Glazes can run down the sides and pool on the kiln shelves for different reasons. A very common reason is simply that the glaze is applied too thickly for that particular pot. The higher and more vertical the sides of the pot, the more likely a glaze is to run down the sides to the shelf. If two different glazes overlap, chances go up. If the two glazes don't have a similar base recipe changes go up. If the glaze is mixing with an oxide stain painted on the sides, chances go up. If the kiln fires to a temperature above what the glaze is meant to be fired at (e.g. cone 7 or 8 when our glaze recipes are for cone 6), chances go up.
When this happens, someone has to chip off the bits of glaze and pottery off the shelf. As most tasks in our pottery studio are done on a volunteer basis, that means that we don't get to buy a new shelf, we fix it. In the years I've been a member, I've lost count of the number of times I've had to do this. Below is my little photo-record of what happens. It's not the only method, but it's the method we use in our studio.
Step one: make sure you're wearing protective glasses. As you chip away, razor-sharp bits can fly in unexpected directions. Prescription eyeglasses don't do a good enough job. Buy and wear eye protection.
Step two: use your hammer and chisel and, as gently as possible to reduce the speed of any pieces that come off, chip away to remove the broken glaze and pottery from the kiln shelf.
If your shelf has a thin layer of kiln wash (more on that below), the pieces should come off relatively easily because the dripped glaze bonds to that instead of the shelf itself. If the shelf is bare, you can gouge chunks of the shelf while chipping away.
Step three: Sand off any remaining rough patches. This could be done many ways, or even skipped in some cases, but it helps to level out the shelf and can take off layers of kiln wash if needed. We try not to have thick layers of kiln wash as it can flake off during a kiln firing, swirl around in the air currents inside and stick into the glaze of your very favourite piece of pottery (it's like magic that way, knowing exactly which piece is the most precious).
I like to use a hard block sander because it minimizes the amount of fine kin wash that can be sprayed into the air. It tends to settle on the shelf and then we sweep it off.
Step four: Apply kiln wash by painting it to the shelf.
As we have an electric kiln and stoneware kiln shelves, we use a mixture of 50% silica and 50% kaolin-EPK. There are different recipes. Mix up the two dry ingredients. I like to put them in a closed, tightly lidded container and then go outside and shake them thoroughly. Let the container contents settle so that when you open it, you don't inhale the fine powder mix (trust me -- as potters we can't avoid all silica dust, but you can minimize your exposure). I've read to add enough water to get it to "pancake mix" thickness, but we get it a bit thinner than that.
Step 5: Make sure the kiln shelf is completely dry before you use it in your kiln. If you start the kiln firing while it's wet, it's more likely to flake off and wreck the glaze of the pieces in the kiln. A great time to reapply the kiln wash is while the shelf is still warm from the previous firing as it dries faster.
And that's it!
Now a big question -- if you're a member of our pottery co-op, would you be willing to help us out and take on fixing the kiln shelves and/or reapplying kiln wash every once in a while? It seems I'm the only person doing it right now and I could use some help. Contact me if you want a hands-on tutorial.
Tips for preventing glaze drips:
- If using a combination of two glazes where you have an overlap region, pick glazes that have the same base recipe. Studios can have a list a glazes with the same base to help you out.
- If you're taking a chance with glazes, make sure they have a minimal overlap area and have it far away from the bottom of the piece.
- Some potters put grooves or lines near the bottom of your pottery piece. If there are drips, they can get caught there.