Firing the Pieces
Potters make stoneware, they work at cone 6-10 (about 2200-2350F). Schools make earthenware at cone 06 (about 1850F). Kilns last longer and fire much easier at cone 06 (a firing can be done in half the time).
- Make sure ware is dry before firing it.
- It is best to program your kiln manually so you know how fast it is heating. Program it to heat up to about 240F and hold there for a few of hours, even over night (to drive out all water) before continuing upward to 1850F at about 300F/hr.
- If you are glazing, bisque fire pieces first (the clay will turn back into mud if children try to paint glazes onto dry pieces). Bisque to cone 04 (1920F). Pieces can be stacked in bisque firings.
- Red-burning clays fire to a stronger product, but many glazes look better on lighter colored ones.
A glaze is just a mix of powdered minerals/glass/clay suspended in water with a little gum (glue) to harden it a little on drying. Glazes melt to a glass during firing (for decorative pieces or models it is OK to bisque-fire ware and then just paint it with acrylics).
- Follow instructions/cautions on the glaze label. Add water to glazes if they are too thick (even newly opened jars can be too thick). It is better to paint on more thin coats than fewer thick ones, this gives more even coverage. Glazes can be freely mixed.
- Don’t glaze the bottoms of pieces or they will glue to the shelf during firing. Sponge off any glaze that gets on the foot.
- Almost all glazes can be fired from cone 06-04 (04 is hotter). If you fire to 04 then you can bisque and glaze pieces in the same firing.
- If the children would like to be able to eat or drink from what they make, glazes are needed.
- The more even the coverage the better the result. Getting a glaze on too thick could cause it to run during firing, err on the side of too thin (e.g. two coats) to start.
- When you become more experienced you might consider mixing your own glazes (since the bottled ones are expensive).
The hazard label: Clay is non-toxic, the label refers to silicosis. This happens when people breathe the dust over a period of many years (thus farmers, gardeners can get silicosis). Keep the floor clean.
Glazes: Use white or transparent glazes on food surfaces, brightly colored elsewhere.
Clay Shrinks When It Dries
So if one part of a piece dries (and shrinks first) then when the other part shrinks a crack will occur.
- Dry things evenly (slow down fast drying sections or speed up slow drying ones).
- Drying things slowly usually means they dry more evenly (e.g. put a piece on a small board, drape a cloth over it and wrap it under, in two days it should be dry).
- To work on a piece again the next day, drape a cloth over it and put it in a plastic bag.
- Encourage the kids not to make things that are too thick or heavy. The more even thickness a piece is the easier it will be to dry without cracks.
Rolling Out Clay
Clay can be sticky, especially if it gets wet. Work on an absorbent surface (e.g. particle board, canvas board), no water is needed (that will just make a big mess).
“Slip” is used to join things (don’t use water, it will cause splits). Slip is just the clay with extra water so it can be painted. Make it by crushing some bone-dry scrap clay (into pieces smaller than marbles), then put this in water. It will slake and settle on the bottom in a few minutes. Pour off the water and stir it. The thicker and more creamy it is the better. Or, make a bowl shape from wet clay, put a little water into it and stir it around with a brush to work up a foamy, thick slurry; you can use that as slip.
Clay Dries Out if You Work To Long
Teach the children to plan what they want to make first. Then use a deliberate set of steps to achieve that. If they do not do this the clay will dry out too much as they work on it too long. Let them know it is OK to attempt a piece multiple times before they get success. Consider watching a few Youtube videos on handbuilding before introducing the clay to the class.
Google pictures for “clay animals”. Or “clay houses”. Or “clay hand building”. It is often a good idea to help the children make something they can use (like a bowl, plate or cup). To do that the clay needs to be rolled (google “rolling clay slabs” for a youtube video). Consider watching videos about ceramic masters to inspire the class (e.g. google “korean pottery masters” or “japanese pottery masters”).