paddled the whole day through, and indeed in the later stages this is
undesirable. The packs remain several days in each liquor and take up to
14 days to tan. Two to three bags of sumach are needed for about 20
dozen goatskins. This method of tanning is efficient and convenient for
bold-grain finishes, on account of the constant tumbling and bending of
the skins which tends to work up a grain. For very soft leathers and
fine-grain finishes, however, the "bag-tannage" or "bottle tannage" is
favoured. In this method the pelt is stitched up by machine to form a
bag, grain outwards, leaving a "neck" in the hind shank. The bag is
nearly filled with a fairly strong infusion of sumach, inflated with air
and tied up at the neck. The bags are then placed into a vat of warm
sumach liquor, in which they just float. The bags are pushed down and
the liquor stirred up, so that the goods are in constant motion. After a
few hours they are piled on a rack, and the tan liquor of the interior
is caused to diffuse through the skins by the pressure due to the weight
of the pile. The bags are refilled with fresh and stronger sumach liquor
and the process is repeated. The skins are thus lightly but effectively
tanned in about 24 hours, and the leather has very fine grain and soft
feel. However tanned the skins are dried out after tanning, and sorted
in the "crust" according to size and colour. The larger skins are
preferred for upholstery and the smaller for fancy goods and
bookbinding.
To illustrate the course of finishing operations, the case of hard-grain
morocco for bookbinding may be given as typical. The goods are wet back
with warm water and drummed for 1-2 hours in warm sumac to prepare for
dyeing. They are then struck out by machine, sammed and shaved. Dyeing
follows, with acid colours, in a drum. The goods are run first in a
little water and the dyestuff added very gradually through a hollow
axle. The acid required (preferably formic) is added later to develop
the full shade. Warm solutions are used, and the dye bath is practically
exhausted. The goods are next placed in cold water to wash off
superfluous liquor and free the skins from acid. They are then horsed to
drain, struck out and hung up to samm. They are seasoned with milk and
water and piled to temper. They are "tooth rolled" in the glazing
machine two ways: right-hand shank to left fore shank and _vice versâ_,
and piled again. After wetting back again they are "wet grained" by hand
with a cork board in four directions: belly to belly, shank to shank,
and across as before, and finally from neck to butt. They are
immediately hung up in a warm shed to dry, and to fix the grain. They
are then softened by "breaking down" with a rubber board, top seasoned,
piled to temper and dry, brushed lightly, piled again, brushed more
heavily, and dried out. They are finally softened by graining in three
directions: shank to shank and across, and neck to butt. They are then
brushed again.
Public-domain text, read in full here on John Shaqi.
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