Many of the freezing mixtures which are to be found described in books are incorrectly so named, for although they themselves are below the freezing point, yet they are not sufficiently powerful to freeze any quantity of water, or other substances, when placed in a vessel within them. In order to be efficient as a freezing mixture, as distinguished from a cooling one, the materials used ought to be capable of producing by themselves an amount of cold more than thirty degrees below the freezing point of water, and this the ordinary mixtures will not do. Much more efficient and really freezing mixtures may be made by using acids to dissolve the salts. The cheapest, and perhaps the best, of these for ordinary use, is one which is frequently employed in France, both for making dessert ices, and cooling wines, &c. It consists of coarsely powdered Glauber salt (sulphate of sodium), on which is poured about two-thirds its weight of spirit of salts (hydrochloric acid).
The mixture should be made in a wooden vessel, as that is preferable to one made of metal, which conducts the external heat to the materials with great rapidity; and when the substance to be cooled is placed in the mixture, the whole should be covered with a blanket, a piece of old woollen carpet doubled or some other non-conducting material, to prevent the access of the external warmth; the vessel used for icing wines should not be too large, that there may be no waste of the freezing mixture. This combination produces a degree of cold thirty degrees below freezing; and if the materials are bought of any of the wholesale druggists or dry salters, it is exceedingly economical. It is open, however, to the very great objection, that the spirit of salt is an exceedingly corrosive liquid, and of a pungent, disagreeable odour: this almost precludes its use for any purpose except that of icing wines.
Fair And Softly Go Sure And Far.
2143. Further Directions
Actual quantities—one pound of chloride of ammonium, or sal ammoniac, finely powdered, is to be
intimately
mixed with two pounds of nitrate of potassium or saltpetre, also in powder; this mixture we may call No. 1. No. 2 is formed by crushing three pounds of the best Scotch soda. In use, an equal bulk of both No. 1 and No. 2 is to be taken, stirred together, placed in the ice-pail, surrounding the ice-pot, and rather less cold water poured on than will dissolve the whole; if one quart of No. 1, and the same bulk of No. 2 are taken, it will require about one quart of water to dissolve them, and the temperature will fall, if the materials used are cool, to nearly thirty degrees below freezing. Those who fail, may trace their want of success to one or other of the following points:—the use of too small a quantity of the preparation,—the employment of a few ounces; whereas, in freezing ices, the ice-pot must be entirely surrounded with the freezing material: no one would attempt to freeze with four ounces of ice and salt. Again, too large a quantity of water may be used to dissolve the preparation, when all the excess of water has to be cooled down instead of the substance it is wished to freeze. All the materials used should be pure, and as cool as can be obtained. The ice-pail in which the mixture is made must be of some non-conducting material, as wood—which will prevent the access of warmth from the air; and the ice-pot, in which the liquor to be frozen is placed, should be of pewter, and surrounded nearly to its top by the freezing mixture. Bear in mind that the making of ice-cream, under any circumstances, is an operation requiring considerable dexterity and practice.
2144. To Make Dessert Ices, both Cream and Water