[Clayart] tony's metaphor tangent - cornwall stone vs custer, mega-pinholes

prickly potter pricklypotter at gmail.com
Thu Mar 6 21:38:36 EST 2014


Laguna used to cut the real G200HP to make it more like the old G200.

I ran both recipes in Insight and it seems like the chemistry is close
enough, regardless of new/old custer.

Moreover the second recipe doesn't have that much more whiting than the
original one!

So why the difference? At the risk of bringing fire and brimstone over my
head, I've noticed that on occasion the substitution rule "neph sy + silica
= feldspar" doesn't work. sometimes it does and sometimes it doesn't and
when it doesn't, the "neph sy + silica" side melts more, shines more, and
runs more.

My guess is that in this case it did and produced a very thick puddle in
the bottom of the pot, out of which the whiting couldn't bubble out
completely, normal behaviour for high whiting glazes if they've had a
chance to pool.

This is just a wild guess. Try and see if the wollastonite version will
work better. or reformulate with a soda feldspar instead of neph sy?


On Thu, Mar 6, 2014 at 3:10 PM, Dick White <tgrcat at verizon.net> wrote:

> Jeff, a couple of thoughts. Yes, whiting is one-third CO2 gas, which under
> some conditions can produce frothing that doesn't make it out of the melt.
> I like to put wollastonite (calcium silicate) in place of the whiting, but
> only if there is a fair amount of silica already in the recipe that I can
> take out because the wolly will bring its own silica with it.
>
> Second thought, you mentioned G200 and Custer. When working with a glaze
> calculation program (I use GlazeMaster too, but let's keep it generic for
> the rest of the audience) to develop substitute recipes, be aware that G200
> has not been available for about 5 years, was replaced by G200HP that is
> higher in potash than the former G200 (check the label on your bag, the new
> name will be shown, though the new bag colors are similar to the old bags).
> And as of a few months ago, they announced the G200HP source mine is now
> exhausted and no more of that will be produced (though you may still have
> some old stock you can use). There was some talk of importing a similar
> feldspar from Spain, but as you put it so aptly, if you're going to swim to
> Spain for the feldspar, you might as well stop in Cornwall, it's closer.
> ;-) If you are developing recipes for your existing stock of G200HP, you'll
> need to be sure you use the G200HP material in your program rather than the
> old plain G200.
>
> Similar to the G200 issue, I see you used Custer in this variant. Sadly,
> Custer isn't what it used to be either. As you may have seen in some
> commentary here and elsewhere in pottery groups (also see Ron Roy's article
> in Nov. 2013 Ceramics Monthly), the current line of Custer is much lower in
> potash and alumina than before, and higher in silica. Pacer (the producer
> of Custer) refuses to acknowledge the change and still has an analysis
> dated 2008 on their website. I feel lucky that back in late 2011 when this
> was first noticed and discussed here on ClayArt, I went to the Pacer site
> and found a new analysis was posted and I wrote it down and put it in my
> GlazeMaster materials. But then Pacer put the old page back and now
> stonewalls it. Anyway, FYI here is the analysis I saw and wrote down:
>
> Na2O - 3%
> K2O - 7% (previously was 10%)
> MgO - .01%
> CaO - .3%
> Al2O3 - 15% (previously was 17%)
> Fe2O3 - .15%
> SiO2 - 73.5% (previously was 69%)
>
> Your recipe doesn't have much Custer in it, so the variance from the old
> to the new doesn't change much (though it might, hard to say without
> testing). Just a data point for your GlazeMaster adventures.
>
> Anyway, I fired up my copy of GlazeMaster to sub wolly for some of the
> whiting and change the Custer to the new analysis, and here is what I came
> up with:
>
> Feldspar Custer 11/11 - 513
> Wollastonite - 579
> Whiting - 660
> EPK - 786
> NephSy - 378
> Silica - 84
> Tin - 120
> Copper Carb - 120
>
> ==================
> Jeff wrote:
>
> A real-life vignette:
> Kathleen: What do you suppose is happening with this glaze? It behaved
> perfectly but now it's  pinholed on one side.
> <Val Cushing's Blue Green
> 1380 Cornwall Stone
> 1020 Whiting
> 600 EPK
> 120 tin oxide
> 120 copper carbonate
>
> Jeff <channeling Tony>: No idea, but it's a long swim for all that Cornwall
> Stone. Let's make it with somethign  more local.
> < Two spot-on oxide matches ensue from Glazemaster, one with G-200 and one
> with custer - here's one:
> VC2
> 298.3 custer feldspar
> 1053.9 whiting
> 772.3 epk
> 421.3 neph sy
> 454.2 silica
> 120 tin oxide
> 120 copper carbonate
>
> <upon opening after the 10R firing  this morning >
> Whoa! Both those tests are gaudy (and runny) turquoise lava glazes on flat
> pieces and run off a test cup (Original was a mat turquoise).  Wuzzup with
> that?
> ...
> <vignette mode off>
> This shows why normal people like stuff that works no matter how long the
> haul to get it, but me, I get sidetracked - why the frozen froth in the
> tests when the oxide balance was so close? What's bubbling? I thought
> whiting lost its CO3 well below  cone 10.
> Theories, please!
> Thanks,
> tangentially yours,
> Jeff
>
>
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-- 
David

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