Pretty new to making plaster molds for slip casting. I have had some success but one recurring problem... sometimes when I take the item I am casting out of the mold, the plaster seems to tear, leaving an uneven surface. Many little bits of plaster are stuck to the original item, leaving lots of little divets in the mold. What is going on here? Am I demolding too quickly? Not mixing the plaster thoroughly enough? Mold release not working?
I demold when the mold is warm to to the touch, about 40 min after pouring.
I mix the plaster with a paint mixer on a drill, 2 min for 0-20lb, 4 min for more. I do not have a good system for sifting the plaster into the water, however.
For mold release I brush on several coats of 50/50 Murphy's soap and let it dry. This has worked well for me, after many early failures with other methods. What I found in those failures though was very hard to remove the item from the mold, not that the plaster would fail in this way.
Any insight appreciated!