767
hydrochloric acid yields chromic chloride, Cr 2 Cl 6 ;water is formed, and oxygen is set free as follows:—
Chromic Hydrochloric Sesquichloride of
Acid. Acid. Chromium.
Oxygen.
2CrO a + 6HC1 = Cr 2 Cl 6 + 3H 2 0 + 30.
The liberated oxygen is seized by the colouringmatter, and the green sesquichloride of chromium,chloride of potassium, &c., are found in solution inwater at the bottom of the vat. The sulphuric aciddirected to be added by the inventor does not appearto be essential to the operation, and is indeed omittedby many bleachers. Oil once bleached by this pro-cess may be preserved from acquiring colour, andthe soap made from it does not become tinged likethat made from oil bleached by other methods.
When palm oil is to bo used in the production ofyellow soap, the most economical mode of bleachingis by the process invented by Arthur Dunn. Thismethod, which is also applicable to other oils andfatty matters of a like nature, consists in the ex-posure of the oil to the combined influence of airand heat. The air is forced beneath the surface ofthe oil through pipes, by means of a blowing appara-tus, and then allowed to rise through the liquid innumerous small streams—the oil being maintainedat a temperature between 170° and 230° Fahr. bysteam pipes or other means. A hood communica-ting with a chimney is placed over the vessel con-taining the oil, for the purpose of conducting awaythe unpleasant vapours which are disengaged.
According to Parnell, the oil should not bemaintained at a temperature exceeding 180° Fahr.during the operation of blowing in the air. Herecommends, however, that at the commencementof the process it be heated to 212° Fahr., and thensuffered to cool a little. This is necessary, inasmuchas the air has but little effect as long as water ispresent; but when this is completely expelled, thebleaching commences immediately.
The oxidising action of the air on the colouringmatter of palm oil seems to be accelerated by com-bining the influence of light with that of heat. Withthis end in view, the oil is sometimes placed in athin layer on the surface of a large shallow uncoveredvat. Several of these vats, each about a foot indepth, may be heated by means of steam passedthrough serpentine leaden pipes proceeding from acommon boiler; one extremity of each tube mayterminate in a receiver for the condensed water, andthe latter may be returned to the boiler. Each vatis first two-thirds filled with water ; and when thisis hot, a sufficient quantity of palm oil is introducedto form when fluid a layer of about 2 inches in depth,and this should be maintained during the wholeprocess at a temperature as near 212° Fahr. aspossible. To preserve an equal temperature through-out the fluid, the steam may be admitted at oppositeends of the vat, and separated in contrary directionsby two distinct serpentine tubes. Both air and lighthaving access, the time required for the decolorationof the oil is from ten to fifteen hours—the thinnerthe stratum of oil, the more rapid is the process.The rapidity of the bleaching action is not sensibly
lessened by loosely covering the containing vessel,provided the removal of the air over the surface ofthe oil is not much interfered with ; but if exposedto a temperature of 212° in a closed vessel, so thatalthough light and heat have access the air is ex-cluded, the oil does not then become bleached. Thebleached oil, which while fluid retains a fawn-colouredtint, is after solidification of a greyish-white colour.
Bosin.— Besides oil and alkali, common rosin orcolophony also forms an important ingredient in theproduction of soap. The kind used is that yieldedby the common turpentine, produced by and ex-tracted from the several species of pine.
Though employed in the fabrication of soap asa partial substitute for oleaginous or fatty material,rosin does not possess a similar constitution to thesematters, and therefore does not duffer an analogousalteration in the process of saponification. Theintroduction of too largo a proportion of rosin intosoaps gives them a very dark colour, and also rendersthem much too soft. Bosin is only used in theproduction of yellow soap. A process for bleachingrosin has been patented by Messrs. Hunt & Pociiinof Manchester. They melt the rosin and then passsteam through it.
Alkalies. —The alkaline ingredients employed inthe soap manufacture are potash and soda. Boththese articles are met with abundantly in commercein the state of carbonate, and in this condition pur-chased by the soap manufacturers.
The sodium carbonate now used by soap manu-facturers is bought as white ash or soda ash.
This ash contains carbonate of soda sometimes tothe amount of 95 or 96 per cent. It is essentiallynecessary in the production of soap that the alkalishould be rendered caustic—that is, set free fromcarbonic acid ; as, if combined with even the weakestmineral acids, these will not be displaced by the fattyacids, and saponification will not ensue. The greatestcare should therefore be exercised in this prelimin-ary process—the preparation of the lyes. The pro-cess for causticizing is the same in principle for thepreparation both of soda and potash lyes. It dependsupon the greater affinity of calcium than of sodiumor potassium for carbonic acid, and the decomposi-tion is one of the most simple, thus—
Sodium Carbonate. Calcium Hydroxide. Calcium Carbonate. ^(Cauaido^Soda )^ 0
Na a C0 3 + CaH 3 0 s = CaC0 3 + 2NaHO
The proportions of soda ash and lime employedvary according to the degree of strength of the soda;similarly with regard to the potash salt. It is, how-ever, always an advantage to have an excess of limeover the alkaline carbonate, as in the contrary caseany undecomposed carbonate is totally lost.
In England the lyes g,re made by boiling togetherthe soda ash and lime: on the Continent the in-gredients are mixed in the cold. The proportionsare 50 lbs. of lime to 100 lbs. of carbonate of soda,80 lbs. of lime to 100 lbs. of carbonate of potash.
The soda ash or potash and lime (slaked with hotwater) are thrown into a vat and boiled by a jet ofsteam for several hours, until the supernatant liquor