The handbook of soap manufacture

by W. H. Simmons; H. A. Appleton

First. On top, a thin crust of soap, with perhaps a little light coloured fob, which is returned to the pan after the removal of the good soap.

Second. The good settled soap, testing 62-63 per cent. fatty acids. The subject of removing and treatment of this layer is fully dealt with in the next chapter.

Third. A layer of darker weak soap, termed "nigre," which on an average tests 33 per cent. fatty acids, and, according to the particular fit employed, will amount to from 15-20 per cent. of the total quantity of soap in the pan.

The quantity of nigre may vary not only with the amount of water added during finishing, but is also influenced by the amount of caustic alkali remaining in the soap paste prior to fitting. If the free caustic alkali-content is high, the soap will require a large amount of water to attain the desired fit. This water renders the caustic into a lye sufficiently weak to dissolve a quantity of soap, consequently, as the "nigre" is a weak solution of soap together with any excess of alkali (caustic or carbonate) and salt which gravitates during the settling, the quantity is increased.

Fourth. A solution containing alkaline salts, mostly carbonates and chlorides, with a little caustic.

The amount of the layer is very variable, and doubtless, under certain physical conditions, this liquor has separated from the nigre.

Utilisation of Nigres.—The nigres are boiled and the liquor separated by graining with salt. Nigre may be utilised in various ways.

(1) It may be used several times with new materials. This particularly refers to soaps of the "Washer" type. The colour of the nigre will determine the number of times it can be employed.

(2) It may be incorporated with a soap of a lower grade than the one from which it was obtained. In this case a system is generally adopted; for example, soap of the best quality is made in a clean pan, the nigre remaining is worked up with fresh material for soap of the next quality, the nigre from that boil, in its turn, is admixed with a charge to produce a batch of third quality, and the deposited nigre from this is again used for a fourth quality soap—the nigre obtained from this latter boil would probably be transferred into the cheapened "washer" or perhaps if it was dark in colour into the brown soap-pan.

(3) The nigre may be fitted and produce a soap similar to the original soap from which it was deposited. It is advisable to saponify a little fat with it.