Christopher Roberts, age 10, of Portage, Ind., for his question:

WHAT IS ALUM?

Alum is the name of a particular group of double salts containing water. These salts form what look very much like crystals of hydrated potassium aluminum sulfate. The compound is called common alum, potash alum or potassium alum.

Most alums are manufactured from aluminum oxide ore.

Potassium alum helps stop bleeding and shrinks human or animal tissue. The product is available in all medical circles.

Other alums are used in making cosmetics, dyes, some baking powders and leather tanning agents. Industry also uses alums to help purify water, to harden plaster of Paris and to help coat or size paper.