Pit firing is an interesting technique used by many ancient cultures, pre dating the development of glazes. It is basically a pit of the appropriate size dug into the ground into which layers of combustible materials and the bisque fired pieces are burned. It can also be done within a stacked brick "pit" built on top of the ground or even in a steel drum in a courtyard (must find a way!) Varied colours and surface effects on the work are achieved depending on the kind of clay body, the types of materials used for burning, amount of heat generated in the fire, where the pieces are placed and where and when the chemicals are added. Towards the end of the burning process, there is the option to cover the fire, which will cut off the oxygen supply and create a reducing atmosphere inside.
In 2012 I decided to do a pit firing in my backyard, This is the results of my first pit firing effort!
1. Firstly an appropriately sized hole was dug - in this case around 60 cm deep 1m long and the materials assembled sawdust, (dampened) twigs, newspaper, cow manure, bark, branches, jarrah and pine firewood.
Let’s take a minute to talk about pit-fire safety. Make sure there is no grass or other things that can catch fire around the opening of the pit. Have a something to put out any unwanted fires, a water source - a hose, failing that at least buckets of water. Pay heed to fire and burn off warnings and do not fire on fire ban days or when it is hot and windy.
2. The first layer was a slow burning layer of sawdust 10- 20cm deep, two different types here and the dark one was damp. When pots come into direct contact with sawdust , a deep black colour develops from carbon trapping in the smouldering sawdust. The finer the sawdust, the deeper the blacks. Copper carbonate and salt were sprinkled around at this stage to encourage the colour development.
3. Next I added my bisque fired pieces (made from burnished smooth, white earthenware paperclay, and BRT raku) are wrapped in newspaper containing copper carbonate, feathers, orange peel, copper wire and other goodies and placed on the sawdust amongst the straw, cow manure and paperbark. Some of the pieces are decorated with washes of black or red iron oxide and or copper carbonate. If a piece is placed in the sawdust it will become black due to the trapped carbon, whereas copper wire, copper carbonate, manure or even salt will result in flashes of reds, browns and even blues and greens if the conditions are right. This is referred to as colour fuming. Many things can be added to the pitfire to get varying colours - salt, seaweed, straw, cow manure, sawdust, banana skins, feathers, horsehair.... Just remember if you are adding chemicals to the fire, avoid breathing in the smoke.
4. Alternate more layers of newspaper, straw, twigs and small branches
5. Stack wood carefully on top using care to evenly distribute the weight, keeping in mind how it will settle as it burns
6. The pyre is lit! The paper and straw layer should allow for the fire to travel across the pit and it will not take long before you have a raging fire.
7. Covering the pit fire creates an atmosphere of reduced oxygen, which results in some lovely colour changes as the fire struggles to burn so it draws the oxygen from the iron in the clay and the sawdust smoulders.
8. It's hard to wait until it's cool enough!
Once the fire has burned down you should start to see the pieces poking through the ashes. They will appear white at first and the colours will appear as they cool and the air hits them.
Leave the pit to cool naturally overnight, and when cool enough to handle remove from the pit
9. Emerging from the ashy aftermath - Rub off the excess ash etc with a soft brush under tap. However, if you have used salt on the pieces don't run under water just brush off excess. The ash and bits stuck to the pots can easily be brushed off with a soft brush. It is not recommended to wash pots that have come into contact with salt because the salt will be pushed further into the clay and eventually the clay will begin to corrode, especially if left unsealed. I washed all my pieces before I knew this and as a result a few of my pieces have started to corrode in the few years since. Once the pieces have been cleaned off, they can be sealed/polished with a warm wax onto the still warm clay or with a sealer - polyurethane varnish, marble and stone sealer (gives that still wet look that deepens the colours and seals surface) or a terracotta sealer.
Most of the colours in these pictures are the result of the addition of copper and iron, as well as carbon trapping.
Check the gallery to find the end results of these pieces!