Section: Glazes, Subsection: Formulation

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 ceramic chemistry?

Article

Perhaps you have a material native to your area and want to create a glaze from it. Logically you want to maximize the amount used in the recipe. It can be difficult to know where to start. Current wisdom suggests doing a matrix of blends with materials like feldspar, silica, kaolin, whiting, etc. After quite a bit of work you would hopefully find a mix that melts well and looks good. However this method has some problems I'd like to address.

Consider a volcanic ash I dug from a local quarry, I spent $30 and had it analyzed. Then I converted the analysis to a unity formula using INSIGHT. I'll dub it Elkwater Ash.

  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 now low the alumina is. This is very unusal. Also the silica 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. Hmmm. No practical material qualifies. That means we will have to tolerate a material with lots of Alumina and minimal silica contribution. Kaolin fits the bill and it will suspend the slurry. Adding feldspar is thus out of the question, it contains far too much silica.

This material has a high CaO content, it will likely make a hard glaze, however high CaO can signal leaching problems. If you know about formulas you'll see from this one that this material is low in flux. This will be worsened after kaolin is added. That means we need to blend it with materials that add fluxes other than CaO. That excludes whiting and dolomite.

As it turns out, it is possible to use up to 60% of this material in a glaze to melt around cone 7 if you use the above guidelines to select what materials to mix it with. Then some line blending could be done to fine tune additions of colorants, opacifiers and variegators to produce something with an interesting surface.

The typical approaches of blind line blending with whiting, dolomite, silica, and feldspar would all have gone in the wrong direction in this case. Even if you could deduce the chemistry by blending techniques, could you achieve close to 60% amount in a glaze? Deal with all the above issues in parallel? Not likely. My point? We need access to many tools when formulating glazes, chemistry is one. Chemistry should come first, then blending; this is the opposite of what is taught in most circles today! Ignore the chemistry and you deny yourself alot of options.

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