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 your glazes or is it better to use glaze chemistry?

Article

You have a material native to your area and want to create a pottery glaze from 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. However, this method has some serious issues when compared to a full-chemistry or chemistry-assisted approach.

First of all, a glaze is much more than 'looks'. There may be a thousand blends that look OK on small test tiles, but only ten that function 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 ones were close to that corner!

Chemistry looks at a glaze as a formula-of-oxides and there is a direct link between the way it fires and that formula. The materials are oxide sources. Looking at glazes this way enables the use of limit formulas. And it knows how to use frits, these 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. This is unusual. And the SiO₂ is very high. The silica:alumina ratio is 60:1 (a glaze is typically 10:1). That means we definitely don't want to blend with materials that add silica but we do want ones that add alumina. That eliminates feldspar! Kaolin fits the bill. And it will suspend the slurry.

This material has a high CaO content, it will likely make a hard glaze, however high CaO can signal leaching problems. This ash is low in flux. This will be worsened after kaolin is added. That means we need to add materials that add fluxes other than CaO. That excludes whiting 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 (see the lesson link below, although dated because it refers to the old desktop Insight software, the principles of the chemistry are valid). Chemistry should come first, then optional blending if needed to fine-tune! One great comment I saw on Facebook by Paul Haigh was: "Chemistry guides, experiment decides".

Related Information

Links

Projects	Oxides
Projects	Materials
Glossary	Wood Ash Glaze Common washed wood ash has a chemistry akin to a ceramic glaze, 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.
Media	Desktop Insight 4 - Add a Native Material to MDT, Build a Glaze Learn to add a native volcanic ash to the INSIGHT materials database (MDT) and then create a glaze from it maximizing its percentage. Learn to impose an LOI on a material and why this method is better than line blending.

By Tony Hansen
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