Toxicity of Tiny Clumps of Colorants in Glaze

[Suki]

Dear Ceramicists,

If a glaze contained tiny clumps of cobalt carbonate and copper carbonate, how would this affect the food safety of the fired product? The glaze in question is itself food safe (taken from Mastering Cone 6 Glazes) and is formulated for cone 6 oxidation. When bisqueware is dipped in the glaze, these clumps of colorant adhere to the surface and are covered in a thin layer of glaze. If rubbed, the clumps break open, and when fired, they become smooth and glossy specks on the surface of the glaze. Some of the fired flecks look metallic (see photographs).  Might the thin layer of glaze over the carbonates be enough to seal them, or is the glaze likely to leach them?

Thank you,

Suki

 

Photos Here:

https://drive.google.com/open?id=187e3WGOvvuuhcDL2GgsKPuL6uPNURgGG

[Dick White]

If the glaze is leaving clumps of colorants on the freshly glazed surface of the pot, it should be sieved again with a finer sieve until there are no more clumps in the glaze slurry.  A stable glaze with cobalt or copper colorants typically remains stable as long as the amount is moderate. Excessive amounts of copper or cobalt can leach out of what otherwise was a stable glaze. In your situation, you are creating tiny spots of unsafe unstable glaze in the midst of what should be a safe glaze.

[JohnnyK]

Hi Suki,

I agree with Dick here. You should do a better job of mixing and sieving the glaze components to get a smoother blend and a better glaze product.

JohnnyK

[neilestrick]

Sieve through an 80 mesh screen. That should take take of any clumps. I've never seen copper or cobalt need a finer screen than that. If, for some reason that doesn't solve the problem, try a finer screen like 150 mesh.

[Magnolia Mud Research]

If you first wet the dry powdered colorants (Cobalt carbonate, copper carbonate, red iron oxide, etc) with alcohol (I use rubbing alcohol, but high end scotch also works) before adding the powder to your glaze mixture, the "clumping' is less likely to occur.  The alcohol lowers the surface tension and quickly wets the particles. The prewetted powder, when added to the glaze slurry, will quickly disperse with stirring.  The alcohol will evaporate and does not negatively effect the glaze.  

LT