Many plans of decolorizing oils are in vogue: - (a) Exposure to sunlight in large white glass bottles; the oil soon becomes colourless, but acquires an almost rancid flavour. (6) Agitation with 2 per cent, of a solution of permanganate of potash; bleaches effectually, but also leaves a bad flavour, (c) The oil is first agitated with water containing gum, and to the emulsion thus formed, is added coarsely crushed wood-charcoal; the whole is then slowly warmed to a degree not reaching 212° F. (100° C), and when cold, the oil is dissolved out by ether or petroleum-spirit, and the latter is recovered by distillation; the result is good, (d) A process much recommended is to pass nitrous acid gas through the oil. (e) The oil (500 parts) is clarified by addition of 50 parts of China-clay and 50 of water. (/) In some cases, it is found advisable to use the coagulation of albumen in clarifying oils. The oil to be treated is mixed by agitation at the ordinary air-temperature with a weak solution of albumen in water.
The whole is then gradually heated, most conveniently by steam, and when hot enough to coagulate the albu-• men, this latter collects in clots, enclosing particles of impurity; after the lapse of sufficient time, these clots subside, and the clarified oil is removed by decantation. The process is analogous to that of the refining of syrups by serum of blood.
Many oils are partially or completely decolorized by filtration through, or agitation with, freshly-burnt animal-charcoal (bone-black). The apparatus for filtering is similar to that employed in sugar-refineries, and consists essentially of tall wrought-iron cylinders filled with bone-black, and provided with a steam-jacket to control their temperature. When the charcoal ceases to decolorize, it should be treated with some solvent (bisulphide of carbon, or petroleum-spirit) to remove the oil, before it is revivified by calcination.
Most processes for the bleaching of oils depend upon the oxidization of the colouring matter by some suitable reagent, chiefly evolving nascent oxygen in some form. There are, however, instances known in which the colour is destroyed by a reducing agent, such as sulphurous acid, in an aqueous solution, as gas, or arising from the decomposition - of an alkaline hyposulphite (e. g. that of soda) by a strong mineral acid. It may be laid down as a general rule that oils which have been burnt or charred by any previous process cannot be satisfactorily bleached. Experiment alone can determine the particular process best suited to any given oil, having regard to the purpose for which it is to be used. The utmost care is required in using any oxidation process for fats intended to be converted into soap, since if the fat be oxidized in any perceptible degree, as well as the colouring matter (i. e. if too much of the bleaching reagent be used), the resulting soap will often be worse in colour than if the fat had not been bleached at all.
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 180° to 200° F. (82° to 93° C). 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 260° F. (127° C), or it may be put into a closed, horizontal, iron cylinder, and heated by a fire beneath up to about 464° F. (240° C), 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.
Palm-oil may also be very suitably bleached by bichromate of potash and hydrochloric acid (Watts' process). The oil is made as free as possible from impurities, and, at about 120°-130° F. (49° to 54° C), is agitated with a strong solution of bichromate of potash, containing about 1 lb. of the salt to every 100 lb. of oil. To this is added enough hydrochloric acid 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. (Spoils' 'Encyclopaedia.')