DOI Seite / Zitierlink:
https://doi.org/10.11588/diglit.1311#0211
178 THE GREAT EXHIBITION
the whole of the fat is generally added at one time. The ebullition is then carried on for
some time, and when the lye has become exhausted of its alkali, it is pumped away,
and a fresh portion of lye is added. After repeated boilings and pumpings, the
saponification is completed, and the soap is brought to strength by boiling down.
Now the soap-boiler may wish to prepare either white or mottled soap. If a white or
curd soap is required, the soap is "fitted," that is, boiled with a certain quantity of
water or weak lye, and allowed to settle, when the black impurities ("nigre") fall to the
bottom, and the soap is then removed to the frames, and allowed to cool. These frames
are composed of a number of separate planks, to facilitate the removal of the soap.
The mottled soap is prepared in a similar manner, except that the operation of fitting is
dispensed with, the " nigrc" is left in the soap. This " nigre" consists chiefly of sulphide
of iron, produced by the action of the lye, which always contains a minute quantity of
sulphate of sodium, on the vessel. In Marseilles and other countries where olive-oil-soap
is made, a quantity of sulphate of iron (green copperas) is added; and in this case the
mottling is produced jointly by the sulphide of iron (the black portion) and a true iron
soap (the red portion). In order that the metallic compound may not fall to the bottom
{as in fitting), the soap has to be much more concentrated; when removed from the boiler
it is of one uniform slate tint, but as it cools, the metallic compounds separate into
nodules, and by trickling off the excess of lye through the mass, they take up certain
forms, which produce the appearance called mottling. Hence mottled soap is of more
value, from its containing a less proportion of water. It is evident, in comparing this
with the cold process, that it is much more scientific ; as an excess of alkali may be
employed to ensure complete saponification, with the perfect certainty that it can be got
rid of in the lyes : the glycerin is also removed, with the impurities contained in the fat,
at each pumping; and a very pure chemical compound is obtained, notwithstanding the
employment of comparatively impure materials. If the soda-ash employed does not
contain sufficient saline impurities to throw up the soap, it is necessary to add a solu-
tion of common salt to effect this object each time the exhausted lye is pumped off.
The detergent property of soap is usually considered to be dependent entirely on the
quantity of alkali which it contains, and hence the question arises, why pure alkali
should not be employed in preference. An objection to this is the caustic character
of the alkali, which is injurious, not only to the hands of the person using it, but also
destructive of the articles wrashed, aDd especially of some colours of dyed goods. By com-
bining with fatty acids, the alkalies are rendered essentially milder in their action, without
being deprived of their capability of entering into combination with various impurities, and
more particularly with certain fatty bodies. The most common explanation of the
washing power of soap is founded upon ChevreuPs observation, that soaps are decomposed
by large quantities of water, giving rise on the one hand to acid soaps, and, on the other,
liberating a quantity of free alkali which remains in solution. According to this view, soap
is a sort of magazine of alkali, which it gives up in the exact quantity required at any
moment when it is rubbed with water. The combination of the alkali with some part of
the dirt cannot he denied. Several constituents of this very indefinite admixture of many
substances are of an essentially acid character, especially those derived from perspiration:
others become acid when exposed on a large surface to the action of the air, in conse-
quence of a sort of spontaneous saponification. This action cannot, however, be the sole
modus operandi of soap, the valuable properties of which, without doubt, arise, in a great
measure, from its power of dissolving substances which are insoluble in water. We
know that certain mineral salts exert a solvent power upon substances which are
entirely insoluble in water: thus it is well known that borax causes shell-lac to dissolve
with great facility, and the chemist will at once call to mind the remarkable solvent pro-
the whole of the fat is generally added at one time. The ebullition is then carried on for
some time, and when the lye has become exhausted of its alkali, it is pumped away,
and a fresh portion of lye is added. After repeated boilings and pumpings, the
saponification is completed, and the soap is brought to strength by boiling down.
Now the soap-boiler may wish to prepare either white or mottled soap. If a white or
curd soap is required, the soap is "fitted," that is, boiled with a certain quantity of
water or weak lye, and allowed to settle, when the black impurities ("nigre") fall to the
bottom, and the soap is then removed to the frames, and allowed to cool. These frames
are composed of a number of separate planks, to facilitate the removal of the soap.
The mottled soap is prepared in a similar manner, except that the operation of fitting is
dispensed with, the " nigrc" is left in the soap. This " nigre" consists chiefly of sulphide
of iron, produced by the action of the lye, which always contains a minute quantity of
sulphate of sodium, on the vessel. In Marseilles and other countries where olive-oil-soap
is made, a quantity of sulphate of iron (green copperas) is added; and in this case the
mottling is produced jointly by the sulphide of iron (the black portion) and a true iron
soap (the red portion). In order that the metallic compound may not fall to the bottom
{as in fitting), the soap has to be much more concentrated; when removed from the boiler
it is of one uniform slate tint, but as it cools, the metallic compounds separate into
nodules, and by trickling off the excess of lye through the mass, they take up certain
forms, which produce the appearance called mottling. Hence mottled soap is of more
value, from its containing a less proportion of water. It is evident, in comparing this
with the cold process, that it is much more scientific ; as an excess of alkali may be
employed to ensure complete saponification, with the perfect certainty that it can be got
rid of in the lyes : the glycerin is also removed, with the impurities contained in the fat,
at each pumping; and a very pure chemical compound is obtained, notwithstanding the
employment of comparatively impure materials. If the soda-ash employed does not
contain sufficient saline impurities to throw up the soap, it is necessary to add a solu-
tion of common salt to effect this object each time the exhausted lye is pumped off.
The detergent property of soap is usually considered to be dependent entirely on the
quantity of alkali which it contains, and hence the question arises, why pure alkali
should not be employed in preference. An objection to this is the caustic character
of the alkali, which is injurious, not only to the hands of the person using it, but also
destructive of the articles wrashed, aDd especially of some colours of dyed goods. By com-
bining with fatty acids, the alkalies are rendered essentially milder in their action, without
being deprived of their capability of entering into combination with various impurities, and
more particularly with certain fatty bodies. The most common explanation of the
washing power of soap is founded upon ChevreuPs observation, that soaps are decomposed
by large quantities of water, giving rise on the one hand to acid soaps, and, on the other,
liberating a quantity of free alkali which remains in solution. According to this view, soap
is a sort of magazine of alkali, which it gives up in the exact quantity required at any
moment when it is rubbed with water. The combination of the alkali with some part of
the dirt cannot he denied. Several constituents of this very indefinite admixture of many
substances are of an essentially acid character, especially those derived from perspiration:
others become acid when exposed on a large surface to the action of the air, in conse-
quence of a sort of spontaneous saponification. This action cannot, however, be the sole
modus operandi of soap, the valuable properties of which, without doubt, arise, in a great
measure, from its power of dissolving substances which are insoluble in water. We
know that certain mineral salts exert a solvent power upon substances which are
entirely insoluble in water: thus it is well known that borax causes shell-lac to dissolve
with great facility, and the chemist will at once call to mind the remarkable solvent pro-