Transparent soap is really not a most desirable soap for toilet purposes, as it contains an excess of free alkali. It has, nevertheless, met with public approval because of the fact it is novel in being transparent. Except for this fact very little merit can be claimed for a soap of this kind.
The transparency of soap is generally due to the presence of alcohol, sugar or glycerine in the soap when it is made. It is very essential in a soap of this character, where lightness and clearness of color are desired, that the material for making the soap be carefully selected as to color and purity. The perfumes also play an important part in the color of the soap and many of the tinctures, balsams and infusions used in perfuming soap may eventually cause trouble by spotting. If the soap is artificially colored, which is almost always the case, the dyestuffs used for this purpose should have careful attention and only those should be used which are known to resist the action of alkalis. Where rosin is used this product must be of the better grade. Distilled water is always preferable for use in transparent soap. The government permits the use of a specially denatured alcohol. This alcohol is not taxed and consists of grain (ethyl) alcohol denatured with 5 per cent. wood (methyl) alcohol. Some soapmakers prefer to use a more expensive refined methyl alcohol, but outside of adding to the cost of the soap, there is no particular advantage. The glycerine should be chemically pure. As to the oils and fats these should be low in acid and of good color. Under no circumstances should the crutcher or kettle in which the soap is made be rusty or unclean in any way. For a light soap enameled utensils are to be preferred.
To obtain transparency in soap the following general methods may be given.
1. Where the transparency is due to sugar.
2. Where alcohol and glycerine produce transparency.
3. Where (1) or (2) is supplemented by the use of castor oil.
4. Where transparency depends upon the percentage of fatty acid in a soap and the number of times the soap is milled.
Under the first method at least 25 per cent. of the charge should be cocoanut oil, the other constituent being tallow or any fat or oil capable of giving a sufficiently hard soap. The soap is boiled and finished as usual, then run to the crutcher to be mixed with a strong cane sugar solution, containing 10-20 per cent. sugar of the weight of the soap. The sugar is dissolved in its own weight of water and the solution heated to 175 degs. F. before being very slowly added to the soap. As the water evaporates, soaps of this type show spots due to the sugar thus being thrown out of solution.
Transparent soap made under the second method may be saponified as usual and consist of any good toilet base. The soap is run to the crutcher and mixed with 95 per cent. alcohol in the proportion of one part alcohol to two parts of fatty acid contained in the soap together with glycerine in the same proportion.
By the third method castor oil alone may be used to make the soap or added to any of the above bases up to 33-1/3 per cent. of the charge. If castor oil only is used, but 2 per cent. or 3 per cent. of sugar is required.