Adventures in Slipcasting: Part 1

A lifetime ago, I was at art college and worked a lot with plaster and casting. I love plaster, smooth slippy, and really useful.

Before that on my foundation I made one slipcast thing (actually two of the same thing). I modified a vase from a charity shop and made a plaster mould. Badly fired in an old waste paper bin, it still survives. I have always been a bit of a bodger. This is one reason why I am so in love with tiny pottery, it’s proper, real ceramics and glazes, but on a domestic scale – bodged to perfection in my kitchen.

So I’m looking for a way to reliably make the same thing over and over. I tried slab building small cups, but after taking 30 minutes each to make, they both exploded. I glazed one back together and use it, but I fell out of love with slab building quickly.

Next: Slip casting. I have always loved when you see plastic cups made out of ceramic, so I set my sights on paper sauce pots from burger joints. I started with Maccy D’s pots. These are small, but a good test. Intended to move on up to 5 Guys pots which are about 42mm high, will hold a good shot of booze, and will fit into my Microwave kiln.

Down my local high street I tried to get some plaster of paris, but could only find fine finishing plaster. Some of the internet said it would work. The rest, no so much. But I only take on information that matched my worldview, so went for it.

Here’s the plan:

  1. Plaster in paper cups so they are stiff enough to cast.
  2. PVA on the cups and a thin clay strip to reinforce the underside of the rim.
  3. Make a 2 part mould with openings in the top that will let me pour slip in.
  4. Make slip by stirring up all my clay offcuts in water.
  5. Dry mould
  6. Secure mould with band, pour in slip and leave to set a little.

My slip was a little runny/thin so it took a while to build up a thick enough skin. Before I started casting, the mould broke into 4 bits, but good enough for a test. Anyhow, 2 casts, one survived, proved that it will work in principle. Moulds in the bin! Amazon order placed.

Version 2: Bigger and better. Not really.

Decided that a one-part mould would also work I followed the same steps above with the plan to chip out the plaster from the cups. Chipping out the plaster took ages and damaged the walls. FAIL! And there was a bubble. In the bin!

Version 3: Softly-Softly.

So the next idea was to fill the cups with expanding foam that I have. This will make them hold their shape under the plaster, but be soft enough to pull out. Lovely, all so good. But then as I’m pouring the plaster I realise they are full of air and will rise up in the plaster. They rose up. Back to the drawing board? Nah, just hold them down for a bit and hope for the best.

Wahoo!! They come out perfectly, left a lovely impression and no bubbles. Brilliant. Really excited.

TODO: trim the rim and dry the mould before using them. That’s for next week.

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