Palm-oil and tallow are the two chief ,fats bleached by the soap-maker. Both may be bleached by pumping air into them in finely divided streams, while they are kept at about 82°-93° F.). The colour of tallow may also be removed by boiling upon a solution of chloride of lime, or of chlorate of potash, to which a strong mineral acid has been added. No more potassic chlorate than 0.1 per cent. on the tallow should be employed.
Experiment has shown that the colour of palm-oil may be quite destroyed by heat. To effect this, the oil may be kept for some hours at about 127° (260° F.), or it may be put into a closed, horizontal, iron cylinder, and heated by a fire beneath up to about 240° (464° F.), at which temperature the colour is destroyed. This process gives rise to most offensive vapours, especially acrolein, and necessitates the conduct of operations in a closed vessel, with suitable means of condensing the vapours and rendering them innocuous, such as have been already alluded to under Floor-cloth (p. 1004), and elsewhere (pp. 1272-6, 1449).
Palm-oil may also be very suitably bleached by bichromate of potash and hydrochloric acid. The oil is made as free as possible from impurities, and, at about 49°-54° (120°-130° F.), is agitated with a strong solution of bichromate of potash, containing about 1 lb. of the salt to every 100 lb. of oil. To thin, is added enough hydrochloric said to form sesquichloride of chromium with all the chromium in the bichromate of potash, the quantity of liquid acid necessary of course varying with the amount of real acid contained in it. A slight excess of acid is rather an advantage than
otherwise. The process occupies about an hour, after which, subsidence removes most of the chemicals, while subsequent agitation with hot water renders the oil quite pure enough for the soap-copper.
Detection and Analysis.—The ordinary solid fats and fixed oils (with the exception of butter and a few others) may be looked upon as mixed glycerides of oleic, stearic, and palmitic acids, in various proportions, the first preponderating in the oils, and the two last (especially stearine) in the fats. For ordinary purposes, there are therefore the following constituents to deal with : (1) Moisture, especially in butter and palm-oil ; (2) organic suspended matter, such as curd in butter ; (3) mineral matters, such as salt in butter ; (4) total fatty acids, in any ordinary oil or fat ; (5) oleic, stearic, and palmitic acids, in any ordinary oil or fat ; (6) soluble and insoluble fatty acids, only necessary in butter, and the few exceptional fats similarly constituted ; (7) glycerine, from which to calculate the glyceryl in the fat ; (8) possible presence of paraffin-wax and mineral oils.