Now that we can describe a glaze in more detail (see
Glaze Description and Notation), the next step is a standardized method for writing a glaze formula. We have all heard the phrase “let’s compare apples to apples,” which is exactly what a standardized glaze notation offers potters. A uniform system of writing or reading glaze formulas is a required element for the accurate transfer of information.
There are many variables when using and firing any ceramic material, which makes it critically important to be specific and exact wherever possible. Clearly, the written glaze formula is one area where uniformity and accuracy can be utilized to reduce the variable qualities inherent in the ceramic process.
100% Batch Glaze
When added up, the individual glaze materials should total 100% in the glaze batch. Gums, suspension agents, dyes, opacifiers, metallic coloring oxides and stains are listed after the 100% batch weight. Using a system where all of the glaze ingredients are listed as percentages permits one glaze to be compared to another.
Examples of additives that are listed after the 100% batch glaze include:
- Coloring oxides/stains: cobalt oxide, cobalt carbonate, copper oxide, chrome oxide, Mason black stain #6600, Alpine Rose #6001, Deep Sea #6244, Teal #6305, etc.
- Opacifiers: tin, superpax, opax, ultrox, etc.
- Gums: CMC, Vee gum CER, etc.
- Suspension agents: bentonite, Vee gum T, Macaloid, Epsom salts, etc.
- Dyes: used to tint raw glaze, dyes are organic colorants that burn out during the firing
This standardized method of glaze description also allows the batch weight of each glaze material to be easily increased. For example, if a glaze formula requires Custer feldspar 53%, the potter can simply add zeros for a larger batch of glaze. Custer feldspar can now be stated as 530 grams or 5300 grams. The same method of adding zeros can be used in other glaze materials to increase the total glaze batch.
All glaze materials listed in the 100% batch glaze (a glaze with no coloring oxides, stains, gums, suspension agents, or dyes) can be rounded to the nearest whole number. If a glaze requires 45.9% whiting, it can easily be rounded to 46% without changing the basic characteristics of the glaze. Any additional materials listed after the 100% base glaze (e.g., cobalt carbonate, bentonite, superpax, CMC, green dye, etc.) should be weighed out exactly as stated in the glaze formula. They should not rounded to the nearest whole number, as small changes can greatly influence color development.
Whenever possible, the mesh size of any raw material should be listed, as there is a great difference in the effect on a glaze whether using 200 mesh, 325 mesh or 60 mesh silica sand in a glaze formula. As a general rule, finer-mesh raw materials having more surface area will combine into a glass more easily than coarse materials. Also, several raw materials such as whiting can be processed in different mesh sizes, which can affect the ability of the glaze to remain in suspension. In addition, coarser-mesh whiting causes transparent glazes to become semi-opaque. In many instances, the generic name of a raw material is listed but not its mesh size. When possible, list the processor of a raw material or trade name.
Following these guidelines, a typical glaze formula should look like Table 1.
Simple Recalculation to 100% Batch Weight
One of the most misunderstood areas of glaze notation can occur when shifting an existing glaze formula into a 100% batch glaze. For this procedure, a simple pocket calculator is all that is needed. An example of changing a glaze to a 100% batch can be found in Table 2.
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