Predicting Ceramic Glaze Durability by Chemistry

How to spot out-of-balance indicators in the chemistry of glazes that suggest susceptibility to scratching or cutlery marking.

A. Insight-live

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Cone 10 Cutlery Marking Glaze
Gersley Borate 11.364
Dolomite 6.182
Whiting 8.182
Custer Feldspar 26.636
Kaolin (EPK) 6.182
Talc 12.273
Silica 21.909
Superpax 7.273

I have a glaze that is scratching, cutlery marking.
Why? I am going to look at the chemistry to see if we can answer that.

Go to Insight-Live (I am already signed in).
Add new recipe.
Show the Import box, Paste (ctrl-v), Confirm and Interpret.

Two materials are not linking to a material in the materials database.
Search EPK, it is a reference material, show which names can be used.
Gerstley, spelled wrong.
Edit, fix names, Save and Done editing.

What does the chemistry of a typical cone 10 glaze look like?
Advanced Search, Limits, Search, choose Green and Cooper.

These are materials, these are oxides.
Materials decompose to oxides during firing.
Oxide arrange themselves into the glass structure as the kiln cools.
Stable structures happen when their relative amounts are within some sort of reasonable proportions.
These limits are about what a strong, wear resistant glass looks like.

MgO: High
Al2O3: Super low (half the minimum)
SiO2: Far below the minimum

Compare with a base cone 6 transparent and matte (even lower than them).

Why is it not melting and running down off the ware? The high MgO is stiffening the melt.
This is likely a special purpose reactive glaze, but it is clear best for decorative surfaces, not functional.

Is this likely to leach? Yes.
The strength and hardness and SiO2 and Al2O3 impart also produces a more stable and therefore less-leaching surface.

Cutlery marking is directly related to the chemistry of the glaze

Cutlery marking is directly related to the chemistry of the glaze

This is an example of cutlery marking in a cone 10 silky matte glaze lacking Al2O3, SiO2 and having too much MgO. Al2O3-deficient glazes often have high melt fluidity and run during firing, this freezes to a glass that lacks durability and hardness. But sufficient MgO levels can stabilize the melt and produce a glaze that appears stable but is not. Glazes need sufficient Al2O3 (and SiO2) to develop hardness and durability. Only after viewing the chemistry of this glaze did the cause for the marking become evident. This is an excellent demonstration of how imbalance in chemistry has real consequences. It is certainly possible to make a dolomite matte high temperature glaze that will not do this (G2571A is an example, it has lower MgO and higher Al2O3 and produces the same pleasant matte surface).

Out Bound Links

  • (Glossary) Cutlery Marking

    In glazes with this fault, rubbing a metal knife or spoon on the surface will leave black marks that cannot be completely rubbed off. This is a common fault in glazes, especially mattes. Even commercial tableware often exhibits this problem. It usually happens because the micro-surface of the glaze ...

  • (Troubles) Glaze Marks or Scratches

    Questions to ask and strategies to try to deal with glaze cutlery marking, that is, glazes that are too easily scatched by metal.

  • (Oxides) Al2O3 - Aluminum Oxide, Alumina
  • (Oxides) SiO2 - Silicon Dioxide, Silica

By Tony Hansen

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