Gypsum’s main use is in the construction business, as gypsum board, or “drywall”.   “Drywall” come in large sheets, and is used to coat the walls of many, many houses and office buildings.  Drywall is composed of about a quarter inch of gypsum between two sheets of paper (“lath”).  But what makes the gypsum plaster adhere to the paper and wood (termed “gypsum lath”)? The surface of the lath appears relatively smooth, yet when the wet plaster is mixed with twice its weight of sand and applied to lath, a pull of almost half a ton per square foot is required to remove it. About 1/5 the weight of gypsum rock consists of water, chemically combined in its crystalline structure. In making plaster, the rock is cooked or calcined until its water content is reduced to1/4 the original. The answer to the above question is, then, this: When the plaster is applied to lath the solution of plaster is absorbed into the pores of the lath paper, and when the gypsum crystals grow they interlock with the lath/paper to establish a tenacious bond.