Leaching Cone 6 Glaze Case Study

Description

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

Article

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.

Related Information

Links

Articles	Are Your Glazes Food Safe or are They Leachable? Many potters do not think about leaching, but times are changing. What is the chemistry of stability? There are simple ways to check for leaching, and fix crazing.
Articles	Is Your Fired Ware Safe? Glazed ware can be a safety hazard to end users because it may leach metals into food and drink, it could harbor bacteria and it could flake of in knife-edged pieces.
Materials	Copper Carbonate A source of CuO copper oxide used in ceramic glazes to produce a variety of colors (used only or with other colorants).
Materials	Copper Carbonate Basic This form of copper carbonate is the article of commerce, a mixture of theoretical copper carbonate and copper hydroxide.
Materials	Copper Oxide Black The purest source of CuO copper oxide pigment used in ceramic glazes.
Materials	Copper Oxide Red
Glossary	Leaching Ceramic glazes can leach heavy metals into food and drink. This subject is not complex, there are many things anyone can do to deal with this issue

By Tony Hansen
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