Alyssa Ettinger on tue 21 jun 05
i have glazes that are cracking. cone 6, on porcelain. i fired them at my
studio and they started cracking within a few days. i fired tests at another
studio (with added flint silica, starting at 2%, up to 12%, hoping to stop
the crackiling, and none cracked--and i have tried to make them crack:
dishwasher, freezer, oven, microwave...)
could my kiln at the studio be firing too high or low, causing cracking?
could 2% silica make this difference? glaze recipe below, advice needed and
wanted asap!
kiln at my studio is oxidation, and electronic. kiln at friend's studio is
also oxidation, but uses a kiln sitter and is fired super fast.
--alyssa
EPK Kaolin 10%
Fusion Frit F38 32%
Whiting 14%
Nepheline Syenite 26%
Flint 18%
mason stains: either 1% or 4%, depending on glaze color needed
Dave Finkelnburg on wed 22 jun 05
Alyssa,
I am sorry to hear about the cracking (crazing) of your glaze. :-(
Certainly, adding 2% silica to that recipe will lower the expansion, and depending on how close you are to matching the clay body, may be enough to prevent crazing. I would expect to see less crazing as you add more silica and at some point the crazing will go away entirely. The best way to test for this is to freeze a glazed, fired test tile, then drop it into boiling water. Next, cool and dry the tile and look at the glaze with a magnifying glass. 10X magnifiers are cheap to buy and will show you a lot. Thanks to Ron Roy for teaching me this!
Firing does also affect the thermal expansion mismatch causing crazing. For starters, crazing happens because a glaze has a higher Coefficient of Thermal Expansion (CTE) than the clay body the glaze is on. At room temperature the glaze has shrunk more than the body, so the glaze is in tension. ALL brittle ceramics are weak in tension. So, the glaze cracks (crazes).
In general, the more vitrified your clay body, the lower it's CTE will be. So, for a crazing glaze applied to a given clay body, the more vitrified that body is, the worse the crazing will be. Fast firing in another kiln may have resulted in a less-vitrified clay body and the reduced crazing you observed.*
The solution, though, is not to underfire the ware. The solution is to fix the glaze so it fits the ware when the ware is properly vitrified.
Good potting!
Dave Finkelnburg
*Note: The above assumes no crystobalite is formed in the firing, and also assumes enough free quartz is present that firing the body higher dissolves more quartz (relatively high CTE) into the glass phase (relatively low CTE) of the clay body.
Alyssa Ettinger wrote:
i have glazes that are cracking. cone 6, on porcelain. i fired them at my
studio and they started cracking within a few days. i fired tests at another
studio (with added flint silica, starting at 2%, up to 12%, hoping to stop
the crackiling, and none cracked--and i have tried to make them crack:
dishwasher, freezer, oven, microwave...)
could my kiln at the studio be firing too high or low, causing cracking?
could 2% silica make this difference? glaze recipe below, advice needed and
wanted asap!
__________________________________________________
Do You Yahoo!?
Tired of spam? Yahoo! Mail has the best spam protection around
http://mail.yahoo.com
Ron Roy on thu 30 jun 05
Hi Alyssa,
The fast firing leaves more crystaline silica in the clay and that helps
cure crazing.
I agree with Dave - don't fire fast to cure the problem - fix the glaze.
Are you firing with cones? - if you say you may be over firing but don't
know that indicates you are not using cones - you should!
Here is a revision of your glaze with the expansion lowered quite a bit - I
am assuming enough to cure the crazing - mix up a test batch of 100 grams
and see if it still crazes when fired in your kiln - with a cone 6 righ
beside it.
EPK - 15.0
Fusion frit 38 - 33.0
Whiting - 13.5
Neph Sy - 13.0
Silica - 25.5
Total 100.0
I lowered the Neph Sy and raised the clay as well - the glaze will stay
suspended better.
Let me know if it works - if it does not I can lower the expansion more.
RR
>i have glazes that are cracking. cone 6, on porcelain. i fired them at my
>studio and they started cracking within a few days. i fired tests at another
>studio (with added flint silica, starting at 2%, up to 12%, hoping to stop
>the crackiling, and none cracked--and i have tried to make them crack:
>dishwasher, freezer, oven, microwave...)
>
>could my kiln at the studio be firing too high or low, causing cracking?
>could 2% silica make this difference? glaze recipe below, advice needed and
>wanted asap!
>
>
>kiln at my studio is oxidation, and electronic. kiln at friend's studio is
>also oxidation, but uses a kiln sitter and is fired super fast.
>
>--alyssa
>
>EPK Kaolin 10%
>Fusion Frit F38 32%
>Whiting 14%
>Nepheline Syenite 26%
>Flint 18%
>mason stains: either 1% or 4%, depending on glaze color needed
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Alyssa Ettinger on fri 1 jul 05
ron,
thank you for this. have already upped the silica in my recipe, which seems
to be working. but, i trist your recipe more so will do some retesting as
soon as there's a kiln in my studio that's being fired to 6 (the perils of a
studio share...w aiting)
in the meantime, i had a major "glaze malfunction" last weekend and lost two
kilns of work. (i think i really had an "alyssa malfunction" and
accidentally swapped two ingredients while mixing...) glaze slumped off the
pots like melted cheese. gah. so that's why i'm doing retests of the org
tweaked recipe to see if it was me or a bad ingredient.
back to chipping kiln shelves... thank goodness for the dremel. will let you
know how the tests work out.
and THANK YOU
alyssa
Ron Roy on sun 3 jul 05
Hi alyssa,
If I can see the recipe for the glaze that is running I can at least
express an opinion about what it should do.
RR
>thank you for this. have already upped the silica in my recipe, which seems
>to be working. but, i trist your recipe more so will do some retesting as
>soon as there's a kiln in my studio that's being fired to 6 (the perils of a
>studio share...w aiting)
>
>
>in the meantime, i had a major "glaze malfunction" last weekend and lost two
>kilns of work. (i think i really had an "alyssa malfunction" and
>accidentally swapped two ingredients while mixing...) glaze slumped off the
>pots like melted cheese. gah. so that's why i'm doing retests of the org
>tweaked recipe to see if it was me or a bad ingredient.
>
>back to chipping kiln shelves... thank goodness for the dremel. will let you
>know how the tests work out.
>
>and THANK YOU
>alyssa
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
| |
|
