Found in limestone formations or shell mounds, naturally occurring lime is calcium carbonate. Gypsum is also the ingredient in moulding plaster, a finer plaster used to create decorative moldings in ornamental plaster work. For finish coats, gauging plaster was added to lime putty it causes the lime to harden. While gypsum was used in base-coat plaster from the 1890s on, it has always been used in finish coat and decorative plaster. When mixed with water, it becomes calcium sulfate again. When calcined (or heated), one-and-a-half water molecules are driven off, leaving a hemi-hydrate of calcium sulfate. Once mined from large gypsum quarries near Paris (thus the name plaster of Paris), gypsum in its natural form is calcium sulfate. Some time in the early 19th century, a transition occurred, and plasterers applied their wall finish before woodwork was installed. On early buildings, the woodwork was installed and primed before plastering began. Early plaster work, however, did not use grounds. These grounds help keep the plaster the same thickness and provide a stopping edge for the plaster. Plasterers use metal or wood strips around the edges of doors and windows and at the bottom of walls. Wire mesh used on inside corners of adjoining walls and ceilings. Installing the corner bead plumb is important.Ĭornerite. Installing the corner bread plumb is important. Wire mesh with a rigid metal spline used on outside corners. Like a wood ground, they indicate the proper thickness for the plaster.Ĭorner Bead. In the 19th century, metal casing beads were sometimes used around fireplace projections, and door and window openings. Fine sand can be added for a sanded finish coat.Ĭasing Bead. Pure lime, mixed with about 35 percent gauging plaster to help it harden, is used for the very thin surface finish of the plaster wall. With gypsum board lath (rock lath, plasterboard), it is the only base coat needed.įinish coat. The brown coat is the second application of wet, base-coat plaster with wood lath or metal systems. The wet plaster is “scratched” with a scarifier or comb to provide a rough surface so the next layer of base coat will stick to it.īrown coat. The first base coat put on wood or metal lath.
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