Sal soda or washing soda is obtained by recrystallizing a solution of soda ash in water. Large crystals of sal soda containing but 37% sodium carbonate are formed.
Potassium carbonate is not extensively used in the manufacture of soap. It may be used in the forming of soft soaps by uniting it with free fatty acids. The methods of manufacture are the same as for sodium carbonate, although a much larger quantity of potassium carbonate than carbonate of soda is obtained from burned plant ashes. Purified potassium carbonate is known as _pearl ash_.
Water is indispensable to the soap manufacturer. In the soap factory _hard_ water is often the cause of much trouble. Water, which is the best solvent known, in passing through the crevices of rocks dissolves some of the constituents of these, and the water is known as hard. This hardness is of two kinds, _temporary_ and _permanent_. Temporarily hard water is formed by water, which contains carbonic acid, dissolving a portion of calcium carbonate or carbonate of lime. Upon boiling, the carbonic acid is driven from the water and the carbonate, being insoluble in carbon dioxide free water, is deposited. This is the cause of boiler scale, and to check this a small amount of sal ammoniac may be added to the water, which converts the carbonate into soluble calcium chloride and volatile ammonium carbonate. Permanent hardness is caused by calcium sulphate which is soluble in 400 parts of water and cannot be removed by boiling.
The presence of these salts in water form insoluble lime soaps which act as inert bodies as far as their value for the common use of soap is concerned. Where the percentage of lime in water is large this should be removed. A method generally used is to add about 5% of 20° B. sodium silicate to the hard water. This precipitates the lime and the water is then sufficiently pure to use.
_Salt_, known as sodium chloride, is used to a large extent in soap making for "salting out" the soap during saponification, as well as graining soaps. Soap ordinarily soluble in water is insoluble in a salt solution, use of which is made by adding salt to the soap which goes into solution and throws any soap dissolved in the lyes out of solution. Salt may contain magnesium and calcium chlorides, which of course are undesirable in large amounts. The products on the market, however, are satisfactory, thus no detail is necessary.