Glaze Explanation

Cones are used in ceramics instead of temperature. A cone is a special piece of clay shaped like a triangle that you put in your kiln. As it heats up it softens, and it is calibrated bend over when your clay and glaze have received the right amount of heat. So for example, if you use a glaze that says "fire to Cone 04", your glaze will be ready when a Cone 04 bends. The Orton Cone Chart gives the approximate temperature equivalents for the various cone, however, the cone actually falls based on the total amount of heat it absorbed, not the maximum temperature it reached, so the temperatures can vary.

Low fire glazes are typically fired at cone 06-04. High fire glazes are typically fired at cone 9-10. Mid fire glazes fall in between, and are fired at cone 4-6.

Low fire was historically used to achieve very vivid, bright colors. Mid and high fire were historically used to achieve more muted, earthy colors. But glazes have continued to improve and now many different glaze characteristics can be achieved at each of the firing temperatures. High fire is not often used in electric kilns because it is harder on the kiln and takes so much more power. Therefore, few commercial glazes are available in High fire.

Low fire takes less power, but mid-high fire which is fired using stoneware and porcelain clay, is stronger and more durable.

Glazes come in WET form (pre-mixed in pints or gallons), and DRY form which you mix with water and put through a sieve. WET is more popular in most cases and almost all Low fire glazes, but mid-High fire glazes are sometimes available in DRY. Typically WET glazes are designed for brushing and DRY glazes are designed for dipping or pouring. DRY glazes can be made brushable with the addition of a brushing medium.

Underglazes are used to get consistent colors that stay where you put them (regular glazes have a tendency to move around during firing). Sometimes they are left as is, and sometimes are covered with a clear or translucent glaze and refired.

Overglazes are primarily accent products applied over pieces that have previously been glaze fired. They typically are lustrous or have special pearlescent qualities. The piece is refired with the overglaze at very low temperatures, such as cone 016-018. When using Overglazes, apply them to surfaces that are free of dust, skin oils, and lotions to ensure proper adhesion. Wipe surfaces with rubbing alcohol prior to application (except when used over crackle glazes.)

Stains and Oxides can be wedged into clay to color it, added to clear glazes, or made into a thick liquid and used for drawing on or coloring clay, similar to a slip.

Slips are basically watered down clay, usually with color added. Engobes are similar.

Acrylic Paints are used to paint bisque (clay that has already been fired once). Paints are not re-fired in a kiln.

There is oxidation firing, and reduction firing. Reduction keeps the oxygen from reaching your glazes. Oxidation is typically done in an electrical kiln, basically because it is hard on the electric kiln elements to do reduction. Temperatures range from low fire to high fire. Reduction is typically done in gas firings, where it is usually high fire, and is used in other firings types such as Raku which is low fire.

Most of the prepared glazes you can buy are for oxidation. However, some can be used in reduction.

Most of the prepared glazes you can buy are formulated for brushing on. Chemicals are added to keep the particles in suspension and the glaze easy to brush on. Usually 3 coats of glaze are required.

Many commercial glazes are also available DRY, to be mixed with water and sometimes a suspension agent. A 25 lb bag of DRY glaze will typically make about 3 gallons of glaze.

Although some commercial High fire glazes are available, most people use hand-made glazes for high fire work. These glazes are based on recipes and use glaze chemicals such as those sold at BigCeramicStore.com. These glazes are typically mixed to be used for dipping or spraying. The Excellent Ceramic Links (to be linked) page lists many sources for glaze recipes. Making your own glazes can be an intense scientific exploration, as there are many factors that affect how well they perform: firing type, firing speeds, clay body used, etc. To make your own glazes you need a good mask (many of the chemicals are poisonous), a variety of sieves and large buckets, an accurate scale, and a great deal of patience!

Please note that there are many factors that affect how well any glaze performs. Getting the right fit of the glaze to the clay body is extremely complex and fills many books. If your glaze and your clay don't "fit": bond together, expand at the same rate, etc, your glaze will craze or crack or the piece can even crack. The benefit of commercially prepared glazes is that they have been formulated and tested to work in a variety of conditions, they are very stable (don't run all over your kiln shelves), and they are easy to get and use.

We definitely recommend that you get some basic books on ceramics as you get started!

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