Predicting Glaze Durability by Chemistry in Insight-Live

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

A. Insight-live

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I have a glaze that is scratching, cutlery marking. Why? Could the reasons be the chemistry? In this video I am going to look at the oxide makeup (using Insight-live) to see if I can answer that. Following is a transcript of the video.

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

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.

Links

Glossary	Cutlery Marking Ceramic glazes that mark from cutlery are either not properly melted (lack flux), melted too much (lacking SiO2 and Al2O3), or have a micro-abrasive surface that abrades metal from cutlery.
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

Cutlery marking here 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).

Click here for case-studies of Insight-Live fixing problems

You will see examples of replacing unavailable materials (especially frits), fixing various issues (e.g. running, crazing, settling), making them melt more, adjusting matteness, etc. Insight-Live has an extensive help system (the round blue icon on the left) that also deals with fixing real-world problems and understanding glazes and clay bodies.

By Tony Hansen
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