Stoneware glazes are formulated to melt somewhere between about 1180 and 1300 degrees, which is where most functional studio pottery is fired. Because the body underneath vitrifies at that temperature too, the glaze is doing less work sealing the pot and more work deciding how it looks and feels.
What stoneware temperature changes
At stoneware heat the body itself becomes dense and largely non-porous, so a pot is watertight whether or not the glaze is perfect. That frees the glaze to be matt, satin, broken or textured without making the ware unusable, which is why the stoneware glaze shelf is so much more varied than the earthenware one. It also means glaze faults show as appearance problems rather than as leaks.
Fluidity and the kiln shelf
Stoneware glazes are melting harder and further than low-fire glazes, and the interesting ones are often the most fluid. A glaze that pools attractively in a bowl will run off a vertical wall and weld a pot to the shelf. Leave the bottom centimetre bare, wash the shelves, and test any new glaze on a tile with a vertical face before putting it on work.
Choosing a working set
A transparent, an opaque white or off-white, an iron dark and one colour covers most needs, and buying those four in the same range means they behave similarly and can be layered. Layering is where stoneware glazes earn their reputation: two ordinary glazes over each other frequently give something neither does alone, and a grid of test tiles is the cheapest way to find out which pairs work.
Buying wet or dry
Wet glaze is convenient and heavy to post; dry glaze is cheaper to buy and to ship and needs mixing with water and sieving. For the volumes a studio uses of transparent and white, dry is usually the better purchase. Weigh and mix dry glaze in a ventilated space with a fitted respirator, because everything in the bag is a fine silica-bearing powder.
Questions people ask about stoneware
What temperature are stoneware glazes fired to?
Broadly 1180 to 1300 degrees, and each glaze prints its own range. The glaze and the clay must overlap.
Can I use a stoneware glaze on earthenware?
No. It will not melt at earthenware temperature and the result is a dry, unmelted surface.
Why do stoneware glazes run more?
They are melting harder at a higher temperature, and the most attractive ones are often the most fluid. Leave bare clay at the foot and wash the shelf.