The term low-fire in pottery refers to firing processes that are done at a relatively low temperature, typically cone 04 to cone 06. The term also describes clay bodies and glazes that are suitable for low-fire firing. The basic steps involved in firing low-fire pottery include drying the greenware, bisque-firing the pot, glazing, then firing again.
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Fully Dry Your Green Pottery
When pottery is first made, it is loaded with water. All atmospheric water must be evaporated as much as possible before the pottery is ready to be put into the kiln. Otherwise, problems with the clay body are quite likely to occur. Be sure to dry your pots slowly and evenly until they are completely bone-dry. The stage before bone-dry is leather-hard; this is when the moisture has nearly left the clay. It's important to be aware that ceramic work is at its most fragile during this time, when it is drying out.
Keep in mind that glazes and clay bodies must be matched for temperature maturation as closely as possible. In nearly all circumstances, low-fire clay bodies should be used with low-fire glazes. Mid-range or high-fire clay bodies will remain too porous or "punky" if combined with the wrong type of glaze. The wrong glaze also can have an undesirable effect on the color of the finished piece.
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The second step is to bisque fire your pottery. Load the kiln carefully and be sure to set the kiln sitter before placing the upper kiln shelves. (Most electric kilns are equipped with these helpful devices.)
When bisque firing low-fire clay bodies, you can fire the clay slightly higher than the glaze. Usually, this means you will bisque fire to cone 04 (make sure you get the zero in there!), which will tighten the clay body just a bit more than a lower temperature will. After bisque firing is complete, the clay contains no chemically bonded water.
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Once the pottery has been bisqued, cooled, and removed from the kiln, you can decorate it. Glazing your pottery can be an adventure of discovery, especially when you are using new glazes or using familiar glazes over a clay body of a new color. The possibilities with glazing are endless, and you can apply the glazes by dipping or painting. You can also experiment with effects such as marbling, transfers, or underglaze painting.
Remember to use wax resist and coat the pot's bottom and up it's outer sides 1/4 inch before using underglazes and glazes. Wax resist prevents the pot from sticking to the bottom of the kiln, which can crack the pot or ruin the kiln shelf.
Raw glazes are not at all the same color they will be once fired, which can stretch your imagination as you work to envision what the finished piece might look like. If you have examples of the fired glazes, you can use them for reference as you glaze your new pots.
It's important to note that raw firing is not as robust as glaze firing and is best suited to decorative pieces because it is not food-safe.
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Glaze Fire Your Pottery
Stack your glazed pieces in the kiln after they are completely dry (usually just a few hours). Always stack carefully, giving the pots 1/4 inch of clearance space on all sides and at the top.
The firing schedule for glazing is faster than for bisqueing. This is because the bisque firing has already transformed the clay into a ceramic material. Low-fire glaze firings are usually between cone 06 and 04, with cone 05 being very common. A typical firing schedule includes:
- two hours with ramp at 150 degrees F/hr
- three hours with ramp at 400 F/hr
- ramp at 120 F until the desired temperature is reached
Allow the kiln to cool down at its own pace until it is below 200 F. At that point, the kiln can be opened and your new, finished pottery can be unloaded.