Molding a Plaster of Paris disk for use as backing for a grinding tool or pitch lap.

pla01.jpg (113535 bytes) We have two 4 1/2 pound boxes of Plaster of Paris ( About 4 kg ), a sheet of poster paper, some masking tape and automotive grease.  Lard will work also.
pla02.jpg (594529 bytes) We cut the poster board into strips and grease the mirror.
Careful, its very slippery.
pla03.jpg (115680 bytes) And tape the strips around the mirror with masking tape.
pla04.jpg (135028 bytes) Turn off your cell phone then add water slowly to the Plaster of Paris so as to avoid thinning too much. Plaster of Paris is not too bad on your hands, unlike cement. Pour into the mold and smooth or shake down by shaking your hand.  Measure from the top of the paper strip to make sure the plaster is not too thick on one side. You only have about ten minutes.
pla05.jpg (146131 bytes) After an hour the Plaster of Paris is warm and hard. It slips easily from the mirror. We cleaned the grease with paint thinner. Don't try to pick up the mirror with grease and paint thinner on it, it is incredibly slippery.
pla06.jpg (113548 bytes) The next day we sand the bumps and the edge with 40 grit sand paper.
pla07.jpg (104921 bytes) It takes almost a week for the plaster to dry then we paint the back and sides with poly urethane. The curved mirror side will get epoxy or pitch later. The Plaster of Paris disk is about half as heavy as cement and that is an advantage when doing fine figuring with the pitch lap on top.

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