There are many kinds of kilns for sale today; you can get a reduction kiln, an oxidation kiln, a wood fire kiln, and many others.
The two main kilns I know about are
Oxidation kilns are more affordable and they do not have open flames, instead they have a series of coils, which radiate heat to cause the firing.
Firing by oxidation causes:
- lighter clay color
- brighter glaze colors
- glaze is more of a coating
- less change from the unfired product to the fired product
- limitations on colors. There are NO real reds, the closest thing to red you can get is a red brown.
Reduction kilns are different from oxidation kilns in that when they fire they use too much fuel and not enough oxygen to fire. Since there isn't enough oxygen in the kiln while it's firing the clay is chemically forced to give up the oxygen in it in order to burn; what results is a form of firing. Reduction firing results in:
- a darker clay body
- metallic specks in the glaze
- less detail (this form of firing is not good for art pieces that rely on detail glazing)
- earthier colors including REDS!
- a more fused appearance
- drawings will be fuzzy
- a glassier body
When choosing a kiln that is right for you, you must consider what type of glaze you want. With an oxidation kiln you get more stability, and products that don't change very much, but you are also more limited on colors. With a reduction kiln you get less detail, but you get more colors, deeper blues and greens, rich yellows, oranges and RED! I keep emphasizing this point because the kilns I worked with at school were oxidation and it really started to bother me when I would ask the teacher about a certain color and she would have to tell me "No we do not have that color because of the kilns." Consider what you want, red is important when you cannot have it.
Common Temperatures and Stages:
- Start the kiln at a very low heat, and it slowly soaks for four hours, after four hours the kiln heat should be at 220 Fahrenheit 890
- 220 Fahrenheit moister is all gone, make sure to keep your kiln unplugged so that the extra water can escape.
- 500 Fahrenheit silica melts
- 842 to 1112 Fahrenheit water starts smoking, chemical evaporation, all water is gone.
- 1063 Fahrenheit Quartz inversion, silica turns to glass.
- 1922 Fahrenheit Mulitite crystals form
- 2012 Fahrenheit clay matures completely.
Cones are used to measure relative heat index, while pyrometers measure the temperature. Since the humidity and lack there of throws off the pyrometer cones tend to be more accurate in that they are set to melt at certain points. In kilns heat is measured in cones, so be forewarned that a certain clay may be fired at 06 cones and a glaze may fire at 6 cones. Make sure you know the difference and are aware of that when you are firing.
Kiln shelves are usually made of ceramic that were mixed with a lot of alumina, and then coated with a mix of alumina that looks a lot like whitewash. The mixture helps keep the glaze that runs off your piece from sticking permanently to the shelf, since grinding some white stuff off the bottom is easier than chiseling the piece off the shelf.
Oxidation kilns have special linings to keep the heat in, they use a system of coils made of kanthol which is very easy to break, and bricks line the kiln; those bricks are made of a special clay that holds the heat in, but it's also very porous and easy to break. When loading and unloading your kiln take extra care not to knock the walls, as you will break them. Spun kalyoline can also be used to insulate the kiln, though it's hard to handle and biodegrades very quickly.
When loading a kiln you want to put the small things toward the bottom and heavy bigger things toward the top, this increases stability in your kiln. You also want to make sure there is enough room between things for airflow. Keep in mind that when firing bisque objects can touch, and even be carefully stacked, but when firing a glaze load objects cannot touch as they will melt together.
Infromation gleaned from a ceramics class. You can find out more on teh interweb, and have fun with ceramics!