OCR Text |
Show First Plaster Was Mud ; Now It's Delicate Item Mankind, In all probabilities, used mud as its first plastic material, applying ap-plying the mud over a framework of woven reeds. Then, further along In the world's history, (he burning of lime and the calcimining of gypsum were discovered. Up until about 30 years ago lime was the standard plastering material. But lime plaster, because of its chemical chem-ical composition, takes a long time to reach its ultimate hardness. And it Is dangerous to apply a decorative material ma-terial over a lime-plastered wall until the plaster has completed the chemical chem-ical process known as the "set." Here's why : While still In the process proc-ess of "setting" lime Is liable to blister blis-ter or to burn through a decorative material. Gypsum plaster, on the other hand, "sets" so rapidly that retarder material ma-terial to slow up the process of "setting" "set-ting" has to be Introduced. Thus the "setting time" Is actually controlled and gypsum plaster that "sets" in two hours Is shipped to those cities where a two-hour "set" Is required; a plaster plas-ter that "sots" In four hours Is shipped where a four-hour "set" Is required, etc. Poth gypsum and lime plasters are incombustible, and thus add to the fire safety of a structure in which they are used. Often a lime-putty finish-coat finish-coat Is used over bnse-coats of gypsum plaster. In this case, the lime is "gauged" with gypsum gauging plaster plas-ter to prevent shrinkage and to give the lime an Initial set. Such a lime finish coat Is thinly ap plied, because of this thinness the lime-coat can get enough carbon dioxide diox-ide out of the air to complete its "set" within a reasonable length cf time. |