An unglazed pot is chalky and porous. Run a thumb over it and it drags. Pour water in and the water slowly leaves. It is, functionally, a slightly useful rock.
Glaze is what turns it into something you can eat from. And the strange part is that glaze does not look like glass when it goes on. It looks like thin grey porridge.
What glaze actually is
Glaze is finely ground rock and mineral suspended in water. Silica for the glass itself. A flux — often feldspar — to lower the temperature at which that silica will melt. Alumina to stop the whole thing running off the pot and welding it to the kiln shelf.
Mix those, add water, and you have a liquid that dries to a dull powder coating. Nothing about it suggests the finished surface.
Two firings, not one
Most stoneware goes through the kiln twice.
The first firing — the bisque — takes the raw clay to around 1000°C. It drives out the last chemical water and turns the piece hard enough to handle, but leaves it porous enough to drink up glaze. A bisque-fired pot dipped in glaze absorbs the water and leaves the mineral behind, evenly, in a second or two.
The second firing is where the work happens. Taken to about 1280°C, the fluxes melt, the silica dissolves into the melt, and the whole coating becomes molten glass sitting on a clay body that is itself beginning to vitrify. As it cools, the two shrink together and lock.
Why 1280°C matters
That temperature is not arbitrary. Below it, the clay body stays porous — fine for a plant pot, poor for a mug that lives in a dishwasher. At stoneware temperatures the body itself becomes glassy throughout, absorbing almost no water at all.
This is the practical difference between earthenware and stoneware. A chipped earthenware plate will slowly take on moisture and stain. A chipped stoneware plate is inconvenient but inert.
The glaze break
Look at a well-made mug where the glaze meets the raw clay foot. The glaze does not stop at a hard line. It thins out, and as it thins it changes colour — often going lighter, sometimes showing the clay through it.
That transition is called the break, and it happens because glaze thickness changes how the colour reads. Thick glaze pools in throwing rings and shows deeper; thin glaze on a rim or an edge shows the body beneath.
It is also why two pieces from the same batch are never identical. A fractional difference in dip time, a slightly thicker wall, a different position in the kiln — any of it shifts where the break falls.
What goes wrong
Plenty.
- Crazing — a web of fine cracks in the glaze, caused by the glaze and body shrinking at different rates as they cool. Cosmetic on decoration, a real problem on a food surface, because the cracks hold what you put in them.
- Crawling — glaze pulling away from the surface in the melt, leaving bare patches. Usually dust or oil on the bisque.
- Pinholing — gas escaping the clay body through the molten glaze and leaving craters behind.
Most of these come down to something invisible at the moment of glazing: a fingerprint, a fractional difference in mix, a kiln that cooled faster on one side.
Why any of this is worth knowing
Because it explains the thing people notice first and struggle to name: why a good piece feels different in the hand.
A properly fired, properly glazed pot has a surface that is glass fused to stone, not paint applied to clay. It will not scratch off. It will not stain. It will outlast the kitchen it was bought for.
That is the whole promise, and it comes down to a few hours at 1280°C.