For the removal of fraudulent admixtures from commercial oils, no general rule can be given ; but subsidence, filtration, and boiling with weak sulphuric acid, will generally effect the desired result. Special methods ace best sought under the head of Detection and Analysis, pp. 1462-77.
Methods for correcting rancidity In oil are as follows :—(a) Agitation with 5 parts of good vinegar, repeating the operation several times. (b) Agitation (5-6 time) of 50 parts of oil with 80 parts of water at 30° (86° F.) holding 12 parts of common salt in solution. (c) To 100 litres of oil, are added 2 kilo. of calcined magnesia ; the mixture is agitated 4 times daily for hour each time for 6 days ; the oil is then filtered ; it must be quickly used, or it will become rancid again. (d) Agitation with a weak solution of caustic alkali, or a moderately strong one of an alkaline carbonate. (e) Prolonged agitation with water.
Most of the processes for refining and bleaching oils also deodorize them to a certain extent. As many of the odorant principles are more volatile than the oils, they may occasionally be removed by merely beating the oil in a closed vessel provided with an exit-pipe. For destroying the disagreeable smell of coco-nut-oil for soap-making, it is recommended to boil it in a wooden vessel by free steam on water containing 6 lb. sulphuric and 12 lb. hydrochloric acid to each ton of oil. Prolonged steaming will sometimes remove the unpleasant odour characteristic of oily distilled products.
Many plans of decolorizing oils are in vogue :—(a) Expoaure to sunlight in large white glass bottles ; the oil soon becomes colourless, but acquires an almost rancid flavour. (b) Agitation with 2 per cent. of a solution of permanganate of potash ; bleaches effectually, but also leavea 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 100° (212° F.), 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. (f) 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-tempera turo with a weak solution of albumen in water. The whole is then gradually heated, most conveniently by steam, and when hot enough to albumen, 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 de'cantation. 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 or bone-black. The apparatus for filtering is similar to that employed in sugar-refineries (see Sugar), and conaista 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, instanoes 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 beat suited to auy 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 ai 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.