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Chemistry vs. Matrix Blending to Create Glazes from Native Materials

Section: Glazes, Subsection: Formulation


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 Text

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. Or you are a student and have an assignment to create a glaze from a selection of materials available to you. It can be difficult to know where to start. Current wisdom suggests doing a matrix of blends with materials like feldspar, silica, kaolin, calcium carbonate, etc. After quite a bit of work you would hopefully find a mix that melts well and looks good. 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 are a million blends that look well, but only a thousand that function well. Function? What about hardness, resistance to leaching, fit on your clay body, suspension and application properties, compatibility with coloring oxides, tendency to devitrify, blister, crawl, cloud, run, etc. Material level blenders tend to look at the visual and deal with this dizzying array of potential problems later. But a good glaze is a complex balance between working properties in production and physical properties of the fired result, this balance does not easily happen by tunnel-vision blending focused on the visual only. You cannot do blending to fix a crazing problem, for example; the high thermal expansion is a product of high Na2O or K2O, the materials in the glaze that contribute these oxides also contribute others that are critical to other aspects of the glaze That means fooling around with their percentages affects a lot more than just the thermal expansion. An example: the ceramic world is full of high-feldspar glazes that craze and settle like a rock in the bucket. Why? Because feldspar is the melter, and the trial blends employed to formulate these tended to crow-bar high percentages into the blend to get the rest of the concoction to melt into reactive glazes with good visuals. That produces crazing glazes. 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. When one learns how to recognize what a formula should look like, he does not waste time going down blind allies that deviate wildly and bring a host of issues. Chemistry-blind approaches shut out the key characteristic we benefit most from knowing.

Consider an example of developing a recipe to use your own native materials in the highest possible percentage. It is a volcanic ash that I dug from a local quarry, I spent $30 and had it analyzed. Then I converted the analysis to a unity formula using Digitalfire desktop INSIGHT software. 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 unusual. 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. Also, 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 (see the lesson link below, although dated because it refers to the old desktop Insight software, the principles of the chemistry are valid). 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 the obvious calcium carbonate, dolomite, silica, and feldspar would all have gone in the wrong direction in this case. Could you possibly have achieved close to 60% of this material in a recipe by chemistry-blind-blending? Deal with all the above issues in parallel? Not likely. My point? We need access to many tools when formulating glazes. Chemistry should come first, then optional blending if needed to fine tune; this is the opposite of what is taught in most circles today! One great comment I saw on Facebook by Paul Haigh was: "Chemistry guides, experiment decides". Ignore the chemistry and you are missing a key guide, you deny yourself and your glaze alot of options. There are a lot of dots to be connected, if you take away the chemistry dots you can no longer visualize the whole picture, they are the majority. You can fix some problems simply by changing firing, but you can adapt to almost anything, including variability, with the control of the chemistry.

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By Tony Hansen

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