In making biscuit, etc., our grandmothers used saleratus, an alkali prepared by exposing pearlash, i. e., purified potash, to carbonic acid gas. Potash is a fixed alkali made from wood ashes. Now soda bicarbonate, made from common salt, is generally used.
Crude soda is known as sal-soda or soda-saleratus; when refined, it is carbonate and bicarbonate of soda. But the purest soda obtained to-day is that made from Kryolith, a mineral found only in Greenland. It is called Natrona Bicarbonate of Soda, and is an ingredient of the purest baking-powders.
Pure, strong alkalies are powerful corrosive poisons, eating the coats of the stomach perhaps quicker than any other poisonous agent. This caustic or burning property is somewhat weakened by the carbonic acid united with them, and is therefore less in bicarbonate of soda than in the potash compounds. The latter are now seldom used. Alkalies when properly combined with acids lose this poisonous property; the carbonic acid gas is liberated, and the compound formed by this union is called a neutral salt, being neither acid nor alkaline. When not properly combined, if the acid be stronger than the alkali, the salt is acid; and if the alkali be in excess, the salt is alkaline and still poisonous.
Soda has a great affinity for water; and when wet, a combination takes place which allows some of the carbonic acid gas to escape. This may easily be seen by the effervescence which occurs when soda is dissolved in hot water. This, the old way of using soda, was theoretically wrong, as much of the gas was lost; yet practically good results were obtained, because the saleratus formerly used was much stronger than the bicarbonate of soda of to-day, and could well be weakened.
Soda alone, when mixed with wet dough, will give off gas enough to raise the dough; but it leaves a strong alkaline taste and a greenish yellow color, and, being poisonous, must be neutralized by an acid, or else its use is not admissible. The best acid for this purpose is one which does not liberate the gas instantly on contact with the soda, before the heat can fix the air cells, and also the one which leaves no unwholesome residue.
Muriatic Acid, which is sometimes used, would be the best, as it leaves only common salt as a residue; but the gas is liberated instantly, and only a skilled hand can mix the bread and place it in the oven without losing much of the gas.
Cream of Tartar, which is tartaric acid combined with potash, and is obtained from the crystals or argols which collect in wine casks, is preferred by chemists. Being only slightly soluble in cold water, it unites with soda only when heated, and the gas is not all liberated until the mixture is in the oven. The residue from the union is Rochelle salt, which is not injurious taken occasionally in small quantities. The objections to cream of tartar are these: being very expensive (the price varying with the grape crop), it is often adulterated with cornstarch, flour, or other substances; and the careless cook guesses at the proportions of soda needed instead of measuring accurately. The only safe way to use these chemicals is to purchase cream of tartar of a reliable grocer, and to measure carefully one level teaspoonfal of soda to two slightly rounded teaspoonfuls of cream of tartar for one quart of four. It takes a trifle more than twice the quantity of cream of tartar to make the reaction complete. The soda must be finely pulverized before measuring; rub it on the board with a knife, measure, and then sift through the finest wire strainer into the flour. Sifting with the flour through an ordinary flour sieve is not enough. Cream of tartar does not become lumpy like soda; but it is better to sift it, and salt also, into the flour, and then sift all together two or three times.