[Clayart] Unstable glaze & toxic ingredients

[ronroy at ca.inter.net]

Hi Madeleine,

John and I are reasonably sure that covering an unstable glaze with a  
stable glaze would result in a stable coating. Having said that I  
could imagine that you could do it in such a fashion that you could  
render the combination unstable. You would have to work hard at it  
though.

I have to warn you - if one glaze is short of silica and the other has  
enough or an abundance you could have a serious running problem. The  
excess flux in the silica short glaze could over flux the combination.

I encourage every potter to learn glaze calculation - what appears to  
be a mystery will become understandable. I should add that  
understanding the role of the different oxides in clay and glazes will  
be one of the most rewarding endeavors a potter can experience.

RR



Quoting Madeleine Hall-Arber <arber at mit.edu>:

> Thanks to all who responded to my question about the white (^6 oxi)  
> glaze.  I haven't done a glaze firing yet, so will report later.  I  
> did have a suspicion that it was unstable, so I do use it on  
> exterior surfaces.  Being primarily self-taught, however, I guess I  
> do have to ask you glaze gurus to repeat yourself.  If I were to use  
> a stable glaze with this unstable glaze (over or under), if that  
> stable glaze includes some toxic ingredients, would the white make  
> it possible for them to leech out?  I am much more cautious than I  
> used to be with the interior surfaces that are likely to touch food  
> (thanks to many clayart discussions/warnings), but am curious.
>
> Madeleine
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Ron Roy
ronroy at ca.inter.net
Web page ronroy.net