ALUM

Alum is used as an ingredient in baking powder and is used
to give crispness to pickles and maraschino cherries and to
harden gelatin. Alum can be a dangerous substance when not used properly. Ingestion of 30 grams (1 ounce) has killed adults.Alum is legal to use in baking powders. In pickles and cherries,the amount usually left in the packaged product amounts to less than 0.2 percent.
Alum is included in some fermented pickle recipes. Research
shows that it has no effect until after the pickles have been
fermented. Then it is used only in a soak solution. It should
be washed off thoroughly before completing the recipe. Do NOT put alum in the final pickling liquid. Douche alum is not food grade.
While alum may be safely used to firm fermented pickles, it
is unnecessary and is not included in the USDA recipes for
canning pickles.