Staining of Fired Ceramic Glazes

Ceramic glazes are glass. That means they are always easy to clean, right? Wrong. If the surface is not developed it will be difficult or impossible to clean.

Details

Ceramic glazes are glass, but if the glass is not melted enough, its chemistry is not balanced or particles of refractory material remain unmelted the fired surface will not be micro-smooth. That means it will trap particles and stains and make them difficult or impossible to clean off. Glazes fire the way they do because of their chemistry. They need enough flux to melt, enough SiO₂ to develop a good glass and enough Al₂O₃ to be hard and durable. By comparing the chemistry and recipes of glazes that do and do not stain (in your account at insight-live.com) you can develop understanding of how to resolve this issue with any recipe.

We are discussing glazes intended for functional ware. Of course, certain decorative ones depend entirely on not melting completely. Others develop crystallization during cooling, these affect surface smoothness also.

Related Information

G2571A bamboo matte on the left is better, it does not stain

A pottery mug has a glaze the stains easily

These mugs are Plainsman H443. The bamboo glaze on the left (A) has 3.5% rutile and 10% Zircopax added to the base G2571A dolomite matte. The one on the right (B) has the same addition but in a base having slightly less MgO and slightly more KNaO. B stains badly (as can be seen from the felt marker residue that could not be removed using lacquer thinner). Why does A stain only slightly? It has an additional 4% Gerstley Borate (GB). GB is a powerful flux that develops the glass better, making the surface more silky. The differences in the recipe provide another advantage: (A) has a lower thermal expansion and is less likely to craze.

Adding calcined alumina to a ceramic glaze until it stains

The glaze is G2934 cone 6 matte base. Because it was not firing matte enough additions of 2, 4, 6 and 8% super fine calcined alumina were tested (Al₂O₃ is the key to matteness and this material is a pure source, if kaolin were added it would also source SiO₂ and require much more). Each addition made it progressively more matte. But with the mattness comes increasing susceptibility to cutlery marking and staining. To test the latter we marked each using a felt pen and then cleaned off the black ink using Acetone. The only one with noticeable staining is the 8% addition (the 6% addition has a slight stain also). The testing also showed no obvious cutlery marked on any of them. The results are reassuring since only 2% or less alumina is needed to achieve the degree of matteness desired so no danger of either problem is indicated. In addition, the integrity of the fired glass suggests that the alumina is dissolving in the melt - that means it is likely contributing to increased surface hardness and durability.

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.
Glossary	Ceramic Glaze Defects

By Tony Hansen
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