[Clayart] Thanks to Ron Roy for helping me reformulate a glaze

[ronroy at ca.inter.net]

Thanks for posting this Lisa.

Just a few words of caution.

The lemon test will only reveal grossly unstable glazes. This glaze  
will probably still release some copper if in contact with acidic food  
long enough.

One of the clues on how to spot unstable glazes - if the recipe has  
little or no silica. Having enough silica in a glaze (not too much) is  
the key to resistance to acidic food attack.

RR


Quoting Dragonbelly Ceramics via Clayart <clayart at lists.clayartforum.com>:

> I posted a few months back about a glaze that I have long used with weird
> chemistry. Ron was gracious enough to look at it and remind me that not all
> stable glazes exist within a narrow band of chemistry and he gave me an
> alternative formula that (bonus!) used less spodumene and matched the
> original with a slightly higher COE (the origonal was quite low).
>
> I am pleased to say testing it showed me a glaze that completely matches
> the original.
>
> It also passed a 24 hour lemon test.
>
> This is the original formula:
>
> Tom West Green, Cone 6
>
> Ferro Frit 3269: 27
> Gerstley Borate: 15
> Spodumene: 40
> EPK: 18
> Copper Carbonate: 5
> Tin Oxide: 7
>
> COE: 6.1  R20:RO 0.72:0.28 Boron: 0.45
>
> Ron Roy's adjustment:
>
> 3269 .............. 27.0
> Gerstley Borate .... 15.0
> Spodumene ......... 28.0
> Neph Sy ........... 12.0
> EPK ............... 18.0
> Total .............100.0
> + copper carbonate 5
> + Tin oxide 7
>
> COE: 6.6 R20:RO 0.71:0.29 Boron: 0.46
>
> It's a glaze that on iron rich clay when thin is a lovely orange. When
> thick a shimmery metallic green.
>
> On white clay, thin is a mossy green, thick makes a metallic green that
> breaks black.
>
> It also appears to withstand years of exposure to the dishwasher without
> any breakdown in color.
>
> And no one know where the glaze came from. It was in use at a former studio
> I belonged to outside of Boston without any attribution or known history.
>
> Best,
> Lisa
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Ron Roy
ronroy at ca.inter.net
Web page ronroy.net