Volume 4

A dictionary of applied chemistry / by Sir Edward Thorpe ; assisted by eminent contributors.

  • Thorpe T. E. (Thomas Edward), Sir, 1845-1925.
Date:
1912-1913
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: A dictionary of applied chemistry / by Sir Edward Thorpe ; assisted by eminent contributors. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    contain any desired proportion of water. Some 1 kind of purification is efieoted by allowing these soaps to drop a certain amount of ' nigre,' containing a good deal of the impurities in the raw material; but, as a rule, these soaps are filled into frames immediately after being made. The manufacture of these soaps may be con- sidered as entirely obsolete, all the more so as the glycerin contained in the fatty raw material is lost, much as in the making of best soft soap from glycerides ; indeed, these soaps may be likened to soft soaps. At present small quan- tities of this class of soap are still made in the south of Europe from low-class fatty material, rich in free fatty acids. This kind of soap must, of course, be considered an ' adulterated ' soap. In this country ' adulterated' soaps are mostly prepared by the aid of the mixing (crutching) machines into which ' neat' soap settled on a nigre is run, and is there in corpora ted with solutions of salt, sodium carbonate, and (or) sodium silicate to any desired extent, so that aU grades of soap ranging from 60 p.c. of fatty acids downwards to even 10 p.c. can be, and are, made. In order to give those soaps sufficient firmness for handling they are either ' stoved ' or ' pickled,' as the case may be. In mitigation of the practice of ' filling' soaps with solutions of sodium borate, or of sodium carbonate or (and) silicate, is adduced the apologetic explanation that, besides harden- ing the soap, the added solutions possess deter- gent properties. The best that can be said for soaps of this class, which range in their pro- portion of fatty acids between 52 and 60 p.c, is that they might be considered as standing on the borderland between genuine soaps and ' adulterated ' soaps. They should be openly described as ' filled ' soaps. There can, however, be no doubt that adulteration has taken place in the case of those ' filled ' soaps which have been ' loaded ' with weighting substances, such as clay, talcum (sold as ' fillers '), chalk, barytes, asbestos, seed husks, &c., or with solutions of magnesium salts, &c. In the same category fall soaps filled with starch. In the opinion of the aixthor soaps containing less than 50 p.c. of fatty acids should be looked upon as ' adulterated,' whether the ' filling ' have detergent properties or not, unless the admixture be declared openly (' sand soaps '). Special kind of soaps for toilet purposes are remelted .soaps, milled soaps, and transparent soaps. For the manufacture of these the reader must be referred to Lewkowitsch, Chem. Techn. iii. 275. 2. Soft Soap—Potash Soap. In the manufacture of soft soap the base used preponderantly is potassium hydroxide, small quantities only, if any, of caustic soda being employed under certain conditions. Thus in summer it is the custom to saponify with a mixture of caustic potash and caustic soda, according to the desired consistence of the finished product. The manufacture of potash soap is much simpler than that of hard soaps, as it involves practically only one operation, viz. ' boiling ' the glycerides with caustic alkali solutions in a pan, no ' salting out' being required. Hence the resulting product, after being properly ' fitted,' can be run straightway from the boiling-pan into the vessels in which the soap is sold. The method of manufacturing ' genuine ' commercial soft soap for household purposes differs, however, essentially from that employed in the manufacture of hard soap, in that the presence of potassium carbonate is required so as to produce a soap of clear translucent appear- ance which possesses the desired consistence for handling. Soft soap used to be made in fire-heated pans ; but during the last two decades these have been replaced in modern works by pans similar to those used in the manufacture of hard soap, and fitted with a ' close ' steam coil in addition to an open steam coil or coils. The fatty raw material which for the best soap consists chiefly or preponderantly of linseed oil, is introduced into the soap-pan and churned up with steam, when a caustic potash solution of atjout 20° Tw. is run in. Whilst the saponification proceeds, lyes of higher strength can be used. When all the glycerides have been converted into soap, and the latter is just slighly alkaline, potassium carbonate solution is introduced. The soap will then contain an excess of water ; this is removed by turning off the open steam and heating the mass with the aid of the close steam coil. At the same time the mass is agitated by a suitable mechanical arrangement so as to promote the rapid evaporation of the excess of water. By taking samples and examining their appearance and consistence on a glass plate, the operator is able to judge exactly the point at which the soap is ' finished.' The mass then represents practically a mixture of potassium salts of fatty acids with a solution containing all the glycerin from the fats, a slight excess of caustic potash, and a certain amount of potas- sium carbonate. The two component parts of this mixture are so carefully balanced that, on cooling, a homogeneous mass is obtained which retains its transparency. If the soap is made from linseed oil only, the transparency will be retained even in winter, and the soap will not ' congeal'; but if the soap stock contains notable amounts of cotton seed oil, or even maize oil, and frost sets in, then the soap is liable to become duO (' bliiid'). Hence the choice of raw material is conditioned by the seasons. If some tallow is used in the stock, the harder potassium stearate or palmitate. or a mixture of both, crystallises out in star-like clusters, producing what is known in commerce as ' figging.' The same effect can also be obtained by replacing a portion of caustic potash by its equivalent of caustic soda. In the soft soaps prepared in the manner described above all the glycerol is retained, and, as has been pointed out already, a certain excess of caustic potash and notably of potassium carbonate, necessary in order to give the soap the inviting appearance the public demands. By operating carefully, potash soaps can be made which are devoid of free alkali and potas- sium carbonate. Such soaps are, however, only manufactured for the purposes of the textile industry, or ' liquid ' soaps for cosmetic purposes, when absence of free alkali or of carbonate is essential, and the appearance of the soap is of 1 no importance. In this case it is indispensable