ed on thu 10 aug 00
Dear John,
You offered a calculation; any possibility of telling me what I should use
to replace g. borate for this turquoise raku crackle??? I would be deeply
appreciative. Sincerely, Ed Gould
Gerstley Borate 80
Cornwall Stone 20
Copper Carb. 2
.com.
John Post on fri 11 aug 00
Hi Ed,
There is no easy way to calculate this and fix it. Let me explain why.
The gerstley does 2 things for this glaze.
First it supplies it with an enormous amount of boron (B2O3). There are no
frits that I use that contain as much boron as the gerstley has in it. So
when you try to substitute a frit for the GB, you end up with a glaze that
has less boron. Or it has many other oxides added into it in order to have
the frit supply the same amount of boron that's in the original recipe.
The second thing that the GB does is to flocculate the glaze. This makes
it brushable. You'll notice that your glaze has no clay in it and it still
brushes on great. That is one of characteristics of GB.
When the announcement came that the GB mine was closing, I recommended that
potters who really need the stuff should order enough to last them the rest
of their lives. It may have seemed overkill at the time but that's what I
did. I only use a little of the stuff in a couple of recipes, kind of like
a chef might use seasoning in a recipe, so a lifetime supply for me was
only a couple hundred pounds.
There are some alternatives though. You could check out Tony Hansen's
website
http://www.digitalfire.com/education/material/gerstleyborate.htm.com to see
what he has to say about the GB alternatives. You could try some of his
suggestions.
The other thing that you could do is to start from scratch with some
fritted raku glazes. Try Ferro frit 3134 or 3110. I would start at 80%
frit and add 20% clay to the recipe. The frit 3134 is a boron frit so it's
fired properties may be close to your original. Frit 3110 is a soda frit
so it won't be exactly like your original but with either of these frits it
is easy to get a turquoise crackle. The 3110 will be bluer than the 3134.
Add the 2% copper carb to the recipes you test.
The problem with frit glazes is that they like to settle to the bottom of
the bucket. The 20% clay is in the recipe to try and remedy that.
On the bright side, it seems that there are several manufacturers working
on alternatives to GB that will match it's fired properties and it's glaze
slurry properties. You may just have to wait for them.
Sorry I couldn't just post an easy reformulation for you. That works best
for glazes that have less than 20% GB in them.
>You offered a calculation; any possibility of telling me what I should use
>to replace g. borate for this turquoise raku crackle???
>Ed Gould
>Gerstley Borate 80
>Cornwall Stone 20
>Copper Carb. 2
J. Post
waverock@c3net.net
Paul Lewing on fri 11 aug 00
ed wrote:
>
> Dear John,
> You offered a calculation; any possibility of telling me what I should use
> to replace g. borate for this turquoise raku crackle??? I would be deeply
> appreciative. Sincerely, Ed Gould
> Gerstley Borate 80
> Cornwall Stone 20
> Copper Carb. 2
> .com.
Ed,
I don't have time to try a calculation for you, but if I were you, I
wouldn't expect to ever get a duplicate of this glaze with anything else
but GB, at least not with a single other ingredient. And this is going
to be true for a lot of raku artists. I think they'll be hardest hit by
the demise of GB.
The reason I say that is that there's just too much GB there. Even if
you can juggle other materials to get the same (supposedly) chemistry as
the GB version, it's a very complex material, whose composition varied a
lot from batch to batch, or maybe even from bag to bag. And calculation
is only as good as your analysis. Often it's the trace elements that
may even be under the threshold of accuracy of your analysis method that
give those wonderful rich effects
The other problem is the physical characteristics of GB. There is no
clay in this recipe to keep the stuff suspended. It relies on that
property of GB to make it usable at all. So even if you exactly
duplicate the chemistry with fritted or even other mined materials, the
application qualities may be quite different. They may even render the
new mix unusable. Or you may have to go through some gyrations to keep
the stuff suspended with gums or something.
Recently someone (Mary Simmons, I think) put together a web site about
GB substitutions that went deep into GB properties and the new materials
that people are trying to use instead. I forget the address, but it was
in conjunction with a new product called BORAQ, and it was announced on
Clayart by Tony Hansen, so I'm sure you can find it through the
digitalfire.com web site. One of her points was that no one material
would be the perfect replaement in all situations; that different
materials may be better alternatives in different situations.
One thing you may try to start with is doing a line blend with the
Cornwall Stone the same and going from maybe 10% Gb/90% frit for the
rest to 90% GB/10% frit. You may find a point at which you can get the
same effect with significantly less GB. This won't replace the GB, but
it will make your remaining stock last that much longerI hope this
helps.
Paul Lewing, Seattle
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