It's sticky but contains a lot of water and takes up to 24 hours to dry. Joint compound, made primarily of gypsum and water, is used to spackle seams between sheets of drywall. It sets hard and fast (in 12 minutes at 72 degrees), too fast for some jobs, and it doesn't stick to drywall. Traditionally, plaster is mixed with lime putty (autoclaved lime). This mix of plaster and ''mud" - builders' slang for joint compound - is the modern part of modern plaster.
#Plaster vs joint compound free
(The exception is wood, which has to be covered with burlap or nylon screening and a layer of plastic or tar paper, so it's free to shrink and expand underneath without cracking the plaster.) The gist was that when plaster of Paris (calcium carbonate) mixes with an equal volume of joint compound (calcium sulfate), it will stick to just about anything, from sheet metal to drywall to glass. Ferver, a general contractor and 20-year plastering veteran, discussed the chemistry of plaster. This two-day workshop, ''Modern Plaster Techniques," had started that morning with five women and six men squeezed around a table at Yestermorrow Design/Build School. ''It's nice to see the hand of the maker." ''It doesn't have to be perfect," he said. Ferver took over, his arm making a seamless arc, first up and outward, then lifting the trowel before swiping it back the other way. I did what he said, and my trowel skimmed the surface, but at the end of the stroke it dug in, leaving marks. ''Angle your trowel and bear down hard." This time it was the instructor, Buzz Ferver, who stood behind me. Their trowels glided over the surface like spatulas frosting a cake. This already felt like an outtake from a bad sitcom.
When I finally pried it off, meringue-like peaks stippled the plaster. I tried to smooth out the lines, but the trowel stuck flat to the wet surface. I shmeared the plaster on the wall in a shaky arc, leaving gouges from the awkward new tool. I dipped the trowel into the dollop of plaster balanced on my hawk, a sheet of metal a foot square with a peg handle on the bottom. ''You just shmear it on," coached Dave Thurston, our co-instructor.
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