Do you know the top five reasons why pots stick to kiln shelves? Knowing can save you hundreds of dollars in shelf replacement, as well as spare you the unhappiness and frustration of finding your pots welded to your kiln shelves. Most of the time, a bit of careful inspection before the firing save you hours of head-and-heart ache.
1. Glaze on Pot Bottoms
The absolute number one reason pottery sticks to kiln shelves is simply that their bottoms were not completely pristine and glaze-free. Even the smallest particle of glaze (and some underglazes and stains) can weld a pot to a shelf. This is why it is important to wax pot bottoms or else use extreme care when glazing to ensure the bottoms remain glaze-free.
Some potters advocate the use of sponges on their wheelheads to clean pot bottoms, but I do not suggest this. It is much safer and a more certain method to simply avoid glaze on the bottoms at all.
Also remember to clean waxed bottoms. Glaze beads up on the wax, but it must be completely wiped away or it will inevitably be caught between the pot and the shelf, leading to sticking.
2. The Glaze Was Too Fluid
Glazes that drip or run down to the kiln shelf will also weld a pot to the shelf. In these cases, the pot may come off the shelf relatively easily, but the damage to the shelf will continue to grow worse with every firing. Eventually, a kiln shelf can lose months or even years off its normal working life.
In addition, the pots are also damaged. Grinding the drips may allow the pot to be put into service (even if just as a "second"). If grinding, use proper eye and dust protection. Glaze is a form of glass and needs to be treated with respectful acknowledgement of its dust hazards.
3. The Kiln Shelves Were Not Clean
Any glaze particles left on a kiln shelf will re-melt and weld anything on top of them to the shelf were the particles are. Even small glaze particles that are left on the shelf can cause a pot to stick.
Covering a glaze drip with kiln wash will not protect pots in future firings! The glaze will flux and can eat through the kiln wash, both downwards into the shelf, and upwards into the pottery.
After every firing, kiln shelves need to be inspected for cracks, damage, and any glaze or clay particles. Kiln shelves should be thoroughly cleaned, with any old kiln wash scraped off as much as possible before the shelves are stored away for the next firing.
4. Improper Kiln Shelf Protection
Different kiln washes are designed for different types of firing. Salt glazing, for example, needs a different formula than a straight-forward bisque.
Many potters are trying a non-kiln wash protective coating. Alumina hydrate mixed with water seems to work for some potters, but it is not an all-around solution. It can be messy in the kiln and can interfere with electric kiln elements. It is also not as good at stopping wholesale glaze drips or protecting shelves from less-than-pristine pot bottoms. I would not suggest using alumina hydrate to protect your shelves unless you have absolutely clean pot bottoms.
Let's face it. If a kiln is significantly overfired, the pottery within it will almost certainly be welded to the shelves. Remember, in this case it may not just be the glaze that is welding to the shelves. Clays will also flux out if fired high enough. For example, if you fire a low-fire white clay body to cone six, you are almost certain to have a pot welded to the shelf, no matter how well the shelf was covered with kiln wash or alumina hydrate.