The principal difference between soaps with base of soda, and soaps with
base of potash, depends upon their mode of combination with water. The
former absorb a large quantity of it, and become solid; they are
chemical hydrates. The others experience a much feebler cohesive
attraction; but they retain much more water in a state of mere mixture.
Three parts of fat afford, in general, fully five parts of soda soap,
well dried in the open air; but three parts of fat or oil will afford
from six to seven parts of potash soap of moderate consistence. This
feebler cohesive force renders it apt to deliquesce, especially if there
be a small excess of the alkali. It is, therefore, impossible to
separate it from the lyes; and the washing or _relargage_, practised on
the hard-soap process, is inadmissible in the soft. Perhaps, however,
this concentration or abstraction of water might be effected by using
dense lyes of muriate of potash. Those of muriate or sulphate of soda
change the potash into a soda soap, by double decomposition. From its
superior solubility, more alkaline reaction, and lower price, potash
soap is preferred for many purposes, and especially for scouring woollen
yarns and stuffs.
Soft soaps are usually made in this country with whale, seal, olive, and
linseed oils, and a certain quantity of tallow; on the continent, with
the oils of hempseed, sesame, rapeseed, linseed, poppy-seed, and colza;
or with mixtures of several of these oils. When tallow is added, as in
Great Britain, the object is to produce white and somewhat solid grains
of stearic soap in the transparent mass, called figging, because the
soap then resembles the granular texture of a fig.
The potash lyes should be made perfectly caustic, and of at least two
different strengths; the weakest being of specific gravity 1·05; and the
strongest, 1·20, or even 1·25. Being made from the potashes of commerce,
which contain seldom more than 60 per cent., and often less, of real
alkali, the lyes correspond in specific gravity to double their alkaline
strength; that is to say, a solution of pure potash, of the same
density, would be fully twice as strong. The following is the process
followed by respectable manufacturers of soft soap (_savon vert_, being
naturally or artificially green,) upon the continent.
A portion of the oil being poured into the pan, and heated to nearly the
boiling point of water, a certain quantity of the weaker lye is
introduced; the fire being kept up so as to bring the mixture to a
boiling state. Then some more oil and lye are added alternately, till
the whole quantity of oil destined for the pan is introduced. The
ebullition is kept up in the gentlest manner possible, and some stronger
lye is occasionally added, till the workman judges the saponification to
be perfect. The boiling becomes progressively less tumultuous, the
frothy mass subsides, the paste grows transparent, and it gradually
thickens. The operation is considered to be finished when the paste
ceases to affect the tongue with an acrid pungency, when all milkiness
and opacity disappear, and when a little of the soap placed to cool upon
a glass-plate, assumes the proper consistency.
A peculiar phenomenon may be remarked in the cooling, which affords a
good criterion of the quality of the soap. When there is formed around
the little patch, an opaque zone, a fraction of an inch broad, this is
supposed to indicate complete saponification, and is called the
_strength_; when it is absent, the soap is said to want its _strength_.
When this zone soon vanishes after being distinctly seen, the soap is
said to have _false_ strength. When it occurs in the best form, the soap
is perfect, and may be secured in that state by removing the fire, and
then adding some good soap of a previous round, to cool it down, and
prevent further change by evaporation.
200 pounds of oil require for their saponification--72 pounds of
American potash of moderate quality, in lyes at 15° B.; and the product
is 460 pounds of well-boiled soap.
If hempseed oil have not been employed, the soap will have a yellow
colour, instead of the green, so much in request on the continent. This
tint is then given by the addition of a little indigo. This dye-stuff is
reduced to fine powder, and boiled for some hours in a considerable
quantity of water, till the stick with which the water is stirred,
presents, on withdrawing it, a gilded pellicle over its whole surface.
The indigo paste diffused through the liquid, is now ready to be
incorporated with the soap in the pan, before it stiffens by cooling.
M. Thenard states the composition of soft soap at--potash 9·5, + oil
44·0, + water 46·5, = 100.
Good soft soap of London manufacture, yielded to me--potash 8·5, + oil
and tallow 45, + water 46·5.
Belgian soft or green soap afforded me--potash 7, + oil 36, + water 57,
= 100.
Scotch soft soap, being analyzed, gave me--potash 8, + oil and tallow
47, + water 45.
Another well-made soap--potash 9, + oil and fat 34, + water 57.
A rapeseed-oil soft soap, from Scotland, consisted of--potash 10, + oil
51·66, + water 38·33.
An olive-oil (gallipoli) soft soap, from ditto, contained--potash with a
good deal of carbonic acid 10, oil 48, water 42, = 100.
A semi-hard soap, from Verviers, for fulling woollen cloth, called
_savon économique_, consisted of, potash 11·5, + fat (solid) 62, + water
26·5, = 100.
The following is a common process, in Scotland, by which good soft soap
is made:--
273 gallons of whale or cod oil, and 4 cwt. of tallow, are put into the
soap-pan, with 250 gallons of lye from American potash, of such alkaline
strength that 1 gallon contains 6600 grains of real potash. Heat being
applied to the bottom pan, the mixture froths up very much as it
approaches the boiling temperature, but is prevented from boiling over
by being beat down on the surface, within the iron curb or crib which
surmounts the cauldron. Should it soon subside into a doughy-looking
paste, we may infer that the lye has been too strong. Its proper
appearance is that of a thin glue. We should now introduce about 42
gallons of a stronger lye, equivalent to 8700 gr. of potash per gallon;
and after a short interval, an additional 42 gallons; and thus
successively till nearly 600 such gallons have been added in the whole.
After suitable boiling to saponify the fats, the proper quality of soap
will be obtained, amounting in quantity to 100 firkins of 64 pounds
each, from the above quantity of materials.
It is generally supposed, and I believe it to be true, from my own
numerous experiments upon the subject, that it is a more difficult and
delicate operation to make a fine soft soap of glassy transparency,
interspersed with the figged granulations of stearate of potash, than to
make hard soap of any kind.
Soft soap is made in Belgium as follows:--For a boil of 18 or 20 tons,
of 100 kilogrammes each, there is employed for the lyes--1500 pounds of
American potashes, and 500 to 600 pounds of quicklime.
The lye is prepared cold in cisterns of hewn stone, of which there are
usually five in a range. The first contains the materials nearly
exhausted of their alkali; and the last the potash in its entire state.
The lye run off from the first, is transferred into the second; that of
the second into the third; and so on to the fifth.
In conducting the _empatage_ of the soap, they put into the pan, on the
eve of the boiling-day, six _aimes_ (one ohm, = 30 gallons imperial,) of
oil of colza, in summer, but a mixture of that oil with linseed oil in
winter, along with two aimes of potash lye at 13° B., and leave the
mixture without heat during eight hours. After applying the fire, they
continue to boil gently till the materials cease to swell up with the
heat; after which, lye of 16° or 17° must be introduced successively, in
quantities of one quarter of an aime after another, till from 2 to 4
aimes be used. The boil is finished by pouring some lye of 20° B., so
that the whole quantity may amount to 9-1/2 aimes.
It is considered that the operation will be successful, if from the time
of kindling the fire till the finish of the boil, only five hours
elapse. In order to prevent the soap from boiling over, a wheel is kept
revolving in the pan. The operative considers the soap to be finished,
when it can no longer be drawn out into threads between the finger and
thumb. He determines if it contains an excess of alkali, by taking a
sample out during the boil, which he puts into a tin dish; where if it
gets covered with a skin, he pours fresh oil into the pan, and continues
the boil till the soap be perfect. No wonder the Belgian soap is bad,
amid such groping in the dark, without one ray of science!