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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 GlazeWhen 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
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 WeightOne 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.
Step 1. XXX feldspar (340) divided by total (790) = .4303
Step 2. Move the decimal point two places to the right: 43.03
Step 3. Round to the nearest whole number: 43
Step 1. Flint (200) divided by total (790) = 2531
Step 2. Move the decimal point two places to the right: 25.31
Step 3. Round to the nearest whole number: 25
Step 1.Clay (100) divided by total (790) = .1265
Step 2. Move the decimal point two places to the right: 12.65
Step 3. Round to the nearest whole number: 13
Step 1.Dolomite (150) divided by total (790) = .1898
Step 2. Move the decimal point two places to the right: 18.98
Step 3. Round to the nearest whole number: 19
As a mathematical check, the recalculated totals of 43, 25, 13, and 19 should add up to 100%. In some instances, the rounding process will not yield exactly 100% but 99.9% or 100.2%, which is perfectly acceptable for the glaze notation method.
Working with clay, glazes and kilns is often described as frustration waiting to happen. While it is true that ceramics can at times produce variable results, it is important to realize that some areas of inconsistency can be reduced or eliminated. Using the glaze description and notation methods enables potters to understand and compare various glazes. It also allows potters to use the same language and share information accurately.