An example of how we can use INSIGHT software to determine of a glaze is likely to leach
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
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.
A step-by-step process to put a liner glaze in a mug that meets in a perfect line with the outside glaze at the rim.
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.
Synthetic Malachite, CuCO3
Cupric Carbonate, Copper (II) Carbonate, Azurite
Copper (II) Oxide, Black Copper Oxide, BCO, Cupric Oxide
Cu2O, Red Copper, RCO, Copper (I) Oxide, Cuprous Oxide
In Bound Links
Glazes are not as inert and stable as many people think. All are slightly soluble and will thus leach to some extent, even if minute, into liquids they come into contact with. However some glazes are dramatically more soluble than others. The subject of leaching and glaze safety can be complex, but ...
By Tony Hansen