John L Worner on sun 16 mar 97
Hi,
I have some ^5 and ^10 porcelain and need to know how high to bisque these
clays.
In Val Cushing's handbook he states that porcelain is fired to ^04. Are you all
going
that high ?
Should both the ^5 and ^10 go to the same teperature in bisque?
Your responses are much appreciated.
John
SolvejgMa@aol.com on mon 17 mar 97
Hi John,
I've used both ^10 and ^7 (4-7) porcelain, originally bisquing at ^06.
With both clays I had a lot of problems with pinholing in the glaze fire, so
I upped the bisque to ^04. Haven't had any pinholes since then.
Good luck!
Kat in Salem, Oregon watching the grass grow in the swamp that's supposed
to be a lawn.
Tony Hansen on mon 17 mar 97
John L Worner wrote:
> I have some ^5 and ^10 porcelain and need to know how high to bisque
I've always bisqued as high as possible but not so high that the ware is
not absorbant enough to glaze easily. Some people bisque at cone 010 and
others who know how to gel a glaze bisque at cone 10 and glaze at 04.
--
Tony Hansen, Plainsman
Ron Roy on tue 18 mar 97
>----------------------------Original message----------------------------
>Hi,
>
>I have some ^5 and ^10 porcelain and need to know how high to bisque these
>clays.
>In Val Cushing's handbook he states that porcelain is fired to ^04. Are
>you all
>going
>that high ?
>
>Should both the ^5 and ^10 go to the same teperature in bisque?
>
>Your responses are much appreciated.
Hi John,
I strongly advise cone 04 for cone 10 porcelain. Most porcelain bodies have
lots of free silica. Firing the bisque to 04 helps to melt more of this
free silica which helps when the ware goes through the quartz inversion at
573C. The higher bisque also makes the bisqued ware stronger. You won't
notice dunting with smaller ware but when trying to fire larger pieces,
especially footed platters, you can eliminate bisque dunting by firing
higher. It also helps to keep those pieces which are subject to dunting up
in the top half of a kiln because that half cools slower. Bisque dunting
happens because one part of the pot cools faster than the rest - goes
through the quartz inversion before the rest, gets suddenly smaller and
cracks. You can recognize this type of crack - after the crack happens the
other part of the piece goes through the quartz inversion and the crack
closes up. Some times you don't see them but a flick of the rim will
produce no ring.
I have worked at Cone 04,6,8 and 10 over the past 35 years and have always
fired all my bisque to 04.
If you are used to bisque firing at lower temps and decide to fire higher
you need to get your glazes "thicker" in the bucket to get the same
application thickness. I have never had a problem getting the "right"
thickness on a clay body bisqued at 04 but I can imagine some low fired
bodies approaching vitrification might present an application problem.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
David Donica on wed 19 mar 97
Hello Ron. Thanks for your continuing wisdom about firing. I've never
bisqued to 04 but now I'm a believer and will bisque to that temp next
time. My question is- how thick is thicker? Latex paint thickness? Heavy
cream? Or maybe sheetrock mud?? No really, this was never a problem for me
in the past, must be advancing age. I'm considering purchasing a hydrometer
to measure my glaze specific gravity. Would you recommend that? What
guidelines exist for their use? Thanks again. Maggie from Mt. Shasta who's
determined to get it right this time. Ha!
Maggie Shepard
FireWorks Pottery
Mt. Shasta, Calif.
Roger King on thu 20 mar 97
John L Worner wrote:
>
> ----------------------------Original message----------------------------
> Hi,
>
> I have some ^5 and ^10 porcelain and need to know how high to bisque these
> clays.
> In Val Cushing's handbook he states that porcelain is fired to ^04. Are you a
> going
> that high ?
>
> Should both the ^5 and ^10 go to the same teperature in bisque?
>
> Your responses are much appreciated.
>
> John
Some (not all) potters prefer to high fire the porcelain as to vitrify
it and bring out the white translucency in the bisque (to the maturation
point of the clay), then glaze it at a lower fire or equal temp. . While
I am by no means an expert here, I have encountered the same question
you pose before, ant this is the amswer I have been given. Best of luck.
Kevin King
Ron Roy on sat 22 mar 97
>----------------------------Original message----------------------------
>Hello Ron. Thanks for your continuing wisdom about firing. I've never
>bisqued to 04 but now I'm a believer and will bisque to that temp next
>time. My question is- how thick is thicker? Latex paint thickness? Heavy
>cream? Or maybe sheetrock mud?? No really, this was never a problem for me
>in the past, must be advancing age. I'm considering purchasing a hydrometer
>to measure my glaze specific gravity. Would you recommend that? What
>guidelines exist for their use? Thanks again. Maggie from Mt. Shasta who's
>determined to get it right this time. Ha!
>Maggie Shepard
>FireWorks Pottery
>Mt. Shasta, Calif.
Hi Maggie,
If you have some old (low fired) bisque around glaze it - scratch through
and take a look (you can use a magnifying glass) at the thickness. Then
take the clear water off you glaze after it has settled - save the water.
Mix your glaze and glaze a piece of high fired bisque. scratch through it
to see the thickness - it will probably be to thick - add some of the water
back in - mix test for thickness again. Keep doing that till you get the
same thickness as on the low fired bisque. You now have a way of guaging
the thickness of your glaze.
In most cases a dry finger dipped into a mixed glaze will be about right if
you cam just see your skin through the glaze - like just on the ridges
aroung your finger nail. You will find the right thickness and then you can
adjust your glaze each time you use it - as long as you take some water out
before your stir it up.
After all that some glazes like to be thicker than others and some thinner.
I only work with a few glazes and am able to guage thickness by just
stirring - not everyone can do that - you will find a way. The hydrometer
may work - I never have used one.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
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