Chemistry vs. Matrix Blending to Create Glazes from Native Materials

Description

Is it better to do trial and error line and matrix blending of materials to formulate glazes or is it better to use glaze chemistry right from the start?

Article

Suppose you have a material native to your area and want to create a pottery glaze using it. You want to maximize the amount used in the recipe. Popular wisdom suggests doing a matrix of blends with materials like feldspar, silica, kaolin, calcium carbonate, etc. Let's compare this method to a chemistry-assisted approach (one where we start at an existing recipe and move its firing properties in a certain direction using discretionary changes to its chemistry).

A glaze is much more than 'looks'. There may be ten blends that look OK on small test tiles, but only one that functions well. Function? That is about hardness, resistance to leaching, fitting your clay body, suspension and application properties, compatibility with coloring oxides, tendency to devitrify, blister, crawl, cloud, run, etc. Fixing these issues in a blend-created recipe almost always compromises the appearance. Consider crazing: It happens because of high thermal expansion, generally imparted by high Na₂O or K₂O in the chemistry. These come from feldspar. Where do all those high-feldspar recipes online come from? Feldspar was one of the corners on the triaxial blend and all the best-looking glazes were close to that corner!

Chemistry looks at a glaze as a formula-of-oxides. There is a direct link between how it fires and that chemistry. The materials in the recipe are thus oxide sources. Looking at glazes this way enables the use of limit formulas. Frits find a comfortable home in this approach, they are keys to melting glazes at middle and low temperatures.

Consider an example: A volcanic ash that I dug from a local quarry near Elkwater, Alberta. I had it analyzed at a lab. I converted the analysis to a unity formula using desktop Insight.

  CaO    8.7% 0.86 molar
  Na₂O   0.1  0.11
  K₂O    0.3  0.02
  Fe₂O₃  1.1  0.04
  MgO    0.8  0.11
  SiO₂  78.7  7.29
  Al₂O₃  2.2  0.12
  LOI   14.0

Notice how low the Al₂O₃ is. And the SiO₂ is very high. The silica:alumina ratio is thus 60:1 (a glaze is typically 10:1). So I don't want to blend this with materials that add more silica but I do want ones that add alumina. That eliminates feldspar! Kaolin fits the bill. And it will suspend the slurry, a real bonus.

This ash has a high CaO content quite a bit higher than a typical glaze (this is coupled with being very low in other fluxes). That means I need to add materials sourcing fluxes other than CaO. That excludes calcium carbonate and dolomite (imagine the waste of time it would have been to triaxial blend this ash with calcium carbonate and feldspar!).

As it turns out, it is possible to use up to 60% of this ash in a glaze to melt around cone 7 (the lesson link below, although dated and using the old desktop Insight software, demonstrates the principles of this). So, for this project, chemistry belonged at step one. Any blending would be to fine-tune things after that! One great comment I saw on Facebook by Paul Haigh was: "Chemistry guides, experiment decides".

There is an elephant in the room here: Firing. Potters generally seek reactive glazes and the firing schedule is often the key to the way they fire. But still, their chemistry still provides the main way to explains the results.

Related Information

Links

Projects	Oxides
Projects	Materials
Glossary	Wood Ash Glaze Common washed wood ash can supply important ceramic oxides when melted, so it can comprise significant percentages in a recipe. Plus it can produce unique visual effects.
Glossary	Feldspar Glazes Feldspar is a natural mineral that, by itself, is the most similar to a high temperature stoneware glaze. Thus it is common to see alot of it in glaze recipes. Actually, too much.
Glossary	Glaze Chemistry Glaze chemistry is the study of how the oxide chemistry of glazes relate to the way they fire. It accounts for color, surface, hardness, texture, melting temperature, thermal expansion, etc.
Glossary	Triaxial Glaze Blending In ceramics many technicians develop and adjust glazes by blending two, three or even four l materials or glazes together to obtain new effects
Glossary	Native Clay A clay that a potter finds, tests and learns to process and use himself. To reduce the costs of importing materials manufacturers, especially in Asia, often develop processes for clays mined in their locality.

By Tony Hansen
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