoroughly amalgamated, and when the steam is shut off the contents
of the pan are ready for cleansing.
Mottled soap is run into wooden frames, which, when full, are covered
over and allowed to cool very gradually. On cooling slowly, large
crystals are produced which result in a distinct bold mottle; if the
cooling is too rapid, a small crystal is obtained and the mottle is not
distributed, resulting in either a small mottle, or no mottle at all,
and merely a general coloration. In fact, the entire art of mottling
soap consists in properly balancing the saline solutions and colouring
matter, so that the latter is properly distributed throughout the soap,
and does not either separate in coloured masses at the bottom of the
frame, or uniformly colour the whole mass.
A sample of the soap should test 45 per cent. fatty acids, and the
amount of salt would range from 1/2 to 1 per cent.
Some of the English mottled soaps, especially those made from materials
which give a yellow-coloured ground, are bleached by soaking in brine,
or pickling in brine containing 2 per cent. of bleach liquor. The
resultant soap has a white ground and is firm. The bleach liquor may be
made by mixing 1 cwt. bleaching powder with 10 cwts. of soda ash
solution (15° Tw., 10° B.), allowing to settle, and using the clear
liquid, or by mixing 2 parts soda ash solution with 1 part of bleaching
powder solution, both solutions being 30° Tw. (18.8° B.).
_Milling-base._--The materials generally used are tallows and cocoa-nut
oils of the finest quality. The tallow is thoroughly saponified first,
and the graining is performed by the aid of caustic soda lye in
preference to salt. The half-spent lyes are withdrawn, and the cocoa-nut
oil added to the pan. This is saponified, and when the saponification is
complete, "boiling-on-strength" is proceeded with. Special care should
be devoted to the "boiling-on-strength" operation--its value in good
soap-making cannot be over-rated--and perfect saponification must be
ensured. The half-spent lyes are allowed to deposit during the night,
and the soap must be carefully examined next morning to ascertain if any
alkali has been absorbed. If the caustic taste is permanent the
strengthening operation is complete, but should any caustic have been
absorbed, further addition of alkali must be made and the boiling
continued. These remarks apply equally to all soaps.
The soap, when ready, is fitted.
Bleached palm oil, olive oil, castor oil and lard are also employed in
the production of special milling soap bases, a palm oil soap being
specially suitable for the production of a violet-scented toilet soap.
_Yellow Household Soaps._ (_a_) _Bar Soaps._--These are made from tallow
with an admixture of from 15-25 per cent. rosin. The best quality is
known in the South and West of England as Primrose Soap, but is
designated in the North of England by such names as Golden Pale,
Imperial Pale, Gold Medal Pale, etc. Tallow alone produces a very hard
soap of inferior lathering qualities; but rosin combines with alkali to
form a soft body, which, although not a soap in the strict sense of the
term, is readily soluble in water, and in admixture with the durable
tallow soap renders it more soluble in water and thereby increases its
lathering properties.
The rosin may be added to the soap-pan after a previous partial
saponification with soda ash, and removal of colouring matter, and
finally saponified with caustic soda lye, or, as is more generally
adopted, as a rosin change. The pan is opened with caustic soda lye and
saturation of the rosin takes place rapidly; when completely saponified
it is grained with salt, and the coloured lye allowed to deposit and
finally withdrawn.
The four operations already detailed apply to this soap.
Cheaper pale soaps may be made from lower grades of tallow and rosin and
are generally silicated.
(_b_) _Tablet or Washer Type._--A demand has arisen for soap of free
lathering qualities, which has become very popular for general household
use. This soap is usually made from a mixture of cotton-seed oil,
tallow, and cocoa-nut oil, with a varying amount of rosin. The tallow
yields firmness and durability whilst the other constituents all assist
in the more ready production of a copious lather.
As to what amount of rosin can be used to yield a finished soap of
sufficient body and satisfactory colour, this naturally depends upon the
grade of raw material at the soap-makers' disposal. Those fats and oils
which yield firm soaps, will, of course, allow a greater proportion of
rosin to be incorporated with them than materials producing soaps of
less body. Rosin imparts softness to a soap, and also colour.
This is a fitted soap and full details of manufacture have already been
given.
Cheaper soaps are produced from lower grade materials hardened with
alkaline solutions.
_Resting of Pans and Settling of Soap._--The fitted soap is allowed to
settle from four to six days. The period allowed for resting is
influenced, however, not only by the size of the boil, and the season,
but also by the composition of the soap, for if the base has been made
from firm stock it is liable to cool quicker than a soap produced from
soft-bodied materials.
On subsidence, the contents of the pan will have divided into the
following:--
First. On top, a thin crust of soap, with perhaps a little light
coloured fob, which is returned to the pan after the removal of the good
soap.
Second. The good settled soap, testing 62-63 per cent. fatty acids. The
subject of removing and treatment of this layer is fully dealt with in
the next chapter.
Third. A layer of darker weak soap, termed "nigre," which on an average
tests 33 per cent. fatty acids, and, according to the particular fit
employed, will amount to from 15-20 per cent. of the total quantity of
soap in the pan.
The quantity of nigre may vary not only with the amount of water added
during finishing, but is also influenced by the amount of caustic alkali
remaining in the soap paste prior to fitting. If the free caustic
alkali-content is high, the soap will require a large amount of water to
attain the desired fit. This water renders the caustic into a lye
sufficiently weak to dissolve a quantity of soap, consequently, as the
"nigre" is a weak solution of soap together with any excess of alkali
(caustic or carbonate) and salt which gravitates during the settling,
the quantity is increased.
Fourth. A solution containing alkaline salts, mostly carbonates and
chlorides, with a little caustic.
The amount of the layer is very variable, and doubtless, under certain
physical conditions, this liquor has separated from the nigre.
_Utilisation of Nigres._--The nigres are boiled and the liquor separated
by graining with salt. Nigre may be utilised in various ways.
(1) It may be used several times with new materials. This particularly
refers to soaps of the "Washer" type. The colour of the nigre will
determine the number of times it can be employed.
(2) It may be incorporated with a soap of a lower grade than the one
from which it was obtained. In this case a system is generally adopted;
for example, soap of the best quality is made in a clean pan, the nigre
remaining is worked up with fresh material for soap of the next quality,
the nigre from that boil, in its turn, is admixed with a charge to
produce a batch of third quality, and the deposited nigre from this is
again used for a fourth quality soap--the nigre obtained from this
latter boil would probably be transferred into the cheapened "washer" or
perhaps if it was dark in colour into the brown soap-pan.
(3) The nigre may be fitted and produce a soap similar to the original
soap from which it was deposited. It is advisable to saponify a little
fat with it.
(4) Nigres from several boils of the same kind of soap can be collected,
boiled, and fitted. The settled portion may be incorporated with a new
charging to keep the resultant soap uniform in colour--unless this is
done, the difference in colour between boils from new materials alone,
and those containing nigre, is very noticeable. The nigre settled from
this fitted nigre boil would be utilised in brown soap.
(5) According to its colour, and consistence, a nigre may be suitable
for the production of disinfectant, or cold-water soaps.
(6) Nigre may be bleached by treatment with a 20 per cent. solution of
stannous chloride--1 cwt. of this solution (previously heated) is
sufficient to bleach 20 tons of nigre.
_Transparent Soaps._--The production of transparent soaps has recently
been fully studied, from a theoretical point of view, by Richardson
(_J. Amer. Chem. Soc._, 1908, pp. 414-20), who concludes that the
function of substances inducing transparency, is to produce a jelly and
retard crystallisation.
The old-fashioned transparent soap is prepared by dissolving, previously
dried, genuine yellow soap in alcohol, and allowing the insoluble saline
impurities to be deposited and removed. The alcoholic soap solution is
then placed in a distillation apparatus, or the pan containing the
solution is attached by means of a still head to a condenser, and the
alcohol distilled, condensed and regained. The remaining liquid soap,
which may be coloured and perfumed, is run into frames and allowed to
solidify.
The resultant mass is somewhat turbid, but after storage in a room at
95° F. (35° C.) for several months, becomes transparent.
The formation of the transparency is sometimes assisted and hastened by
the addition of glycerine or a solution of cane-sugar.
A patent has been granted to A. Ruch (Fr. Pat. 327,293, 1902) for the
manufacture of transparent glycerine soap by heating in a closed vessel
fatty acids together with the requisite quantity of alcoholic caustic
soda solution necessary for saponification, and cooling the resultant
soap. It is also proposed to add sugar solution.
Cheaper qualities of transparent soaps are made by the cold process with
or without the aid of alcohol and castor oil, and with the assistance of
glycerine or cane-sugar.
With the continual demand for cheaper production, sugar solution has
gradually, in conjunction with castor oil, which produces transparency,
superseded the use of alcohol and glycerine.
For a small batch, 56 lb. Cochin cocoa-nut oil and 56 lb. sweet edible
tallow may be taken, melted at 130° F. (54° C.), and carefully strained
into a small steam-jacketed pan. It is imperative that the materials
should be of the highest quality and perfectly clean. Twenty-three lb.
of pure glycerine and 56 lb. of bright caustic soda solution made from
high grade caustic and having a density of 72° Tw. (38° B.) are crutched
into the fat; the alcohol, which would be 45 lb. in this example, is
then added. The whole must be most intimately incorporated, and the pan
covered and allowed to rest for one hour or one and a half hours.
Saponification should ensue.
To produce a transparent glycerine soap with the aid of castor oil, and
with or without the use of alcohol, the following is the procedure:--
Cochin cocoa-nut oil, sweet edible tallow, and castor oil, of each 56
lb. are taken, warmed to 130° F. (54° C.), and carefully strained into
the jacketed pan. If it is desired to use glycerine and cane sugar
solution, and no alcohol, the glycerine (25 lb.) is now stirred into the
fats together with the requisite (83 lb.) caustic soda solution 72° Tw.
(38° B.). If it is intended to use alcohol and sugar, and no glycerine,
the latter is replaced by 47 lb. of alcohol, and added after the
incorporation of the caustic soda lye.
The whole being thoroughly crutched, the pan is covered and
saponification allowed to proceed for one hour or one and a half hours.
Should the saponification for some reason be retarded, a little steam
may be very cautiously admitted to the jacket of the pan, the mass well
crutched until the reaction commences, and the whole allowed to rest the
specified time.
Whilst saponification is proceeding, the "sugar solution" is prepared by
dissolving 50 lb. cane sugar in 50 lb. water, at 168° F. (76° C.), to
which may be added 5 lb. soda crystals, and any necessary colouring
matter. The water used for this solution should be as soft as possible,
as hard water is liable to produce opaque streaks of lime soap.
It is absolutely necessary before proceeding further to ensure that
saponification is complete. A greasy, soft feel and the presence of
"strength" (caustic) would denote incomplete saponification--this can
only be remedied by further heating and crutching. Deficiency of caustic
alkali should also be avoided, and, if more lye is required, great care
must be exercised in its addition.
Saponification being completed, the sugar solution is carefully and
gradually crutched into the soap; when the contents of the pan have
become a homogeneous and syrupy mass, the crutching is discontinued, and
the pan is covered for one hour. The heat of the soap in the pan should
not exceed 170° F. (77° C.).
Having rested the necessary period, the soap will have a slight froth on
the surface, but will be clear underneath and appear dark. Samples may
now be withdrawn, cooled, and examined prior to framing. If the process
has been successfully performed the soap will be firm and transparent,
of uniform colour, and possess only a faintly alkaline taste.
If the sample be firm but opaque, more sugar solution is required; this
should be added very carefully whilst crutching, an excess being
specially guarded against. If the sample be soft, although transparent,
and the alkaline taste not too pronounced, the soap evidently contains
an excess of water, which may be remedied by the addition of a small
quantity of soda ash; too much soda ash (carbonates) must be avoided,
lest it should produce efflorescence.
Having examined the soap and found it to be correct, or having remedied
its defects, the soap in the pan is allowed to cool to 145° F. (63° C.)
and perfume added. The soap is now quickly filled into narrow frames and
allowed to cool rapidly.
The blocks of soap should not be stripped until quite cold throughout,
and they should be allowed to stand open for a while before slabbing.
When freshly cut into tablets, the soap may appear somewhat turbid, but
the brightness comes with the exposure it will receive prior to stamping
and wrapping.
_Saponifying Mineral Oil._--This sounds somewhat incongruous, as mineral
oil is entirely unsaponifiable. Most of the suggestions for this purpose
consist of the incorporation of mineral oil, or mineral oil emulsified
by aid of Quillaia bark, with a cocoa-nut oil soap, and in all these
instances the hydrocarbon merely exists in suspension.
G. Reale (Fr. Pat. 321,510, 1902), however, proposes to heat mineral oil
together with spermaceti and strong alkali, and states that he
transforms the hydrocarbons into alcohols, and these, absorbing oxygen,
become fatty acids, which are converted into soap by means of the
alkali.
In this connection may be quoted the interesting work of Zelinsky
(_Russ. Phys. Chem. Ges. Zeits. Angew. Chem._, 1903, 37). He obtained
substances, by acting with carbon dioxide upon magnesia compounds of
chlorinated fractions of petroleum, which when decomposed by dilute
sulphuric acid, yielded various organic acids. One of these acids on
heating with glycerine formed tri-octin, which had the properties of a
fat.
Dr. Engler, in confirmation of the theory of the animal origin of some
petroleums, obtained what might be described as petroleum (for it
contained almost all the hydrocarbons present in the natural mineral
oil) by distilling animal fats and oils under pressure.
_Electrical Production of Soap._--Attempts have been made to produce
soap electrically by Messrs. Nodon, Brettonneau and Shee (Eng. Pat.
22,129, 1897), and also by Messrs. Merry and Noble (Eng. Pat. 2,372,
1900).
In the former patent, a mixture of soda-lye and fat is agitated by
electricity at a temperature of 194°-212° F. (90°-100° C.), while in the
latter caustic alkali is electrolytically produced from brine, and
deposited on wire-netting in the presence of fat, which is thereby
saponified.