Firing a Test Kiln

Small "L" shaped test tiles were dipped in the glazes to determine variances with application thickness. The glaze was thicker near the tops of the tiles by only dipping the second layer half way up the first dip. Depressions were made in the tiles to have some recesses for the glazes to settle in. Some glazes can be one color when applied thin, and completely different when applied thick.

Take good notes so you can reference them later.
You can use an underglaze pencil or an iron oxide wash to write numbers, letters, or some other designation on each of your test tiles. This will allow you to match the tiles to your notes accurately. When testing raku glazes, in the reduction stage the clay body may turn completely black, so you will need to carve, scratch, or impress your symbols into the tile before it is bique fired, otherwise the carbon will cover up any underglaze pencil or oxide. You could also use some white raku glaze to designate your tiles.

Depending on the kiln, size of piece, glaze, and temperature to be reached, the firing program will change.
Ceramics often undergo several different firings before the piece is complete. The first firing is called the "bisque" firing, and is usually lower than the final firing. Temperature is usually cone 05 (1915°F) or cone 04 (1958°F).
The next firing, sometimes called "glaze firing" will be determined by the clay body and the glazes used. Cone 6 (2232°F) is common in electric oxidation firings, while cone 10 (2377°F) is most common for gas fired pottery.
China paints, lusters, decals, and overglazes are fired a third time to around
cone 018 (1350°F) to cone 020 (1180°F). Be sure to follow the instructions on the product about kiln venting and proper temperatures.

Clay is used to hold these larger cones at a slight angle. The first cone to melt should be closest to the depression made in the cone pack. The highest temperature cone will be on edge of the cone pack. Holes were poked into the clay using a fork, to eliminate any air pockets and to speed drying. Be sure you can see the conepack through the peephole before as you load the kiln. You can use several conepacks in a large kiln to check for variances in the kiln. Be sure to use the appropriate clay body to form your cone packs, you wouldn't want to use low-fire terracotta to hold cones for a cone 10 firing. When possible, use a soft brick or piece of broken shelf under the conepack, and always coat your shelves with kilnwash in case a glaze wants to run. Remember, glaze is glass, when it sticks to a shelf it must be removed with a chisel or ground off with an angle grinder. The broken glaze on your shelf can be very sharp, so wear gloves, or believe me, you will eventually cut yourself. Always wear a respirator when grinding.

A cone is inserted between three prongs on the inside of the kiln. A weighted lever is raised with the hand, and the cone physically keeps one prong up, in turn keeping the lever up. The small white button is pressed, activating the kiln sitter. The control knob is adjusted, and firing begins.

These are the pyrometric cones for use with electric kilns equipped with a "kiln sitter". It is a mechanical device that shuts off the kiln when a determined temperature is reached. Special pyrometric cones are used as the measuring tool for high temperatures of kilns. Each cone has a number according to a temperature chart. In larger kilns, the cones are watched through the peepholes for "sagging". When the cone begins to melt, the temperature is met and the kiln must be manually shut off. The kiln sitter does this watching and shutting off for you.

A small test kiln is ideal for quick firings. This kiln has one control knob, and two peep holes. The peep holes are usually plugged during firing, but sometimes left out to vent the kiln, or provide a quicker cooling time. Firing decals is one example of when the peeps are left open.

This larger electric kiln uses metal coils instead of carbon fuels. The temperature is reached with oxygen present, so this is called an oxidation atmosphere. Low firing commercial glazes are designed for this atmosphere. Each coil of wire has its own control, fuse, and neon indicator light. This kiln has three seperate layers that can be stacked or unstacked for delicate loading of awkward pieces. The seperate units then plug into the control box. Kilns of this sort usually run on 240 volts, although smaller kilns may use a standard 120 volts. There is most always a metal plate on the control box that will give you the voltage and amperage rating. If the rating is only in watts, you can do some simple math to calculate the amperage used. be sure the breaker is rated for more than your kiln will be using, you don't want to burn your studio down with an electrical fire within your walls! The metal plate will also have

Firing a test kiln is a good idea before committing a full kiln load to chance. A large gas kiln like this one can hold several hundred pieces. Natural gas is used to reach cone 10 (2377°F). The fuel supply is increased beyond the level of oxygen in the kiln, and flames will begin to come out of the peepholes. This is called a reduction atmosphere. Amazing glazing effects are achieved on handmade pottery using high temperature glazes in a reduction atmosphere. Unfortunately, reduction atmospheres are not possible in electric kilns, so the only opportunity you might have to test new glazes will be in a fully loaded gas kiln. Small gas kilns are a rarity, so search out someone that repeatively fires theirs, and help them load or unload it sometime in trade of a small corner of a shelf to place some of your test tiles.

Some glazes will react differently with different speeds of firing. If you are having problems with pinholes or crazing (small hairline cracks) of the glaze surface, try slowing your firing schedule down a bit to give the glaze more time to become fluid. Always allow your kiln to cool down slowly to prevent ware breakage, glaze crazing, and prevent kiln brick cracking. Electric test kilns only provide oxidation firings, so take that in mind if testing raku glazes.

The kiln program for cone 06 glaze testing using the this small test kiln and 1/4 inch thick tiles would be something like this:
Low 1-2 hours
Med 1-2 hours
Hi until cone drops (within 2 hrs), for a total of 4-6 hours.

Raku glaze testing can be somewhat tricky, due to the nature of the firing. Each raku firing produces somewhat unique results, and can be hard to recreate even with careful reproduction of the exact process. Ideally, there would be at least two seperate treatments for the testing of a single raku glaze. Oxidation firing that could either encompass allowing the test tile to oxidize and cool slowly in the open air, or pull the oxygen from steam from rapid cooling by plunging into water. The reduction firing is the most varied. Different levels of reduction occur within a small surface area, causing the same glaze to yield a dramatic collection of possible metallics, irridesents, glossy, and matte finishes on the same piece. Commercial glazes not normally labled or intended for raku use can be used in combination with "true" raku glazes and unglazed portions of a piece with excellent contrasts.