Daniel Semler on tue 19 sep 06
Hi Randy,
This is an interesting point.
Generally calculated COEs are most useful with gloss glazes. Mattes
which are either underfired or which are the result of crystallisation
are not necessarily so well approximated. Much of the COE work that we
use for glaze calculation is based on glasses which are well melted,
and which don't have a clay substrate (but that's another whole bag of
worms).
So yes, I'd start at the COE of the gloss glaze that fits but I'm
not dead sure which way I'd go from there. You'll likely do well to do
a line blend extending either side of that COE estimate - my guess is
that the low side would be where you'll succeed but its only a guess.
If the glaze is an under melted clay matte or higher cone glaze, I'd
suggest getting it to melt properly and matte through crystallisation.
This is going to be better for durability anyway.
Let us know how it goes.
Thanx
D
Steve Slatin on tue 19 sep 06
Randy --
It's much easier to see crazing on glossy glazes. It's harder to see them
on matte glazes; sometimes you can't see even with a loupe and you
have to use liquid ink or something similar to reveal the crazing.
You've got a possible hint in your differing results -- the later crazing
on one test may tell you something. Did that glaze have a different
calculated COE than the others? If so, that test may be bordering on a
good fit. Another possibility -- are you firing to clay maturity? If the
clay hasn't been fired to maturity, it might be absorbing moisture and
expanding, and that could result in the late crazing.
Good luck -- Steve Slatin
McCall wrote:
I have been testing some clear matt glazes and initially all the glazes I
tested crazed except one. Over a period of a couple of months that glaze
has now crazed on the test tubes.
Most of the glazes in the test are clear with a thin coat, but are milky
with two coats.
If I look at the coefficient of expansion of a glaze that does not craze on
my clay is that what I need to shoot for with the clear matt glaze in the
adjustment?
All the glazes that don't craze are shiny.
Steve Slatin --
The angel lay in a little thicket. It had no need of love; there was nothing anywhere in the world could startle it ...
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Randy McCall on tue 19 sep 06
I have been testing some clear matt glazes and initially all the glazes I
tested crazed except one. Over a period of a couple of months that glaze
has now crazed on the test tubes.
Most of the glazes in the test are clear with a thin coat, but are milky
with two coats.
If I look at the coefficient of expansion of a glaze that does not craze on
my clay is that what I need to shoot for with the clear matt glaze in the
adjustment?
All the glazes that don't craze are shiny.
Randy
South Carolina
Pottery Web site
http://members.tripod.com/~McCallJ/index.html
Ron Roy on wed 20 sep 06
Hi Randy,
The calculated expansion of a glaze that does not craze could also be
too low and cause dunting or shivering.
The best thing to do is run a 5 part line blend of a glaze that is just
crazing (large spaces between cracks) and glaze that has never crazed -
that will give you a good idea what the calculated expansion number
should be for other glazes - on that clay body. Best to do the same
thing on all the clay bodies you use.
RR
On 19-Sep-06, at 12:39 PM, Randy McCall wrote:
> I have been testing some clear matt glazes and initially all the
> glazes I
> tested crazed except one. Over a period of a couple of months that
> glaze
> has now crazed on the test tubes.
> Most of the glazes in the test are clear with a thin coat, but are
> milky
> with two coats.
>
> If I look at the coefficient of expansion of a glaze that does not
> craze on
> my clay is that what I need to shoot for with the clear matt glaze in
> the
> adjustment?
>
> All the glazes that don't craze are shiny.
>
>
> Randy
> South Carolina
> Pottery Web site
>
> http://members.tripod.com/~McCallJ/index.html
>
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Ron Roy
RR#4
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