If you are having someone else fire your work, you only need to know what cone you want it fired to, and whether it is a bisque firing or a glaze firing. Bisque firing means it is the first firing where clay is changed to ceramic. The glaze firing is a second firing where color and shine are added to the item.
I single fire many of the simple items I sculpt. Meaning I apply color and glaze right to the dry clay. It is then bisque fired and completed in one firing. Single firing is usually regarded as less consistent and prone to problems for pottery. But sculptures seldom need to meet the durability of pots, and the variation in color or slight crackle in the glaze, are not really a problem for many items.
If you are firing your own kiln and you have never done it before, be prepared for a nerve wrecking few hours. You should do a test fire first, with an empty kiln, and then fire a few simple things another day. This will help you get over some of the anxiety of working with such high temperatures. If you know a potter who can spare the time to come over, or a handyman friend, that first firing can be a little less stressful.
If an element fails, you will know because the temperature doesn't go up as it should. Shut it down, let it cool and repair the element ( instructions on you tube and various kiln sites ).
After a few firings you will find it quite a routine procedure and stress will be replaced by the anticipation of seeing the finished product. Just remember, kilns reach high temperatures. They should not be left unattended.
There are a couple of things you will find helpful if you are firing your own kiln. A pair of cutting goggles from your local hardware store will protect your eyes from IR rays, when looking in the peep hole. A pyrometer that will display the temperature if your kiln doesn't have one. It really helps to know what temperature you are at.
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Step 1: Firing
I like to divide my firing sequence into a series of steps.
One … heat to 95C and hold to dry the clay. ( about ½ hour )
Two … slowly heat to 150C and hold there a while, to drive off steam. ( 40 min. to reach 150 and hold 20 min. seems to work okay )
The above times will be longer for thick or complex items and shorter for simple, thin walled items.
Three … gradually raise the temperature to 400C. This is the phase that burns off much of the organic matter. ( usually about 60 min. to reach 400 )
Four … ramp up to the cone temperature at the desired rate and hold there for a few minutes. ( for earthenware I usually ramp at 150C per hour and hold 15 min )
Five … the kiln shuts down and I leave it to cool overnight.
For a second firing I move through the steps more quickly since the work is now ceramic and not prone to exploding from trapped moisture.
You can find cone charts on the net for different firing rates. They will give you a final temperature to aim for. However, in the end you will need to rely on witness cones to know that you are reaching the desired levels of time and temperature. Every kiln set up is different. Adjust your settings to slump the cone over. You will find a lot of information on pottery sites.
In sculpture the durability, strength, and water absorption are not as critical as in pottery. Being a little off on a cone firing is not usually a problem. But it is best to be as close as your equipment allows.
Step 2: Clay
Clay is generally divided into three basic categories. Earthenware, stoneware, and high fire or porcelain. I begin with earthenware or low fire clay. If you have another type, you can use it, and fire it to the low temperatures. It should work fine.
If you already have a red or brown clay, you can use it where I have used white firing clay. Glaze colors may be slightly muted but not much. For brighter reds, yellows and oranges, you can coat the dark clay with a white slip or white under-glaze to give brighter colors. This should not be necessary for beginning and simple sculpts but it is something you can explore as you go.
Clay is purchased at a ceramic supply place and comes in 50 pound boxes. There are two bags to a box and most outlets will sell you a 25 pound bag for about half the price of a box.
For this instruction I mostly use low fire white clay. Sometimes I use a stoneware clay but only fire it to earthenware temperatures. There are different types of white firing clay, made by different companies. Over time you should try several, to see which ones you like. Some fire to an off white color but all will take colored glazes well. How they feel and work, will probably be most important to you.
Some sculptors have a very high regard for paper clay. You can purchase this from suppliers or you can make your own. To mix your own use 1/3 by volume of wet paper fibers to 2/3 clay. You will need to thin the clay down and mix with a paint mixer on as drill. One mixed you need to dry it and wedge it ti make it a workable clay again.
Sources of paper fibers include:
toilet paper shredded in a blender ( small amounts )
insulation that was made from old newspapers from your hardware store
( this may need to be rinsed to reduce the fire retardant chemicals )( irritating to handle )
industrial cellulose fibers used in filtering
( cellite and fibracell are a couple of brand names that I can think of )( hard to find )
Or shredded newspaper ( a lot of work )
I only use paper clay certain projects. I find it can make smooth surface and small detail a little more difficult to achieve.
Step 3: Glaze
For the beginner I suggest using under-glaze colors which can be covered with a clear glaze. The top clear coat gives more depth to the color but the flat mat of an under-glaze is sometimes desirable. Available at ceramics supply outlets.
To begin with, you could purchase the small containers of commercial under-glaze. A few main colors should do. This will keep your cost down and you can buy larger containers once you know what you will use. Get a small container of clear gloss glaze as well, if your budget allows.
The glaze should be for low fire clay. This means it will fit the clay well and is not likely to crack too badly or have bits of it fly off in firing.
One of the appeals of ceramics is the vast array of possible finishes, but it is also a bit daunting to the beginner. I think it is easier to begin with under-glazes.
Once you have some experience, you can spend years exploring glaze and finish affects in more depth.