Just thought I’d talk a little about what I do, when working with plaster. I’ve looked in a fair number of places online, to pick up good techniques with using moulds, places like Hirst Arts (where I bought the moulds) and TerraGenesis.

Long term, I’m probably going to pick up a flexible mixing bowl for working with the plaster. More space, and easy to clean. You just wait for the plaster to dry, then flex it out. I’ve read all the warning about putting plaster down your drain, after all. 😀 Right now, I’m using disposable plastic cups. As long as there’s not much plaster left, it flakes our easily enough.

I’m mixing it with a plastic spoon, though I think, long term, I’ll move to something more like wooden tongue depressors. bigger and less likely to add air to the mix.

The plaster I’m using is Herculite stone, which I got from Maragon Arts and Crafts. I’ve ordered from them a couple of times, and it reached me quickly. Shipping’s not cheap, but what do you expect for a 10 kilo bag of plaster. They’re the same people I got the RTV Silicone from. Anyway, the plaster’s a tough one, and perfectly capable of working to the degree of detail I want. Air bubbles are always a problem, but that’s life.

Anyway, the basic moulding method I’ve cobbled together is:

• Take the mould and soak it in a tub of warm water (not hot), mixed with a little dishwasher rinse aid.
• Use a blunt toothpick to dislodge any obvious air bubbles,while soaking. After casting three or four times, you get to know where the bubbles are likely to form.
• Set out the moulds on a plastic sheet. A plastic bag works, it’s just to catch the plaster and protect the surface.
• Measure out the water and plaster. Measure them separately, rather than adding them together. The Herculite stone is 1 water to 3 plaster.
• Add the plaster to the water. Don’t do it the other way round. Stir while adding it. you want to keep it well mixed. Keep it a slow mix though, or you’ll introduce more air than you want.
• Let it sit for a minute or so. give it a quick mix afterwards, just to make sure it’s not settled too much.
• Pour the plaster into the moulds. Advice seems to be, pour it onto a bit between the actual moulds, rather than directly into them. Eitherway, don’t move it around too much. Let the plaster flow into place. Let it go a bit higher than the edges, so it can settle and still be flat.
• If you don’t have a vibrating table (I don’t), drum a bit on the surface with your fists. The idea is to vibrate the mould, to make the bubbles rise and pop.
• After 6 minutes or so, use a straight edge to scrape off the top of the moulds. You’ll have to judge this, depending on the mix, and the plaster type used. I’ve seen it described as supposed to be like toothpaste.  I’ve been using a small steel ruler, but I’ll be picking up a wall paper scraper later. Less flexible.
• Let it set. You should have instructions for the time it’ll take. I tend to leave it an hour or so
• Strip it from the mould. Fairly fun part, really. flex the mould, to let you get an edge of a block, then pull it out.
• Leave them to cure some more. Around 24 hours seems good. Longer won’t hurt. Course, you can start over, with another set of blocks 😀 It’s mildly therapeutic.

The reason for the warm water, and the rinse agent, is to reduce the surface tension. Means air bubbles are less likely to form.The water just flows better. You don’t need to get it all out of the moulds, either; the plaster will displace it. I’ve seen some suggestions about adding it to the water that you make the plaster up with, but I’ve not needed to do this. Maybe with a more detailed mould.