Evan Dresel on mon 2 aug 99
Maybe the glaze wise-ones out there can help me understand the results
of my tests to correct crazing in a glaze. The original glaze is:
Patty's Crystal Clear (from ClayArt)
Cone 6 oxidation
Nepheline syenite 30
Gerstley borate 21
Whiting 8
Kaolin 10
Flint 31
Add: Bentonite 2
GlazeChem gives the expansion as 67.4 x 10e-7 per degree C, but my
original test tile crazed. So I tried to fix this by raising the Si and
Al while keeping the alumina:silica ratio the same. To do this I mixed
59 flint with 41 EPK which has close to the same ratio and essentially
nothing else. Then I mixed a 200g batch of the base glaze, dipped a
tile; added the flint-EPK mixture in 3% increments (6 g) up to 15 %
dipping a tile at each addition. The expansion thus should range down
to 63.6 x 10e-7 per degree C. For comparison my other glazes seem to
fit with an expanion of around 69 x 10e-7 per degree C.
Ok, the original recipie tile showed nearly no cracks after 2
freeze-boil tests but the others did with the worst cracking at about 12
%. The first tile may have been dipped a bit thinner but why would the
crazing get worse with decreased expansion?
The cracks in the tiles don't look quite the same as in other badly
crazing glazes I have seen. They don't run as straight and don't always
seem to meet at 90 degree angles which seems more typical to me (the 90
degree "orthogonal cracking" is seen in mud cracks and tundra frost
polygons and has something to do with how the cracks form but I can't
remember exactly what from my geology student days). My glaze tests
have a moderate number of bubbles (I'm really on a quest for a bubble
free glaze) and that may have something to do with the appearance.
Any ideas of what is going on here? Is this some symptom of too low
expansion and I was just making things worse?
-- Evan in W. Richland WA who hopes he doesn't fry his brain
volunteering at the Art Sale this afternoon
pedresel@3-cities.com
Ron Roy on thu 5 aug 99
Hi Evan,
Sounds like the opposite of crazing to me - here is a similar glaze with a
higher expansion - from the 418 of your original glaze to 511 in my
revision. Do a line blend of the two and see if the cracking stops.
Has anyone else noticed how hard it is to adjust glazes with Gerstley
Borate in them? I am constantly taking it out so I can get the results I
want.
Revision with a higher expansion.
-----------------
NEPH SY............. 15.00
F3134............... 8.00
Gers Borate 9/97.... 8.00
WHITING............. 11.00
EPK................. 8.00
SILICA.............. 20.00
CUSTER SPAR......... 30.00
BENTONITE........... 2.00
----------
102.00
RR
>----------------------------Original message----------------------------
>Maybe the glaze wise-ones out there can help me understand the results
>of my tests to correct crazing in a glaze. The original glaze is:
>
>Patty's Crystal Clear (from ClayArt)
>Cone 6 oxidation
>
>Nepheline syenite 30
>Gerstley borate 21
>Whiting 8
>Kaolin 10
>Flint 31
>
>Add: Bentonite 2
>
>GlazeChem gives the expansion as 67.4 x 10e-7 per degree C, but my
>original test tile crazed. So I tried to fix this by raising the Si and
>Al while keeping the alumina:silica ratio the same. To do this I mixed
>59 flint with 41 EPK which has close to the same ratio and essentially
>nothing else. Then I mixed a 200g batch of the base glaze, dipped a
>tile; added the flint-EPK mixture in 3% increments (6 g) up to 15 %
>dipping a tile at each addition. The expansion thus should range down
>to 63.6 x 10e-7 per degree C. For comparison my other glazes seem to
>fit with an expanion of around 69 x 10e-7 per degree C.
>
>Ok, the original recipie tile showed nearly no cracks after 2
>freeze-boil tests but the others did with the worst cracking at about 12
>%. The first tile may have been dipped a bit thinner but why would the
>crazing get worse with decreased expansion?
>The cracks in the tiles don't look quite the same as in other badly
>crazing glazes I have seen. They don't run as straight and don't always
>seem to meet at 90 degree angles which seems more typical to me (the 90
>degree "orthogonal cracking" is seen in mud cracks and tundra frost
>polygons and has something to do with how the cracks form but I can't
>remember exactly what from my geology student days). My glaze tests
>have a moderate number of bubbles (I'm really on a quest for a bubble
>free glaze) and that may have something to do with the appearance.
>
>Any ideas of what is going on here? Is this some symptom of too low
>expansion and I was just making things worse?
>
>-- Evan in W. Richland WA who hopes he doesn't fry his brain
>volunteering at the Art Sale this afternoon
>
>pedresel@3-cities.com
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Paul Lewing on sat 7 aug 99
Hi, Evan,
I think Ron Roy (the REAL expert on this) has already answered your
question with some recipe suggestions, but I did want to add one
thought.
One of the problems in diagnosing fit problems is that, if the problem
is only a mild one, crazing and shivering look pretty much the same- a
network of fine cracks. If it's severe enough, shivering (too low
expansion) will result in chips of glaze popping off the pot.
Conversely, crazing (too high) will break your pot. But when it's as
close as what you have here, it's difficult to tell just by looking
which it is. Now, usually, especially in the midrange temperatures,
it's that the expansion is too high, because it's difficult to get
enough of the low-expasion silica and alumina into a glaze and still
have it melt to a glossy surface, unless there's also a lot of boron
present.
Good luck with Ron's adjustments.
Paul Lewing, Seattle
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