•The secret to cool bodies and glazes is a lot of testing.
•The secret to know what to test is material and chemistry knowledge.
•The secret to learning from testing is documentation.
•The place to test, do the chemistry and document is an account at https://insight-live.com
•The place to get the knowledge is https://digitalfire.com

Sign-up at https://insight-live.com today.

Leaching Cone 6 Glaze Case Study

Section: Glazes, Subsection: Food Safety


An example of how we can use INSIGHT software to determine of a glaze is likely to leach

Article Text

Following is an example of how we might look at a glaze and determine if it is food safe. Here is a letter we received:

"I have a receipt for a matte turquoise glaze which I am using on your M350 clay and I was wondering if it was food safe. The following is the recipe.

Turquoise Glaze
Dolomite          12
Kaolin             5
Gerstley borate   12
Potash feldspar   12   
Nepheline syenite 23   
Silica           5.5   
Strontium carb    15   
Ziropax           10   
Copper carb        5

Everyone I ask about the strontium carb. gives me a different answer about it's safety. I would like to be sure it is food safe before I use the glaze on functional pottery. I would appreciate an answer from you. Thanks."

It appears that strontium carbonate could be the least of this glazes problems. Here is the chemistry of the glaze as calculated by INSIGHT:

 CaO       0.32*
 MgO       0.21*
 K2O       0.09*
 Na2O      0.12*
 SrO       0.27*
 Al2O3     0.26 
 B2O3      0.11 
 SiO2      1.53 
 ZrO2      0.14 

Other articles on this site talk about balance in the chemistry of glazes and SiO2 and Al2O3 content are of primary concern in this regard. This glaze is extremely low in SiO2 (it is 1.5)(recommended minimum is 2.5), that leads to unstable glasses that leach. You could simply increase the silica in this glaze but it is so low that it needs to be increased to 30 parts in the glaze recipe to supply the minimum amount of the oxide SiO2! That is certainly going to affect the appearance of the glaze.

Copper is a destabilizing influence in many glazes. Often non-leaching glazes will begin to leach after copper is added. This glaze has lots of copper. Leach testing is obviously needed.

The recommend maximum for SrO is around 0.2, this has 0.27, that is likely too high (INSIGHT has built-in limit formulas that assist in determining minimum and maximum amount for oxides).

Perhaps you would agree that the writer should go back to the people who said this glaze is safe and ask them what they were thinking. We cannot guarantee that it is not, but certainly a simple leaching test is advisable. I am guessing overnight in vinegar will leach out the color at least.

Out Bound Links

In Bound Links

By Tony Hansen

Feedback, Suggestions

Your email address


Your Name


Copyright 2003, 2008, 2015 https://digitalfire.com, All Rights Reserved