358

TANNED LEATHER

if rigorously exact results are required, the drying must be carried out in a
vacuum or in a current of carbon dioxide.
2. Ash.—5-10 grams of the substance are carefully charred in a dish
and then calcined at dull redness with addition of a little ammonium nitrate
to facilitate the combustion. With this method complete incineration is
often difficult; in such a case it is preferable to weigh the leather in small
pieces and to throw it, piece by piece, into a crucible heated to dull redness,
each piece being added only after the previous one has burnt away com-
pletely.
With a large amount of ash, it is well to make a qualitative analysis.
The ash of true leather tanned
with tannin consists essentially of
calcium carbonate with traces of
iron and of phosphates. Coloured
leathers may contain metals from
the mordants used (tin, copper,
iron, chromium, aluminium) ; tin
may also be introduced as stannous
chloride used for bleaching. Small
quantities of silicates (talc, kaolin)
may be employed in the treat-
ment of the leather. Finally, other
mineral matters (barium, magne-
sium and lead salts and sodium
chloride) may have been added as
filling to increase the weight. Com-
plete quantitative analysis of the
ash is rarely necessary, but deter-
mination of its calcium content is
sometimes required, this being
made by the ordinary methods.
fig. 66
3. Fatty Substances.—20
grams of the finely divided or
powdered leather are extracted in
an extraction apparatus with car-
bon disulphide or petroleum ether ;
at the end of the extraction the
bulk of the solvent is distilled off and the residue evaporated in a taxed
dish on a water-bath and dried at 100-105° to constant weight.
Some substances, such as d^gras, are not, however, dissolved completely
by solvents.
If the fatty substances are considerable in amount, their constants are
determined to ascertain their nature (see Fatty Substances, Vol. I).
4. Soluble Substances.-—The fat-free leather from the preceding
determination is dried in the air to expel the solvent, then left for 12 hours
in contact with about 200 c.c. of water at the ordinary temperature, and
afterwards extracted for 1-5-2 hours with further quantities of w^ter up
to a volume of a litre. This may be effected by a displacement apparatus,