Decorative pit firing

What could be more enjoyable than spending an evening in the countryside with friends around a fire? Pit firing your pottery gives you the perfect excuse. You will need hole in the ground, sawdust, firewood, newspaper and some chemicals to stain the clay.

The pit

The pit should be about 2ft (60cm) deep, 3ft (1m) wide and long enough to fit all the pottery and firewood.

The pottery

You could probably pit-fire any pottery, but the ideal pot would be a large piece made of a white or light clay body that will show the darker colours of the chemical stains, and with large smooth areas to show off the swirls, smudges and splatters. The pieces must be bisque fired before the pit firing, since the pit firing will be too fast and uneven and greenware is likely to get broken while packing the pit.

Packing the pit

It is best to pack the pit before dusk so you can see what you are doing.

The bottom of the pit should be lined with about 3 inches (10cm) of sawdust. Arrange the pieces on the sawdust. The part of the pot in contact with the sawdust will be black. You may want so sprinkle some chemicals on the pots at this point. I'll talk more about chemicals later.

Next pack the pit evenly with firewood and some tightly crunched balls of newspaper until the pots are completely covered as in the picture above. When packing the wood, think about how it will settle as it burns. Take care to avoid situations where the burning wood pile will collapse suddenly and break the pots underneath. The wood should be softwood (e.g. orange boxes and pine), so that it burns evenly and completely, and is not heavy so that pieces which fall down do not break the pots.

Lighting the fire

It is best to wait until dark. Light the fire by lighting as many of the the balls of newspaper as you can reach. The wood should catch fire in an even and controlled manner. Don't throw gasoline on the pile.

Chemical decoration

As the fire burns down and the pots become visible through the ashes, throw more chemicals on the pots. This is a great deal of fun because the chemicals cause coloured flames. Powders cause cloudy diffuse patterns. Chemicals dissolved in a little water (or beer) make more sharply defined patterns, but need to be more accurately placed on the pottery.

Very important Protect you skin by wearing disposable rubber gloves. Make sure nobody is downwind, otherwise they will be breathing chemical fumes which could be poisonous.

Finishing up

Let the fire burn out and go to bed. Next morning when the pots have cooled, remove them from the pit and wash off the ash and loose chemicals with water.

Pots with smooth surfaces can be made more shiny by polishing with clear floor wax.

Chemicals

Always be careful handling chemicals. Keep them away from, food, drink, children and pets. Wear rubber gloves.

Most of the colours in these pictures are due to copper and iron. The form of copper or iron determines when it should be thrown in.

Oxides and carbonates melt at higher temperatures and can be thrown in at the beginning.
Chlorides are more dramatic, but melt at lower temperatures so they should be thrown in after the apex of the fire to prevent them evaporating off.
Crystaline forms are usually more effective than powders

Other ideas

Experiment with different chemicals, for example, ammonium or potassium chloride. Make sure to find out how toxic the chemicals are. I would avoid using any heavy metals (including lead).
Low fire glazes. Glaze your pots with a low fire glaze (cone 04/05). The pit firing is a reduction firing, so you can get metallic reduction. Pre-fire your glazed pieces to cone 04 to bind the glaze to the piece. This will make sure the glaze doesn't rub or chip off while you are packing the pit.

Problems

Expect some pots to crack due to thermal stresses caused by uneven heating.

Cracking can be reduced several ways. Make stronger pots. Use a clay body that responds well to heat shock (ask if the clay is suitable for raku firing). Do the pit firing in warm weather so the fire cools more slowly.

You can strengthen cracked pots with polyurethane varnish. Varnish the inside by pouring the varnish into the pot and pouring it back into the tin using the same action you would with glaze. Paint the outside with varnish normally.

For some reason, people always want to throw sticks and beer bottles into the fire. Don't. If the beer bottles fail to smash your pots, the glass might melt and stick to the piece. You will have an ugly fused lump (complete with bits of ash) or, more likely, the glass will contract at a different rate to the pot and pop off, leaving an ugly scar.

Don't cover the fire with sand. This causes massive reduction which will turn the pots completely black.

[ Project MAC | Stephen Adams ]

Last updated on 22 January 1998

stephen.adams@iname.com.