Ceramic Industry

Glaze Description and Notation

January 22, 2010

Cone 9 reduction, satin matt, semi-opaque, blue/green glazes.

Glaze description and notation offers clearly defined methods to classify and write glazes formulas. It is also a starting point to compare one glaze with another and adjust glaze formulas for firing temperatures, glaze defects and kiln atmosphere conditions. In short, glaze description and notation is the communication method we use to understand and transmit glaze-related information. Just imagine being in a foreign country and not understanding the language. Such a handicap would certainly limit your ability to travel or communicate.

What can be said about a glaze? Plenty! We have all been in the position of trying to describe our favorite glaze to another potter or to ourselves. If the actual glaze is not present, the verbal account can leave a lot to the imagination, which can produce an inaccurate image. The glaze description language is an agreed-upon meaning to the many characteristics found in glazes. It uses several key words and phrases to define a glaze. The idea, as with all languages, is that everyone understands what is meant by the standardized terms that classify a glaze. The glaze description language allows the listener or reader to formulate an accurate mental image of how the fired glaze looks and functions.

One area of potential error occurs when writing or describing glazes. Many potters still refer to a glaze formula by its popular name. For example, we have all called glazes Randy’s Red or Bob’s Blue. While there is a certain personal and informal quality in referring to glazes in such a manner, it can lead to numerous inaccuracies in exchanging or understanding glaze formulas. Just referring to a glaze by its common name does not tell us enough about its characteristics, such as color, texture or light transmission. As an added potential for error, the actual glaze formula for Randy’s Red could have changed many times since Randy developed the glaze. Identifying glazes only by their names is an inaccurate attempt to transfer information and can indicate a potter’s lack of knowledge about glaze formulas.

Even potters who know better are still drawn into the easy habit of referring to a glaze only by its name. However, there is another method that can transfer information with greater accuracy. Once the vocabulary is known, it makes the task of explaining how a glaze looks much easier, faster and more accurate. We're not talking about deciphering Sanskrit or Egyptian hieroglyphics; just a few simple categories can add to the enjoyment of making and describing ceramic glazes.

Keep in mind there are no set rules about the number of terms to use in describing a glaze. However, it is critical that any terms used have some universal understanding among potters. In addition, too many descriptive terms can be overly cumbersome to use in actual practice. Many potters choose only those descriptions that accurately focus on their particular glaze. Listed are several characteristics that will define a glaze in specific terms.

Firing Temperature: c/06, c/6, c/9
The firing temperature or pyrometric cone rating is critical in the description of any glaze. The three most common temperature ranges used today in ceramics are c/06 (1830°F), c/6 (2232°F) and c/9 (2336°F). Most clay body and glaze formulas have been developed around these temperature ranges. Published glaze and clay body formulas also fall within these widely used pyrometer cone and temperatures.

Preparation: Frit or Raw Oxides
Frits are manufactured from specific raw oxides, which are calcined or fired in a molten mass, cooled and then ground into a fine powder. Frits are frequently used in low-, medium- and some high-temperature glazes, acting as a major flux or glass former. A raw glaze is composed of non-fritted ceramic materials such as feldspars, clays, dolomite, whiting, zinc, talc, magnesium carbonate, and other naturally occurring raw materials.

Composition: Lead, Alkaline and Alkaline Earth
The composition of any glaze can determine its surface texture, light transmission, viscosity when molten, and color development when used with stains or metallic coloring oxides. The major (but not only) ingredients in glazes fall within three broad categories: lead-based glazes containing lead; alkaline-based glazes containing feldspars and/or frits; and alkaline earth-based glazes containing calcium, magnesium, barium, strontium and beryllium.

Texture: Gloss, Satin Matt, Dry Matt
The fired glaze can develop gloss, satin matt or a dry surface texture. A gloss surface is shiny and smooth to the touch. A satin matt surface is similar to a satin ribbon, with a semi-smooth surface texture, while a dry matt glaze can have a gritty surface. The surface texture of the glaze is not to be confused with its light transmission qualities.

Light Transmission: Transparent, Semi-Opaque, Opaque
The ability of light to penetrate the glaze layer determines whether the glaze is transparent, semi-opaque or opaque.

Color: Green, Yellow, Red, Blue, etc.
Glazes can be formulated in any color using stains or metallic coloring oxides. Color is often one of the major defining characteristics in describing any glaze.

Special Effects: Wood, Soda, Salt, Raku, Luster
Glazes can be developed for wood, soda, salt, luster, Raku or any number of applications or firing techniques.

Application: Sprayed, Dipped, Brushed
A glaze can be sprayed, dipped or brushed on the ware. The application method can also play a part in the total fired glaze effect.

Atmosphere: Oxidation, Neutral, Reduction
A glaze can be fired in an oxidation atmosphere (more air than fuel present in combustion), a neutral atmosphere (equal amounts of air and fuel), or a reduction atmosphere (more fuel than air present in combustion). The kiln atmosphere can alter any of the above glaze characteristics depending on the particular glaze formula and kiln firing cycle.

Glaze Hardness: Soft, Hard
While not typically a glaze characteristic that is noted, some fired glazes can be described as either hard or soft in their resistance to abrasion or solubility.

While it is not necessary (or, in many instances, required) to use every glaze description characteristic, an example would be: I am currently firing a c/9, fritted, alkaline, gloss, transparent, blue, soda-fired, sprayed glaze in a reduction atmosphere, and it has a hard-fired surface. More frequently, a description of this same glaze would be stated as: I am currently firing c/9/reduction, gloss, transparent blue, soda-fired glaze.

Standardization in describing glazes leads to accuracy in talking and writing about glazes, all of which can create a greater understanding of ceramic materials. Disseminating accurate glaze information to other potters will also prevent many glaze defects that are caused by the misidentification of glaze characteristics.