Plaster of Old
Most plaster used in the United States since the turn of the century is based on gypsum. 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).
Limestone was burned (calcined) to drive off water and carbon dioxide. The resulting calcium oxide, called quicklime or lump lime, was slaked with water (rehydrated) in sand-lined pits. The mixture reacted explosively, but when the reaction subsided, creamy white lime putty remained. Mixed with sand and animal hair, it was applied over lath—horizontal strips of wood or straw. The first coat, or scratch coat, oozed between the laths to form "keys" that held the plaster in place. Two more layers, the brown coat and the finish coat, usually hairless, had to be "packed"—troweled again and again under pressure. Poorly packed plaster cracked as it dried; homeowners were warned to wait a year before painting.
Plaster made from calcined gypsum rock (calcium sulfate) forms a harder surface than lime without the bother of slaking and packing, but it sets almost immediately. In 1880, a retarding agent made of ground-up horns and hooves was found to slow setting. A quarter of the century later, vitually all the lime plaster in the United States had been replaced by gypsum plasters.
Still, there are some plasterers, like Rory Brennan of Putney, Vermont, who prefer to use lime, espcially for restorations. "Rigid gypsum plaster isn't compatible with the softer lime," Brennan says. He points out that lime takes centuries to harden: "Lime plaster is young at 100 years." he says.
