why is plaster the enemy of clay?

[lerpa]

I've seen a few things online lately advising against bringing plaster anywhere near clay, but no explanations. What's the issue between plaster and clay?

[bciskepottery]

The issue is lime popping: A body firing problem where particles of calcium sulphate in the fired or bisque fired body slowly absorb water from the air, expand, and pop out a section of the body. (from Digitalfire)

 

 

[Kabe]

I've seen a few things online lately advising against bringing plaster anywhere near clay, but no explanations. What's the issue between plaster and clay?

 

 

 

I use plaster for press molds for tile. It is how I replicate tiles. I keep all the clay that comes in contact with the plaster when I press out a tile separate from my good clay. I put the clay that was scraped off of the top of a press mold in plastic bags marked "dirty clay". I use this clay to sculpt a new tile that I want to pour a new cast of. This way I do not have to waste all the clay. I also keep the clays separate for reclaiming. I do not have a pugmill or a mixer so I do it with a drill in a bucket. I stir the clay into slip and pour the slip into a plaster sink I made. I let it firm back up and rebag it. You could not mix your clean clay with your bad in a pug mill for fear of contamination. I also use Plaster bats to throw on and I think they are great. I keep my trimings separate from my throwing clay too just in case I might have trimed some plaster off the top of a bat. So far I have not had any trouble with pop outs but I am very careful to keep the dirty clay away from my good clay and if in doubt, it is dirty clay. If you are going to use much plaster you need to set up a serarate place to mix it for pouring, it can get messy. It it is amazing stuff to work with, I couldn't get by without it, but clay and plaster do not play well together. Don't forget to smile and have fun.

[Marcia Selsor]

Plaster is not the enemy..just bits of plaster getting into the clay. The property of plaster serves the function of absorbing moisture from the clay. Plaster bats are used for throwing and plaster slabs are used for drying wet clay or for wedging. Just don't scrape the surface of the plaster. I have plaster wedging tables and slabs for drying in my studio. Where I am teaching, clay is recycled on about 20 large plaster slabs after the clay (5 types) is slaked down in tubs. Then it is wedged up and reused.

There are also hundreds of plaster bats for throwing on the wheel.

The absorption of moisture also works in tiny chips of plaster that will absorb the moisture when imbedded in the clay...thus, blowing up when fired.

 

Marcia

[Benhim]

Gypsum also expands at a much greater rate than clay as it's heated. As a chunk of plaster that's in your clay begins to heat up the crystal will start to expand, and then when it gets hot enough the crystal will actually begin to grow inside the pot while the. This will cause an explosion or what's also referred to as a pop out. This can be small or larger depending on how big of a chunk of plaster is in your clay. Eventually the gypsum will melt into a streak of bubbly yellowish stuff on the side of your pot or on the kiln shelf. It's not the end of the world, but it will ruin a piece. Plaster is a very useful material for making impressions, stamps, molds etc, because the fine particles take on very fine detail easily. Most clay bodies do not do this nearly as well as plaster, so molds or stamps made of clay will provide less detail in the finished product.