[Clayart] Question on fluxes
Paul Randall via Clayart
I have been reformulating some glazes that call for Custer Feldspar, substituting G200 EU. I use the Glazy target feature which now includes an automated "Solve for Oxides" tool. Quite nice!
I remove all oxides from the base glaze recipe, solve for the new base with G200 EU and then add back in the oxides to the new recipe.
The Glazy tool seems to be quite good at matching the UMF and Extended UMF using this tool. However, once I solve for 100% and then round, slight differences happen. Mostly in the fluxes and it seems mostly with Potassium and Sodium. The rest of the UMF and extended UMF remain similar. For example:
Original UMF w/Custer
0.07 sodium
0.14 potassium
New UMF w/G200 EU
0.05 sodium
0.17 potassium
In this case Silica increased from 2.78 to 2.79, the rest are similar.
Anybody out there who understands the impact on the resulting glaze? Do such small amounts matter?Thank you for comments.
Paul Randall
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David Finkelnburg via Clayart
Good question!
Without the entire recipe it's hard to even guess. However, at the least
you should appreciate that between sodium, Na, and potassium.K, if the Na
is 1/3 of the two in the first case and less than 1/4th of the two in the
second case, those are NOT small differences. You may see differences in
melting, glaze viscosity and coefficient of thermal expansion.
That is not to say one's better than the other or that either is
unacceptable in your work. Glaze thickness, clay body composition, firing
temperature and glaze composition all matter. I would make test pieces and
fire them side by side using both glazes to see how much, if at all, the
new spar matters in your studio. With your calculations you have a strong
hint to watch closely for differences.
Regards,
Dave
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John Post via Clayart
The way that the Glazy calculation feature treats Sodium and Potassium is
it assumes they are interchangeable.
The original Custer recipe has a total of 0.21 of the R2O oxides when you
add them together.
The new G200 EU version has 0.22 of those same R2O oxides.
That's plenty close and I wouldn't even worry about it.
In my experience, I have found that treating those two oxides
interchangeably does not adversely affect glaze results.
I like to make sure that the Alkaline Earths - Calcium, Magnesium, Barium,
Strontium and Zinc numbers are exact matches with the glazes I am trying to
replicate.
I think of those as the chemicals that give a glaze its visual appearance
and have a bigger impact on fired results than Sodium and Potassium.
As far as the Silica goes, if the glaze has silica as an isolated
ingredient, you can simply remove some to make the silica an exact match.
A 1/100 increase in silica is going to be unnoticeable and I wouldn't even
worry about it.
I am betting the glaze calculation you did will work out just fine.
Best,
John Post
> > To all,
> > I have been reformulating some glazes that call for Custer Feldspar,
> > substituting G200 EU. I use the Glazy target feature which now includes
> an
> > automated "Solve for Oxides" tool. Quite nice!
> >
> > I remove all oxides from the base glaze recipe, solve for the new base
> > with G200 EU and then add back in the oxides to the new recipe.
> >
> > The Glazy tool seems to be quite good at matching the UMF and Extended
> UMF
> > using this tool. However, once I solve for 100% and then round, slight
> > differences happen. Mostly in the fluxes and it seems mostly with
> Potassium
> > and Sodium. The rest of the UMF and extended UMF remain similar. For
> > example:
> >
> > Original UMF w/Custer
> > 0.07 sodium
> > 0.14 potassium
> >
> > New UMF w/G200 EU
> > 0.05 sodium
> > 0.17 potassium
> >
> > In this case Silica increased from 2.78 to 2.79, the rest are similar.
> >
> > Anybody out there who understands the impact on the resulting glaze? Do
> > such small amounts matter?Thank you for comments.
> > Paul Randall
>
>
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ronroy--- via Clayart
Much depends on how balanced the glaze is.
If the amounts of silica and alumina are in a "normal" range it
probably will not make much difference. If the glaze is either short
of silica or alumina the fluxes will have much more of an influence.
In this case it is important to use the right analysis for the custer
you used. You can find my article on the subject on my web site -
ronroy.net
When I say "normal glaze I am really saying a glaze with enough silica
and alumina to be deemed a stable glaze. I can go into that further if
you want but our book is all about stable glazes. Think of them as
reliable and functional.
When calculating a glaze to sub in a new material always do the fluxes
first then the alumina then the silica last.
In the case you used as an example - lower the spar to get some of the
KNaO out - till those numbers are right - the others may not need
adjusting but do so if they need it.
Then adjust the clay to get the alumina right then the silica.
If the is boron involved simply keep that the same.
I know it will seem difficult when you are first trying to adjust a
glaze but I always found it to just be a puzzle and an enjoyable
process.
You will be surprised at how effective calculation is when subbing materials.
RR
ron...@ca.inter.net
Web page ronroy.net