Glazing Basics: What Happens Between Dry Clay and a Finished Cup

Shaping a pot is only half the process. The other half, turning that fragile dry form into something glossy and watertight, happens in two firings with glazing in between. Beginners often find this stage confusing because most of it happens inside a kiln they cannot see into. Here is what is actually going on.

The first firing: bisque

Once a piece is bone dry it goes into the kiln for a bisque firing, which is a relatively low-temperature firing that turns clay into a hard, porous ceramic. After bisque the piece no longer dissolves in water, but it is still absorbent, which is exactly what you want. That absorbency is how glaze sticks.

What glaze is

Glaze is finely ground glass and minerals suspended in water. When you dip or brush it onto bisqueware, the porous clay drinks the water and leaves a powdery coating on the surface. In the second firing that coating melts into a thin layer of glass bonded to the clay. The colour comes from metal oxides. Iron gives browns and ambers, copper gives greens, cobalt gives blue.

Applying it

• Wipe the bisqueware with a damp sponge first to remove dust. Dust under glaze causes pinholes and crawling.
• Dipping gives the most even coat. Hold the piece with tongs or your fingers, submerge for a slow count of three, and lift out.
• Brushing takes patience. Three thin coats, each in a different direction, beat one thick one. Let each dry to a matte finish before the next.
• Keep the bottom bare. Glaze that touches the kiln shelf melts to it permanently. Wipe a clean band at least a few millimetres up from the foot.

The second firing: glaze

The glaze firing goes hotter than bisque, to the temperature the specific glaze is designed for. Glazes and clays have to match. A glaze made for a low temperature will run off and pool if fired high, and a high-temperature glaze fired low stays rough and unmelted. Your clay bag and your glaze jar both list a cone number, which is the standard way of describing firing temperature. Match them.

Opening the kiln

This is the best moment in pottery and also the most humbling. Colours will not be what they looked like in the jar. Thick spots will be darker. Thin spots will show the clay through. Keep notes on what you did to each piece, and after a few firings you will start to predict results. Until then, enjoy the surprises. Some of them will be the best things you make.