This section is from the book "The Steward's Handbook And Guide To Party Catering", by Jessup Whitehead. Also available from Amazon: Larousse Gastronomique.
In a powdered state has been extensively employed as an adulterant in bread; it has much the same effect as a slight tendency to sourness in the dough has in whitening the bread; it has some effect in preventing certain constituents in flour from changing into a gummy and transparent appearance, which rfiakes the bread look dark. Bakers find that second-rate and soft flour is corrected by the use of alum, so that it produces a more saleable loaf than better flour would without such treatment. Alum is, however, unwholesome, while not positively poisonous; it is a mineral salt, which, regularly taken into the stomach, causes great injury to health; the bread adulterated with it is damaged also in its keeping qualities, and becomes unduly dry and tasteless in a day after baking. Stringent laws have been passed in various countries to suppress this practice; in Paris as many as forty bakers at one time have been fined, their shops closed and their business suspended by law, as a punishment for using alum in bread; prosecutions, convictions, fines and imprisonment have occurred in most large cities of the United States for the same cause.
The bakers' journals deny now that this practice prevails to any considerable extent, and a state of opinion has been worked up in the trade which causes it to be considered disgraceful and dishonest to resort to the practice. The adulteration of bread with alum seems to be on the decrease. In hotel bakeries there is no need of resorting to such expedients. Alum is one of the injurious ingredients in inferior baking powders. It is useful in pickling, to make the pickles firm and brittle; it is used in making cochineal coloring, in very small amounts, and dyeing, etc., to set the colors. Plain alum, and its derivative, the bisulphate of alumina, have a remarkable effect in clarifying muddy water, and the former is extensively employed for that purpose. A small quantity - a tablespoonful powdered - mixed with a barrel of Mississippi river water renders it quite clear after standing an hour or two, and the impurities are coagulated together by its action so that they can be removed by straining. The immense filtering works which now operate to purify the entire water supply of some cities are based upon this singular property of the mineral salt alum.
Powdered alum is useful in case of a cut to apply to stop the flow of blood.