Kit Shannon on wed 6 oct 99
I hope someone can help.
At the University studio here we fire mostly cone 10 reduction and have
a large number of shop glazes that have been standards for several years
now.
Last year we started to experience problems with one of the glazes.
It used to be a nice blue celedon like glaze. A little darker than most
celedons but very nice glaze to emphasize carvings or throw marks.
Now we can't get it to come out right. Instead of being shiny and
celedony, it has a smooth matte surface. It doesn't look that bad, but
it is certainly not what you want over carved designs, cause it totally
covers them.
here's the recipe:
Custer Feldspar: 25
whiting : 25
Ball Clay 25
Silica 25
Cobalt Carb .25%
RIO 1.2%
We've tried re-mixing it to see if there was a mistake there, but it
comes out the same.
We think it is something to do with the materials. Maybe the Custer
Feldspar? Does anyone have ideas as to how we could fix this glaze.
Maybe substitute some other material?
Or is it possible that our firing times are slightly different now?
Different people have been firing the kiln here so it usually doesn't
get fired the same way twice, but this is the only glaze we are having
real problems with.
I doubt it is just the firing because in the past I've used it on pots
that were fired in the anagama and they came out wonderful
Does anyone have some ideas about this glaze?
Thanks for the help
-Kit
Ron Roy on thu 7 oct 99
What other feldspars do you have - I'll reformulate with a different spar
and then you will know - I have heard (fromTW) that Custer has changed.
I recommend G200 if you have the choice.
RR
>----------------------------Original message----------------------------
>I hope someone can help.
>At the University studio here we fire mostly cone 10 reduction and have
>a large number of shop glazes that have been standards for several years
>now.
>Last year we started to experience problems with one of the glazes.
>It used to be a nice blue celedon like glaze. A little darker than most
>celedons but very nice glaze to emphasize carvings or throw marks.
>Now we can't get it to come out right. Instead of being shiny and
>celedony, it has a smooth matte surface. It doesn't look that bad, but
>it is certainly not what you want over carved designs, cause it totally
>covers them.
>
>here's the recipe:
>
>Custer Feldspar: 25
>whiting : 25
>Ball Clay 25
>Silica 25
>Cobalt Carb .25%
>RIO 1.2%
>
>We've tried re-mixing it to see if there was a mistake there, but it
>comes out the same.
>We think it is something to do with the materials. Maybe the Custer
>Feldspar? Does anyone have ideas as to how we could fix this glaze.
>Maybe substitute some other material?
>Or is it possible that our firing times are slightly different now?
>Different people have been firing the kiln here so it usually doesn't
>get fired the same way twice, but this is the only glaze we are having
>real problems with.
>I doubt it is just the firing because in the past I've used it on pots
>that were fired in the anagama and they came out wonderful
>
>
>Does anyone have some ideas about this glaze?
>Thanks for the help
>
>-Kit
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Craig Martell on thu 7 oct 99
>Kit wrote:
>I hope someone can help.
>. Instead of being shiny and
>celedony, it has a smooth matte surface. It doesn't look that bad, but
>it is certainly not what you want over carved designs, cause it totally
>covers them.
Hi:
This glaze has enough whiting (calcium carbonate) that the glaze could be
affected by the cooling rate. A slower cool would allow pyroxene crystals
to form resulting in a matt. If you drop the kiln fast to about 1800
F. before clamming it up, you would most likely see the return of a shiny
blue celedon. You don't say what sort of kiln you are firing in at
school. In an anagama, you are being aided by the accumulation of fly ash
on the pots which helps flux the glaze. Anagama firings tend to be a bit
hotter than cone 10 too. I know, it depends on who's firing the kiln and
how even the temp spread is.
regards, Craig Martell in Oregon
Kit Shannon on thu 7 oct 99
Thanks for the Info on the glaze problem . I'll try a 100 g batch with G200
and see if that does anything.
Yesterday in the studio there was discussion that the Whiting bin may have
been filled with Dolomite due to different labeling by our supplier. So that
might be the whole problem, but I'm not sure when the error occurred. So
i'll also make up a batch with the new whiting and see.
thanks again,
Kit
>
>
> >----------------------------Original message----------------------------
> >I hope someone can help.
> >At the University studio here we fire mostly cone 10 reduction and have
> >a large number of shop glazes that have been standards for several years
> >now.
> >Last year we started to experience problems with one of the glazes.
> >It used to be a nice blue celedon like glaze. A little darker than most
> >celedons but very nice glaze to emphasize carvings or throw marks.
> >Now we can't get it to come out right. Instead of being shiny and
> >celedony, it has a smooth matte surface. It doesn't look that bad, but
> >it is certainly not what you want over carved designs, cause it totally
> >covers them.
> >
> >here's the recipe:
> >
> >Custer Feldspar: 25
> >whiting : 25
> >Ball Clay 25
> >Silica 25
> >Cobalt Carb .25%
> >RIO 1.2%
> >
> >We've tried re-mixing it to see if there was a mistake there, but it
> >comes out the same.
> >We think it is something to do with the materials. Maybe the Custer
> >Feldspar? Does anyone have ideas as to how we could fix this glaze.
> >Maybe substitute some other material?
> >Or is it possible that our firing times are slightly different now?
> >Different people have been firing the kiln here so it usually doesn't
> >get fired the same way twice, but this is the only glaze we are having
> >real problems with.
> >I doubt it is just the firing because in the past I've used it on pots
> >that were fired in the anagama and they came out wonderful
> >
> >
> >Does anyone have some ideas about this glaze?
> >Thanks for the help
> >
> >-Kit
>
David Hewitt on fri 8 oct 99
When I hear of problems like this and see that the recipe included a
Ball Clay I instinctively wonder if this is the problem.
Ball Clays can vary quite markedly and if the user has just asked for
Ball Clay the supplier can supply from one of a number of different
materials.
In the UK, for example you might be able to choose from materials having
the following % analyses. For comparison the China clay that I use is
added at the end.
SiO2 Al2O3 Other Loss on
Oxides Ignition
BKS(L) 74 17 4 5
HYPLAS 71 70 19 6 5
HVAR 63 19 6 5
TWVA 51 33 5 11
N 50 China clay 47 37 4 12
I wonder, therefore, if you have recently had a new supply of your Ball
clay and if so, do you know its analysis?
David
In message , Kit Shannon writes
>----------------------------Original message----------------------------
>I hope someone can help.
>At the University studio here we fire mostly cone 10 reduction and have
>a large number of shop glazes that have been standards for several years
>now.
>Last year we started to experience problems with one of the glazes.
>It used to be a nice blue celedon like glaze. A little darker than most
>celedons but very nice glaze to emphasize carvings or throw marks.
>Now we can't get it to come out right. Instead of being shiny and
>celedony, it has a smooth matte surface. It doesn't look that bad, but
>it is certainly not what you want over carved designs, cause it totally
>covers them.
>
>here's the recipe:
>
>Custer Feldspar: 25
>whiting : 25
>Ball Clay 25
>Silica 25
>Cobalt Carb .25%
>RIO 1.2%
>
>We've tried re-mixing it to see if there was a mistake there, but it
>comes out the same.
>We think it is something to do with the materials. Maybe the Custer
>Feldspar? Does anyone have ideas as to how we could fix this glaze.
>Maybe substitute some other material?
>Or is it possible that our firing times are slightly different now?
>Different people have been firing the kiln here so it usually doesn't
>get fired the same way twice, but this is the only glaze we are having
>real problems with.
>I doubt it is just the firing because in the past I've used it on pots
>that were fired in the anagama and they came out wonderful
>
>
>Does anyone have some ideas about this glaze?
>Thanks for the help
>
>-Kit
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP18 3DQ, UK. Tel:- +44 (0) 1633 420647
FAX:- +44 (0) 870 1617274
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
Ron Roy on sat 9 oct 99
Tip - never take the material out of the original container - but if you
have to - make sure the part of the bag that gives the details is in the
bin with every batch of new material.
RR
>----------------------------Original message----------------------------
>Thanks for the Info on the glaze problem . I'll try a 100 g batch with G200
>and see if that does anything.
>Yesterday in the studio there was discussion that the Whiting bin may have
>been filled with Dolomite due to different labeling by our supplier. So that
>might be the whole problem, but I'm not sure when the error occurred. So
>i'll also make up a batch with the new whiting and see.
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
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