GlassyClass on tue 30 oct 01
Is the expansion that you are talking about the same as COE (coefficient of
expansion) in hot glass work, in glass it was only a few numbers, sounds
like there are many more in clay if it is the same thing.
On this line, of thought,
Is it possible to use ground glass (looks like fine flour) in a glaze, or
would that work at all, I am sure someone has already tried it.
Of the two glasses I used the most, one was a COE of 92, and the other was a
96, could not mix them, they would fracture.
----- Original Message -----
From: "Ron Roy"
Sent: Sunday, October 28, 2001 7:41 PM
Subject: Re: crazing help
> Hi Jeanne,
> The calculated expansion for #1 is 436 on the scale I use
> #2 is 465
> #3 is 372 ( the boron in this glaze is just a bit over 12% so in reality
> this number will be a bit higher.
> #4 is 399
> #5 is 522
> #6 is 333
> As you can see - the last one - you said came closest is the lowest - but
> With the C6 bodies I have experience with I would only expect #2 and #5 to
> craze - so I have to ask a few questions before I start adjusting #6 to
> stop any crazing.
> Question #1 - Are we sure it's crazing on #6 - sometimes it's what we call
> shivering - when that happens the cracking glaze pattern is usually
> different in that the lines tend to be parallel. Try mixing equal amounts
> of #1 and number 6 - that glaze will have a calculated expansion of about
> 385 - just combine the materials for both and divide by 2
> EPK - 22.5
> 3134 - 22.5
> Silica - 22.5
> Custer - 10.0
> wollast - 15.0
> F4 - 7.5
> Total - 100.0
> There are two other reasons a glaze can craze even though the basic
> expansion is OK for the body,
> Question #2 - How long is your firing, are you using large cones to
> the final temperature and how long do you soak at the end? Not giving the
> glaze a chance to bond well with the clay can give very small fit
> Question #3 How fast is your cooling from dull red heat to kiln opening?
> a glaze is cooled fast from say opening your kiln too soon you can get
> crazing - simply because the glaze cooles before the clay and contracts to
> quickly. We assume you have turned off any venting after shut off. If not
> refire all and turn vent off at end of firing.
> This may result in some of the glazes not being clear (fast cooling
> clears) but if that is the case you have to turn off the vent just before
> your glazes solidify - probably around 700C
> So let me know as much as you can an we will go from there.
> >Help! Am trying to get a clear glaze to work on a white stoneware, fired
> >range ^4-6 ox. (It's called Alpine White, from Seattle Pottery Supply).
> >tested several clear glaze recipes, to no avail, they all craze. What
> >ingredients can I play around with - add more, or less of, to try to get
> >glaze that fits? These are mugs with underglaze decoration.
> >Some glazes I've tried, some from Clayart:
> >5x20 Clear:
> >20 EPK; 20 FRit 3134; 20 Silica; 20 Custer; 20 Wollastonite
> >12.5 Whiting; 12.5 EPK; 20 Neph. Sye; 25 Frit 3134; 30 Flint
> >EZ Clear:
> >50 Frit 3134; 30 EPK; 20 Silica
> >30 Neph Sye; 8 Wollastonite; 21 Gerstley Borate; 31 Silica; 10 EPK
> >Shiny Clear GTB-1
> >8 Whiting; 27 Gerstley Borate; 39 Neph Sye; 8 EPK; 18 Silica
> >G1214W Clear
> >10 Wollastonite; 25 Frit 3134; 25 EPK; 25 Flint; 15 F4 Kona Feldspar
> >The last one came the closest, but still a bit crazed, maybe I should
> >around with that one.
> >I seem to be a glaze klutz, I sure would appreciate any advice you can
> >my way!
> >Thank you!!
> >Jeanne, in cold, wet, rainy Sitka by the Sea, Alaska.
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> Ron Roy
> RR# 4
> 15084 Little Lake Rd..
> Ontario, Canada
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