5 stages in the firing cycle

1. Water smoking- occurs when the temperature in the kiln reaches 212˚F, all remaining surface water is burned off, enough steam is often generated to be seen. Hence the term water smoking. Water boils and turns to steam at 212˚F. If the steam leaves the clay too fast, the pot explodes just like if you boil eggs too fast, they crack.

2. Dehydration- 660˚F, the chemical water, starts to burn off, by 950˚F the clay is completely dehydrated. It is now a chemically different material than it was when put into the kiln. Now it is aluminum silicate known as mullite. The change is non reversible.

3. Quartz Inversion - 1000°F. The quartz crystals in the clay undergo a change called the quartz inversion. The crystals grow and change in shape.

4. Oxidation - the burning off of organic materials occurs at 1600°F to 1700°F. This has no chemical effect on the clay, but it does leave the clay more porous.

5. Vitrification - Partial vitrification begins as the temperature rises. This will be determined by the type of clay body. Clay with a high alumina content (stoneware and porcelain) will vitrify more slowly and at a higher temperature than clay high in fluxes such as iron or talc (earthenware). Vitrification, you will remember, is melting of the clay platelets. Complete vitrification results in a glasslike material; therefore, clay ware is never completely vitrified.

Glazing and Finishing

Glaze- glass like coating put on pottery to make it waterproof, melts when fired, hardens as it cools

Glaze application techniques:1. Dipping

2. Spraying

3. Pouring/dripping

• Brushing/painting

…a little art history• The Chinese discovered wood ash floating through the kiln and when it landed on thepottery it created a shinysurface (but isn’t considered real gaze)

• The Egyptians were the first to use true glazes since about 5000BC. They dug their clay from the Nile river and used sand from the desert to make glaze.

Ingredients of glaze

• Glass former – (silica or sand) creates the glossy surface, silica forms glass all by itself. However, silica melts at about 3100⁰ F, which is much too hot for ceramic kilns. As such, it cannot be used on its own.

• Flux – aids melting, keeps surface of pottery from oxidizing so the glaze can attach, lowers the melting point of the silica.

• Refractory (alumina) – resists melting, aids in high temperature firing, stiffening agent.

Glazing Tips

1. Keep a “dry foot”. (no glaze on the bottom) Why???

2. Use a pencil (regular #2, not a mechanical) to draw a thick line around the bottom of the glaze edge. Why??

3. Glaze “takes” best on the first firing. Why??

4. Dip glazes once, (unless it’s thin enough, or you are crossing colors). Why??5. Brush 2 or 3 layers. Apply coats as soon as the sheen of the coat before isdisappearing. Why??

..Other ways to finish your piece

• Burnish- “to polish by

rubbing to a high sheen” Often with pottery, a slip is applied to the surface of the clay when it is in the late leather-hard stage then polished with a smooth rock or spoon.

• Sgraffito- a colored slip

is applied and often burnishedto the pottery surface then a design is etched or carved out of the applied slip.

…other ways to finish your piece cont...

underglaze

Photo transfer

Slip trailing Glaze transfers