Monday, August 15, 2011

Side Effects, and How to Pour a Plaster Drying Block


There are all kinds of small issues involved in ramping up ceramic production, issues that you don't really think about until confronted with them. Stuff like, where to keep the extra ware while it is in progress. (Answer: Build more shelving, and clean up the studio. A lot of the crap in there is just that, crap.)

I've been noticing that I can no longer keep up with my reclaim clay. I used to scoop out a slabfull of slurry every few days, and that was enough to keep the buckets to a minimum: one wet bucket & one dry. I had to bring in a second wet bucket and then a third; when I make more stuff, I also make more trimming scraps and throwing slurry. Answer: I need to increase my drying space, by making another plaster slab.

I've actually never cast anything in plaster before, as I had heard horror stories about how, if you don't get it exactly, precisely correct, it cracks or crumbles or just never dries at all. But I happened to have a 4-pound box of plaster lying around (can't remember what I bought it for...) so I made a small, test block: no problems at all. Onto the real thing!

First thing: you don't need to get pottery plaster, if you are just making a drying block. Pottery plaster is better for molds because it is finer and takes detail better, but for my purposes plaster of paris from the hardware store is just fine.

Second thing: Prepare the mold before you do any mixing. I used the lid of a case of paper. These are easy to come by -- ask at Staples. Or any office, even the so called "paperless" offices, will still use a case or two of paper every week. I coated the inside of it with vegetable oil (no need for fancy sprays or mold release soap.) In hindsight, I perhaps should have used paraffin; the corners of the mold sprung leaks under the greater pressure of the full-sized block. Paraffin might have prevented this; as it was, I stopped them up with gobs of clay on the outside of the box.

Third: add the plaster to the water, NOT vice versa. I filled up a bucket with cool water: Four pounds of plaster needed 5 cups of water, which is more than the directions stated, but it was obvious once I started that the suggested 3 cups were not going to be enough. So, for 25 pounds, I'll use about 30 cups of water.

Fourth, when you add the plaster, sprinkle a handfull at a time onto the surface of the water, wait for it to sink, and then sprinkle the next handful. Do this as quickly as the material will allow. Toward the end I had to mix with my hand, to squish the lumps.

Have a scraper or smoother ready. The plaster might be thick enough that it won't flatten out by itself.

Plaster sets up very quickly but it takes a long time to finish curing. I won't use my new slab for a week, to make sure it's ready.


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