![]() ![]() The mixture reacted explosively, but when the reaction subsided, creamy white lime putty remained. The resulting calcium oxide, called quicklime or lump lime, was slaked with water (rehydrated) in sand-lined pits. Limestone was burned (calcined) to drive off water and carbon dioxide. Before then, walls were coated with lime-based plaster, which dates to the time of the pharaohs (it’s on the walls of the tombs). Most plaster used in the United States since the turn of the century is based on gypsum. In 30 mintes the patch has set and the job is finished, without any need for sanding. He then uses a sponge to touch up any surface imperfections. (5) After the first coat sets, Marshall mixes another batch of lime putty and gauging plaster (this time with less gauging so it’s easier to work) and skims a thin final coat over the entire area. (4) To reinforce the patch, he cuts a sheet of fiberglass mesh to cover the hole and presses it into the wet plaster. (3) He mixes everything with his trowel into a stiff dough, which he presses firmly onto the exposed lath and around the edges of the hole, leaving a slight depression for the final coat. ![]() ![]() (2) Marshall sifts in several handfuls of gauging plaster to give the putty more body. The putty is formed into a ring on the mixing board and the bonding agent is poured into its center. To make lime putty for the patch, Marshall mixes hydrated dolomitic lime with water until it is the consistency of whole-fat yogurt, then slakes it for at least 20 minutes, leaving a thin layer of water on the surface to ensure even rehydration. He brushes a bonding agent over the lath and all plaster being recoated, then covers all but the smallest cracks with fiberglass mesh tape. (1) The plaster around the hole is scraped smooth with an angle plane or rough drywall screen. John Marshall starts by chiseling out the old plaster from between the lath and brushing away any loose dust and plaster crumbs. Soft, crumbly plaster should be removed down to the lath and out as far as the firmly attached areas in preparation for patching. Firm plaster that has popped off lath can be reattached with plaster washers and drywall screws. While it may be tempting to rip down an old plaster-over-lath wall and replace it with drywall, often the plaster can be saved. ![]()
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