Ceramics Requires Several Hours of Intense Heating
Having covered How to Choose A Kiln, its now time to look at how to fire pottery and ceramic using a kiln. A good number of people are not aware that the beautiful ceramic serving dishes they use everyday are made from clay. Still a good number of people are not aware that the ceramic pieces they use goes several hours of intense heating in a kiln so that they come out shining like glass. Kiln firing is a process that uses a lot of energy and as such kilns are made of light insulating bricks and ceramic fibre which are heat energy efficient.
When clay start drying from the initial working condition, it becomes waxy and then leather-hand. It dries further and becomes chalky and bone-dry. When clayware is in these unfired states of moist state, leather-hard and bone-dry, it is called green-ware. If clay has dried past leather-hand state, wet clay can not be joined to the dry clay. Any incision into the surface of clay can only be done at or before leather-hard state. When clay dries, it losses some water and shrinks. Green-ware can not easily be glazed in that state because they are very delicate and will break if water is introduced to them.
Stacking for Bisque Firing
In order to make the clay pots/figures be porous and able to absorb water in glaze sufficiently, and be strong to handle, clay pieces are first bisque fired to red heat temperatures of 815˚C to 870˚C .In bisque firing, clay pieces in a kiln can be put on top of each other and on stilts but you can also stack them on kiln shelves. It is however important to consider weight distribution and shrinkage of clay pieces before putting them on top of each other – they can break.
Stages through Firing
When ceramic are fired, it involve four stages that clay has to go through, namely:
1. smoking water
Smoking Water and Dehydration
The first two stages, smoking water and dehydration are what bisque firing is about. Smoking water is the water added to clay during handbuilding and that water is removed from clay at boiling point of water. Dehydration is about removing water that is in chemical composition of clay and this is driven off the clay all the way up to 485˚C
Silica inversion is a point in firing cycle where silica expands and contract as it changes from alpha to beta quartz. Silica inversion occurs at 260˚C and again at 540˚C as the temperature goes upward and the same happens again as the ware cools as the temperature goes downward. Does silica inversion matters in firing pottery and ceramics? Silica inversions seem not to matter in all the firings.
Even Temperature in All Corners of the Kiln
A kiln can build very high temperatures within a very short time if all burners are open. The smoking water and dehydration stages have to be done at very low rates of temperature increases to avoid your clay figures breaking apart due to uneven expansion of clay as it contains water – yes, they do blow up into pieces and will sound like popcorns being made. The temperature throughout the kiln should be even in all corners of the kiln.
Bisque Firing Technique
1. With burners lit at the lowest flame possible, and with the kiln’s door partly open, raise the kiln temperature very slowly from a room temperature of say 20˚C to 120˚C at the rate of 1˚C temperature rise per minute. This should take you about 1 hour and 40 minutes.
2. With burners lit at the low flame, and with the kiln’s door partly open, raise the kiln temperature very slowly from 120˚C to 260˚C at the rate of 2˚C temperature rise per minute and at the rate of 2˚C per minute from 260 to 540˚C. This should take you about 3 hour and 30 minutes. Pause still for about five minutes at 260˚C and 540˚C.
3. At 540˚C, a mirror held close to the door’s peephole will not fog up – an indication the clay pieces have no more moisture to emit. Your ware is now out of danger of blowing up. Close the kiln door securely and open the burners more to fire from 540 ˚C to 870˚C in another one hour.
4. Total time taken to bisque fire to 870˚C is 6 hours 20 minutes.
5. After 870˚C is reached, put off the burners, and with the doors and peephole tightly closed, let the kiln cools by itself to below 100˚C before opening the door. This may take another 4 - 6 hours depending on the materials used in making your kiln.
After bisque firing, your ware is now strong, porous enough and ready for glazing. Wipe the ware with a cloth and water to remove chalky dust powder on the ware. Apply the glaze appropriately to your liking.
Stacking Glazed Pottery Pieces in a Kiln
Glazed pottery pieces must be stacked very carefully in the kiln. Pieces should not touch each other as molten glaze will boil and attach itself to the next piece. Leave a distance of 2 cm between glazed pieces. Feet or edges that support the pieces on the shelves should be free of glaze. You may (or you may not) use a kiln wash (a mixture of 50% kaolin and 50% silica) on shelves to protect shelves from molten glaze drops.
Firing Glazed Clay Figures
1. With burners lit at the lowest flame possible, and with the kiln’s door tightly closed, raise the kiln temperature gradually from a room temperature of say 20˚C to 120˚C at the rate of 3˚C temperature rise per minute. This should take you about 30 minutes.
2. With burners lit at moderate flame, and with the kiln’s door tightly closed, raise the kiln temperature at moderate rate from 120˚C to 260˚C at the rate of 4˚C temperature rise per minute and at the rate of 4˚C per minute from 260 to 540˚C, pause temperature still for about five minutes at 260˚C and 540˚C, and fire at the rate of 4˚C per minute from 540˚C to 870˚C. This should take you 3 hours and 20 minutes.
3. The temperature range from 600˚C upward is the oxidation stage where impurities will unite with oxygen and burn out. Vitrification also starts at this temperature range all the way up to 1304˚C.
4. At 870˚C, close the damper slightly more and adjust the burners to raise the temperature from 870˚C to 1037˚C at the rate of 3˚C per minute. This should take you about 1 hour.
5. After 1037˚C is reached, it now starts getting difficult to achieve a temperature rise. You now have to manipulate the burners and amounts of fuel getting into the kiln together with adjustment of the dampers - too little propane gas in the kiln will reduce the fire and too much propane gas will cool the kiln.
6. Reduction firing should start at 1037˚C. This is done by denying the gas flowing into kiln to have air (oxygen). In gas kiln you switch off air jets in your gas kiln (and in electric kiln you introduce moth balls inside the kiln). Iron reduces best at 1037˚C. Pause temperature still for 10 – 15 minutes at 1037˚C to facilitate the best reduction.
7. Move the temperature at the rate of 1˚C per minute after pause from 1037 to 1200˚C. Pause for 5 minute at 1175˚C for another reduction. Gaining a temperature rise in this temperature range can be difficult. Gas without enough air will not burn fast enough - open air jet a bit and then close again to continue with reduction. This should take you no more than 2 hours and 30 minutes.
8. Move the temperature at the rate of 2˚C per minute from 1200 to 1304˚C. This should take you one hour. Spend ten more minutes at the top temperature of 1300˚C to 1304˚C for glaze to mature fully.
9. This kiln firing of glazed pieces should take you about 8 hours 30 minutes.
Cooling the Kiln
Once you are through with firing up to 1304˚C, let the kiln cool gradually until after the temperature inside the kiln is below 100˚C. Kilns made with light insulating bricks and ceramic fibre should be able to cool below 100˚C in about 5 hours. Then open your kiln and inside there you will see your surprises. Take care – for what you see there can sometimes give you what? - A heart…. ?
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WillSteinmetz on July 28, 2011:
Dany on November 30, 2010:
Thank you for the detailed information! :D
Marshall on September 13, 2010:
Great summary and explanation of a difficult process
Hello, hello, from London, UK on September 12, 2010:
A fascinating subject and you explained in such details. Thank you.