This section is from the book "Practical Cooking And Serving", by Janet McKenzie Hill. Also available from Amazon: Practical Cooking and Serving: A Complete Manual of How to Select, Prepare, and Serve Food .
When coarse salt and ice of about the same size are mixed, the salt melts the ice to unite with it, which, in turn, dissolves the salt, so that both solids are changed to liquids. This change requires a large amount of heat, which is absorbed from the can of mixture placed in the ice and salt. By the cold thus produced the mercury will sink below zero. Snow is particularly good for freezing purposes, but a little water should be mixed with it, in order to start the melting process. The best ice is porous, called snow ice. Such ice, being filled with air cells, is more readily acted upon by the salt. As the melting ice and salt are colder than the mixture of ice and salt, it is a mistake to draw off the water as fast as it is formed. But, when the ice floats upon the surface of the water, the melting process , and consequently the freezing process, is at its maximum, and the water, which from this point begins to rise in temperature, should now be drawn off and, if necessary, the mixture should be repacked.
The proportion of ice to salt depends upon the texture desired in the article to be frozen: the larger the proportion of ice, the slower will be the freezing process and the smoother and finer-grained will be the frozen product. Hence, in freezing creams and sherbets, which are to be firm, velvety, and fine-grained, one measure of salt to three of ice is used; while in freezing "frappés" and "granites," which should be granular and coarse-grained, only two measures of ice to one of salt are employed.
Creams freeze in a shorter time than sherbets, the syrup, which forms the foundation of the sherbet, retarding the process. We make use of this fact in adding fruits to frozen dishes, as in puddings, tutti-frutti, etc. If wines and liquors be used, the fruit is soaked for some hours, or over night, in the spirits; and, as alcohol does not freeze, the saturated fruit is thus kept from freezing also. Where spirits are not desired, the fruit may be soaked in a very rich, heavy syrup.