Saturday, June 3, 2017

Mixing Pottery Plaster no. 1

Mixing Pottery Plaster no. 1

The other day I decided I needed some plaster bats, the kind that has a triangle type locking device, lucky for me my brother had the mold for it. You can buy the molds from this company here:

It’s been a rather long time since I mixed up plaster and it’s about as fun as mixing up glazes in my books but even worse. With plaster, you cannot wash your hands in the sink, or pour any of it down a drain; it will immediately harden and clog your drain which will be almost impossible to fix.

I like to use plaster bats when I throw any kind of plates or platters. It encourages even drying and I try to avoid the S cracks. So, as I set out to make some plaster bats I did forgot a few key steps; so my first bat didn’t turn out the best. The rest of the bats turned out better as my memory kicked back in. My method isn’t as precise as it could be, simply because I don’t use plaster a lot in my studio practice. If you need precision I would recommend going to the pottery plaster site and getting more in depth instructions.

First, make sure you have all the equipment you need. A bucket for mixing the water and plaster. (Don’t mix plaster with clay or glazes, it’s essentially lime based and will explode in your kiln, so keep plaster things with plaster things) have a way to measure your plaster, a water container, murphy’s oil or mold soap, a good mixer (you can hand-mix too) and of course your mold. And scrapers to clean up afterward.

First get your molds ready, I use Murphy’s oil as a release soap and it works fine. But you don’t use it straight out of the bottle. Mix it with water until it looks like, um urine or pee. Yes, that is as technical as I get (I am guessing like 1 part oil to 8 parts water). I brush it onto the mold, make sure you get an even coat everywhere, and then wipe all it off lightly. I just use a paper towel to wipe it off. There does remain a fine thin film to release the plaster from the molds, you need to do this step or the plaster will adhere to your mold. Don’t mix the plaster and then get the molds ready, that is a bad idea.

The ratio to plaster to water is 70 parts water to 100 parts plaster. So, I discovered after the first mold I needed 3000 grams of plaster, thus I needed 2100 grams of water (or 2100 ml) 3000x70=210000/100=2100, (or) 70% of 3000=2100 easy peasy.

After you measure your plaster, using tap water (the chemicals in the water effects the plaster so use as clean water as you can, or distilled water) Also, the temperature of the water matters. The warmer it is, the quicker the plaster sets. Water should be at room temperature of slightly above that. I just guessed the temperature by touch, I make it slightly warm – I know technical, right? If it’s too cold it takes too long to set up and I have no patience for that.

Always add plaster to the water, never the other way around. And sift the plaster into the water, don’t pour it in, in big clumps or drop it into the water. Sift slowly, making sure the plaster is evenly distributed and keep adding plaster until you see little islands of plaster emerge on the top of the water. You will know you have the right amount when you see the islands. Then let the plaster absorb the water by waiting for about 2 minutes before you start to mix. If in that time, the plaster starts to react to the water and thickens it means your water is either too hot or your plaster is amazingly reactive. Keep an eye on it, but this has never happened to me; after two minutes’ mix. I use a mixer attached to a drill on low speed because I find when I use a stick or my hands some clumps remain. If you wear gloves you can still mix it up well enough though. Just keep mixing until the plaster thickens, so that when you pick up some plaster with your fingers you can drag it over the surface and the plaster creates a peak up on the surface.

When the plaster is thick, pour into the molds in one spot carefully, not too quickly like a big vomit – you want to avoid getting air bubbles trapped. Even tap the side of the mold to help bring up air bubbles. We want a nice clean, strong mold that will last. Okay now, let it set-up until it has hardened enough to release from the mold, it will get very hot while it begins to dry this is a chemical reaction and you want this. Therefore, you don’t put it directly onto your skin, unless you like the possibilities of burns. I know someone who thought they could cast a kids hand in pottery plaster no. 1 – just no.

Once you get the plaster out of molds try to find a way to dry them out. (It was plus 30C out so I put them out in the sun) Drying them or curing them after is important, the drier you can get the plaster molds the stronger they are. Which translate that they’ll last longer and you will not have to endure making plaster molds for a while, yippee. I personally make such a huge mess when I make them, I get plaster on me, caked on the buckets, on the ground, on tools and it hardens and it’s a mess. I just try to avoid the whole thing if I can, but I really like using them for plates and so forth so I will endure it. I can get 7 or so 12” plaster bats from one large 50lb bag of plaster.

Good luck and happy creating.

On and Upwards.



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