Chemically epeaking, rosin ie an acid, or mixture of acids (picric, sylvie, colophonic, and pimaric) whoee general formula ia C2.113002, and their combining proportion (with 31 parts eoda) is 302. Rosin decompoece carbonate of soda, and combine° instantly with caustic alkali, funning in each case a so-called resin aoap, which ia a thick, elimy, brown mass, containing 15.8 per cent. of dry soda; ite attraction for water ie ao great as to become liquid ou exposure to air, even though previoualy dried artificially.
Rosin is never employed alone for soap, but always in conjunction with fate ; it him been described as an ameliorator, and is aleo a cheapener ; it contributes to the popular qualities of aoap, rendering it more readily eoluble, and forming a copious lather in laundry and household soaps. In them the, propertiou varies frein 15-20 per cent. of the fatty matter ernployed, to an equal weight, or even more. Hard amps for manufacturing purposes rarely contain it. It inny be saponified alone and tho result mixed with a fat-eonp in due proportiou, or the whole may be saponified together. The reeult ie tbe eame, and the choice of methods dependa upon convenience in working ; but the former is preferable with impure rosin, in order to give it as many ohangee of lays aa possible, and thus to wash out auepended impurities, ouch as leaves, bits of stick, ike., all of which discolour the product.
Alkali (Bee Caudle Potash, pp. 251-3, Carbonate of Potash, pp. 253-60, Caustic Soda and Carbonate of Soda, pp. 279-309).—In connection with their application to eoap manufacture, it may be mentioned here that all large Englieh soap-rnakers make their own eoda, either from common ealt, or from " salt-cake" (eulphate of eoda), and e,austicize at once the liquor formed by the lixiviation of the black-ash. Nearly all the soda used in American and Colonial soaperies is bought in the Engliah market, and iraported either as soda-ash or caustic eoda. The process of preparing solution of caustic aoda or " leys " from soda-ash (or black-ash) ia deacribed on p. 307. It is only necessary to add a word of caution to the aoap-maker as to the way of dealing with the solution when rnade. The reason for conducting the opemtion in so weak a solution aa is directed (about 22° Tw., or 1.110 sp. gr.) is, that only under those conditione will carbonate of soda pest with its carbonic acid to caustic lime. At higher apecifie gravities the
reaction may even be reversed, and caustic soda will remove carbonic acid from carbunate of lime. It is of the utmost importance to the soap-maker to causticize his leys as completely ae possible, and to keep them so, eince (unlesa his soap be made entirely from fatty acids, a very rare thing) any carbonate of eoda in hie leys is simply se much soda wasted. The cauatic soda ley should, there fore, only be prepared ea it is wanted for use, aod when atored should be kept in covered vessela to which air has no accese; shallow open tanks should be avoided at all hazards. If, as is frequently the case, etrenger ley is required than that produced iu the original operation, it ehould be concentrated by evaporatiou in open vesseht, or by dissolving in it some solid caustic (loch. It (Mould Ito specially noted that for concentrating or storing solution beyond 30° Tw. (sp. gr. 1.150), east-iron veseele alone ebould be employed, owing to the solvent action of eoda upon wrought-iron.
It may be laid down as an axiom that no ley ie sufficiently causticized which liberates bubbles of gae, when a email portion of the clear cold ley hae an excess (i. e. more than eufficient to neutralize it and to turn blue litmue red) of mineral acid added to it. This is a aufficiently good test for a foreman to work by.
Iu the ease of nearly pure solutions of caustic soda, a very close approxituatiou to their strength, i. 0. their percentage of caustic soda, may he obtained by the observation of their density or by an hydrometer (Fig. 1, p. 2). These instruments are usually made very cheaply of glass, thelower end being weighted with shot or mercury. When floated, they displace their own weight of liquid, and hence the bulk displaced varies with the sp. gr. of the liquid examined, which is indicated on the stem of the instrument. Two scales are in use among soap-makers, those of Twaddell and of Brume. The zero point of both is that to which the instrument sinks in distilled water at 15-5° or F.). It must not be forgotten that increase of temperature in a fluid, by increasiag its bulk, diminishes its sp. gr. All sp. grs. quoted in this article are at 151° (60° F.).