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Kiln Firing: Different Colors
We sometimes get questions from people who are having trouble getting their red, orange and yellow glazes to turn out brightly colored. Here are some tips for achieving these bright colors.
- Oxygen: Most commercial glazes are designed for use in an oxidizing environment (usually in an electric kiln). Red, orange and yellow are particularly sensitive to the amount of oxygen in the air. All clay has carbon in it, much of which burns out in the early stages of firing. This creates carbon monoxide, which will affect the glaze if it hangs around. So you want plenty of air flow, to remove this carbon monoxide as quickly as possible.
If you fire with a vent, you should be getting enough oxygen in the kiln. If not, make sure the top lid is propped and peephole plugs are out.With manual venting it is also best to put colors such as red on the top shelf where they will get more oxygen.Do not crowd the kiln, leave the pieces room to breathe.
- Firing Rate: Fire your bisque very slowly to Cone 04 to burn out the organics. Glaze firing is a little tricky. Sometimes it is recommended to fire fast and sometimes fire slow. The theory of firing slow is to ensure that carbon monoxide isn't being created faster than it can be replaced with fresh air. However, sometimes a fast glaze firing (3-4 hours) is preferred. This particularly with non-inclusion or cadmium red glazes where the color can burn out if fired slow. An example of glaze which should be fast fired is Amaco LG53, LG56 and LG66 which are the Brilliant Red, Orange and Yellow.
- Sometimes a glaze on one piece will affect the color of the glaze on another piece. This is particularly true if you are firing without a vent. Keep colors such as red separated from other colors for best results. Copper glazes are specifically known to interfere with these colors in many cases, so keep green colors away.
- Application: In most cases, it is best to apply the glaze thickly, sometimes as many as 4 coats is required. Amaco says to apply about twice as much as you would apply with a normal color.
- Firing temperatures.Make sure you use witness cones to see that the glazes are fired to the proper temperature. Do not over fire. Often these glazes do not like to go above Cone 06. Sometimes they do better fired at Cone 07 or even 08. On the other hand, bisque should be fired at a mature Cone 04 to make sure organics are burned out.
- Don't use red, orange or yellow glazes on greenware. Only use on bisque.
- It may help to fire your bisque hotter than usual, to burn out more carbon during the bisque firing.
- White clays usually are more pure (have less organic content). Therefore, they may give better results.
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