stry originated in the discovery made
by M.E. Chevreul about 1815, that fats are glycerides or compounds of
glycerin with fatty acids, mostly palmitic, stearic and oleic. The
object of the candlemaker is to remove this glycerin, not only because
it is a valuable product in itself, but also because it is an
objectionable constituent of a candle; the vapours of acrolein formed by
its decomposition in the flame are the cause of the unpleasant odours
produced by tallow "dips." He also removes the oleic acid, which is
liquid at ordinary temperatures, from the palmitic and stearic acids,
mixtures of which solidify at temperatures varying from about 130 deg.
to 155 deg. F., according to the percentage of each present. Several
methods are in use for the decomposition of the fats. In the autoclave
process the fat, whether tallow, palm-oil or a mixture of the two, mixed
with 25 or 30% of water and about 3% of lime, is subjected in an
autoclave to steam at a pressure of about 120 lb per square inch for
eight or ten hours, when nearly all of it is saponified. On standing the
product separates into two layers--"sweet water" containing glycerin
below, and the fatty acids with a certain amount of lime soap above. The
upper layer is then boiled and treated with enough sulphuric acid to
decompose the lime soap, the calcium sulphate formed is allowed to
subside, and the fatty acids are run off into shallow boxes to be
crystallized or "seeded" prior to the separation of the oleic acid,
which is effected by pressing the solid blocks from the boxes, first
cold and then hot, by hydraulic machinery. In another process
saponification is effected by means of concentrated sulphuric acid. The
fat is mixed with 4-6% of the acid and treated with steam in boiling
water till the hydrolysis is complete, when on standing the glycerin and
sulphuric acid sink to the bottom and the fatty acids rise to the top.
Owing to the darkness of their colour, when this process is employed,
the latter usually have to be distilled before being crystallized. The
autoclave process yields about 45% of stearine, one-third of which is
recovered from the expressed oleic acid, but with sulphuric acid
saponification the amount of stearine is higher--over 60%--and that of
oleic acid less, part of it being converted into solid material by the
action of the acid. The yield of glycerin is also less. In a combination
of the two processes the fat may first be treated by the autoclave