Monthly Tech-Tip from Tony Hansen SignUp

No tracking! No ads!

Black Coring
Bleeding colors
Chrome Flashing in Ceramic Glazes
Clouding in Ceramic Glazes
Cracking of Clays During Drying
Dunting and Cracking of Clay Bodies During Firing
Foaming of Ceramicd Glaze Slurries
Glaze Blisters
Glaze Crazing
Glaze is Off-Color
Glaze Marks or Scratches
Glaze Pinholes, Pitting
Glaze Shivering
Glaze Slurry is Difficult to Use or Settling
Leaking of Fired Ceramics
Lime Popping
Orange Peel Surface
Over Firing of Ceramic Glazes and Bodies
Powdering, Cracking and Settling Glazes
Runny Ceramic Glazes
Specking on Ceramic Ware
Splitting at the Plastic Stage

Uneven Glaze Coverage

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.


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 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

Calcined alumina added to matte a glaze

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.


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
Ceramic glaze defects include things like pinholes, blisters, crazing, shivering, leaching, crawling, cutlery marking, clouding and color problems.
By Tony Hansen
Follow me on

Got a Question?

Buy me a coffee and we can talk, All Rights Reserved
Privacy Policy