Rosin, although very soluble in alkaline menstrua, is not however
susceptible, like fats, of being transformed into an acid, and will not
of course saponify, or form a proper soap by itself. The more caustic
the alkali, the less consistence has the resinous compound which is made
with it. Hence fat of some kind, in considerable proportion, must be
used along with the rosin, the _minimum_ being equal parts; and then the
soap is far from being good. As alkaline matter cannot be neutralized by
rosin, it preserves its peculiar acrimony in a soap poor in fat, and is
ready to act too powerfully upon woollen and all other animal fibres to
which it is applied. It is said that rancid tallow serves to mask the
strong odour of rosin in soap, more than any oil or other species of
fat. From what we have just said, it is obviously needless to make the
rosin used for yellow soaps pass through all the stages of the
saponifying process; nor would this indeed be proper, as a portion of
the rosin would be carried away, and wasted with the spent lyes. The
best mode of proceeding, therefore, is first of all to make the hard
soap in the usual manner, and at the last service or charge of lye,
namely, when this ceases to be absorbed, and preserves in the
boiling-pan its entire causticity, to add the proportion of rosin
intended for the soap. In order to facilitate the solution of the rosin
in the soap, it should be reduced to coarse powder, and well
incorporated by stirring with the rake. The proportion of rosin is
usually from one-third to one-fourth the weight of the tallow. The boil
must be kept up for some time with an excess of caustic lye; and when
the paste is found, on cooling a sample of it, to acquire a solid
consistence, and when diffused in a little water, not to leave a
resinous varnish on the skin, we may consider the soap to be finished.
We next proceed to draw off the superfluous lyes, and to purify the
paste. For this purpose, a quantity of lyes at 80° B. being poured in,
the mass is heated, worked well with a rake, then allowed to settle, and
drained of its lyes. A second service of lyes, at 4° B., is now
introduced, and finally one at 2°; after each of which, there is the
usual agitation and period of repose. The pan being now skimmed, and the
scum removed for another operation, the soap is laded off by hand-pails
into its frame-moulds. A little palm oil is usually employed in the
manufacture of yellow soap, in order to correct the flavour of the
rosin, and brighten the colour. This soap, when well made, ought to be
of a fine wax-yellow hue, be transparent upon the edges of the bars,
dissolve readily in water, and afford, even with hard pump-water, an
excellent lather.
The frame-moulds for hard soap are composed of strong wooden bars, made
into the form of a parallelogram, which are piled over each other, and
bound together by screwed iron rods, that pass down through them. A
square well is thus formed, which in large soap factories is sometimes
10 feet deep, and capable of containing a couple of tons of soap.
Mr. Sheridan some time since obtained a patent for combining silicate of
soda with hard soap, by triturating them together in the hot and pasty
state with a crutch in an iron pan. In this way from 10 to 30 per cent.
of the silicate may be introduced. Such soap possesses very powerful
detergent qualities, but it is apt to feel hard and be somewhat gritty
in use. The silicated soda is prepared by boiling ground flints in a
strong caustic lye, till the specific gravity of the compound rises to
nearly double the density of water. It then contains about 35 grains of
silica, and 46 of soda-hydrate, in 100 grains[57].
[57] By my own experiments upon the liquid silicate made at Mr. Gibbs’
excellent soap factory.
Hard soap, after remaining two days in the frames, is at first divided
horizontally into parallel tablets, 3 or 4 inches thick, by a brass
wire; and these tablets are again cut vertically into oblong nearly
square bars, called wedges in Scotland.
The soap-pans used in the United Kingdom are made of cast iron, and in
three separate pieces joined together by iron-rust cement. The following
is their general form:--The two upper frusta of cones are called curbs;
the third, or undermost, is the pan, to which alone the heat is applied,
and which, if it gets cracked in the course of boiling, may easily be
lifted up within the conical pieces, by attaching chains or cords for
raising it, without disturbing the masonry, in which the curbs are
firmly set. The surface of the hemispherical pan at the bottom, is in
general about one-tenth part of the surface of the conical sides.
The white ordinary tallow soap of the London manufacturers, called curd
soap, consists, by my experiments, of--fat, 52; soda, 6; water, 42; =
100. Nine-tenths of the fat, at least, is tallow.
I have examined several other soaps, and have found their composition
somewhat different.
The foreign Castile soap of the apothecary has a specific gravity of
1·0705, and consists of--
Soda 9
Oily fat 76·5
Water and colouring-matter 14·5
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100·0