The Firing
After the pots have been made using a grogged stoneware clay and have been bisque fired to 1000 degrees Celcius, they are glazed using our own Raku glazes and allowed to dry.
When dry, pots are placed on a kiln shelf which is supported on three columns of broken kiln bricks to allow sufficient space underneath for the flame to circulate freely without hitting the pots directly. Only a few pots are fired at a time as the process requires you to remove the orange-hot pots from the kiln very quickly.
The gas burner we use is simply a conventional blow torch with a nozzle size of about 1 inch delivering 1250 grams per hour at 1 bar (if this technical bit is too much to take in your gas equipment supplier will advise you). Propane gas is supplied via a regulator connected to two linked 11 kg propane gas cylinders.
The burner nozzle is placed about an inch from the burner port and lit.
A piece of kiln shelving is placed partially covering the flue hole on the top of the kiln to act as a crude damper and a battery operated pyrometer is pushed through a hole about halfway up the side of the kiln. This isn't absolutely necessary as the correct temperature is judged by eye! but it does indicate how fast the temperature in the kiln is rising and allows you to adjust the damper opening to get the fastest temperature rise.
While the temperature is rising put the next load of pots on top of the kiln to thoroughly dry and warm up which seems to minimise cracking. Prepare a combustion container by putting a layer of sawdust in the bottom and laying a single sheet of newspaper on top of the sawdust. (I make my combustion container very simply, by laying a few rows of bricks on top of each other, without any mortar, to build a rectangular box on which I can place a sheet of metal as a cover)
As the temperature rises through red heat to orange heat keep an eye on the pots by looking down through the flue hole (Watch your eyebrows!!) and observe the changes in the appearance of the glaze surface
The first change you should see is the glaze surface taking on an almost sugary appearance called sintering. Next, the glaze surface will start to blister and bubble, a really ugly stage which is a bit disconcerting when you first see it happening. After a while the glaze surface will start to flow and smooth and you will know you are approaching the critical stage of the firing.
Make sure you have protected your hands with heat resistant gloves and that you have your raku tongs handy. Remove any drying pots from the top of the kiln and observe the glaze surface carefully. It should now present an orangepeel appearance, smooth but with tiny dents and pinholes. You will find a good torch useful at this stage to observe the way the beam of light is reflected from the glaze surface. Quite suddenly the glaze surface will smooth out, all the pinholes will disappear and the whole surface will look shiny and wet.
This is the time to quickly turn off your gas supply (at the cylinder not the burner!), lift off the kiln and place it safely to one side. Then as quickly as you can, use your long handled raku tongs to lift the still glowing pots from the kiln shelf and place them in the combustion chamber. The sawdust in the chamber will burst into flame and this is where you will need a helper to cover the pots with more sawdust until all the flames are covered and very little smoke is seen. Cover the container with a lid and leave.
This whole process will have taken about 30 minutes in an efficient kiln and you can now load the next lot of pots in the kiln and start your next firing.
After about 15 to 20 minutes use your tongs to remove the still hot pots from the combustion chamber and immerse them quickly into a large container of cold water (something like a dustbin/trashcan is useful). Again this is not strictly necessary but we have found that this seems to help fix the lustres and develop the crackles. When the pots have cooled enough to handle with bare hands, fish them out of the water and have a look at them. The first time you do this you will probably be bitterly disappointed. All you will see is a dull blackish pot. Don't worry! This is just a covering of carbon from the sawdust and you can soon remove this by scrubbing with warm soapy water to reveal those wonderful lustrous colours and crackle effects.
By now your next kiln load is probably ready for your attention. You don't have time to sit back and relax when you are using this exciting technique.
There isn't the space in this short article to cover everything so if you have any questions or run into problems don't hesitate to drop us an email.