Plaster Isn’t So Hard to Use

by Brian Crocker

Quite some time ago a Potter Friend and I had a brief discussion, at our Studio Potter’s Club, about using Pottery Plaster, he was setting up to make a clay slops drying container and knowing that I had been a tutor in the Ceramic Design Dept., at the then S.A.C.A.E. Underdale South Australia. He asked me a few questions about Mold Making with Pottery Plaster. It was only then that I realized how long ago it was since I had been involved with plaster.

It was 1981 when I last taught students, scraps of information came out of the memory bank bit by bit, with a few gaps here and there, my friend said ” O.K. I’ve got some idea of what you’re on about”.

So I got into the Car and headed for the Hills and home. Isn’t it great the way driving gets the gray matter working; it’s like thinking of all the things you could have said in a discussion after the event. I mentally sorted out the sequence and timing of mixing and casting pottery plaster on the way home and when I got there I gave him a phone call at the Club to add a few things.

I’m sure I still wasn’t as lucid as I should have been, “Oh yeah is that how it goes?” was his reply.

So out came the notes that were down in the cellar, from my teaching days, out came the pottery plaster, model, cottles, buckets etc., and I settled down to make a one piece sprig mold from scratch. This cleared a lot of fog from the brain.

And so from that exercise here are the basics of using plaster and casting a Mold, I hope it will be of some help to you.

Plaster MoldOne important point to start with, for the best results, — USE ONLY FRESH POTTERY PLASTER (no other plaster will do) it should be “NO OLDER” THAN 6 MONTHS FROM A GOOD DRY BAG,, AND USE ONLY CLEAN RAIN WATER or SOFTENED WATER. Sounds extreme? But that’s the way it works best.

Decide what you are going to use the mold for, because this determines the water to pottery plaster percentage (%), the more water, the more absorbency the plaster will have but also the weaker it will be .

So here are a few percentages:

  • Clay Slops Drying 8 – 9 kg of water to 10 kg of plaster. = 80 – 90 %
  • Slip Casting Molds 7 – 8 kg ” 10 kg ” = 70 – 80 %Sprig Molds 7 – 8 kg ” 10 kg ” = 70 – 80
  • Jigger / Jolley Molds 6 – 7 kg ” 10 kg ” = 60 – 70
  • Press Molds 6 – 7 kg ” 10 kg ” = 60 – 70
  • Case Molds 3 – 4 kg ” 10 kg ” = 30 – 40

Now having decided, the next requirement is to make a soap mixture of bar soap (the kind Grandma used to use, no color no perfume no filler, as pure as you can buy, or it could eventually attack the plaster and spoil the detail ) grate it into a bowl or large glass jar, not a tin,, and add warm water [ 50 gm of grated soap to 110 gm of warm water ] mix well and leave to soak and soften, give an occasional stir over a day or two and add a little warm water if it looks too thick, it has to be brushable with a brush similar to a soft shaving brush. It is very important that there are no lumps, spend time feeling the mixture with your fingers, lastly pass it through a sieve.

NEVER USE GREASE, VASELENE, OILS OF “ANY KIND”, OR WAXES, they all clog the pores of the plaster. It is the absorbency of the plaster ( the ability of the plaster to draw moisture out from the clay ), that you require.

Now after making sure the model is brushed {with a large Artists Brush} to thoroughly clean it, soap up the model with the prepared soap mixture.

The model can be made of many materials, plaster, pottery [ glazed or bisqued ] timber, plastic or metal etc., etc. Brush on the soap gel, not too thick, feel it with your fingers to be sure there are no lumps, let it dry a little and wipe off any excess with another brush. Let it dry thoroughly then burnish it with a cloth similar to an ” old ” child’s cotton nappy [diaper] ( no fluff or lint ) , don’t use toweling, it is abrasive and can remove the dry soap. Each time the model is burnished there must be a shine created on the soap.

Soap and burnish 3 to 4 times, but no more or it may clog any detail.

There are times when the detail on the model is so fine and complex that a decision must be made regarding the number of times to soap and burnish, it is best then to feel it with your fingers to “see” if the detail is being rounded with the soap. (Don’t hesitate at any stage to wash off all the soap let it dry and start all over again.)

When the soap is thoroughly dry give it a last gentle shine, keep your fingers off the shine now, a good Pottery Plaster can pick up even a finger print. Cover with a soft dry cloth, not plastic, and leave to harden.

Now get “all” the equipment ready, you will have no spare time once you start, you need a timer or clock, a weighing scale [ is it large enough? ], a clean dry bucket for plaster [ old plaster or plaster dust will seed the new mix and cause problems ], a bucket for water, a bucket for slops and bubbles a dry bucket to rest the mixing tools in and a clean dry bucket for mixing, a clean stirring paddle or a slow electric drill fitted with a clean mixing propeller, a large spoon to skim off bubbles a rubber hammer and something to pour the excess plaster in, preferably another small shape to use as a sprig mold.

And last but not least the base to hold the model and four cottles to retain the wet plaster, all soaped to prevent the new plaster from sticking, also about 1 kg of soft clay to stop up any gaps in the cottles.

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