Sue Hintz on fri 13 nov 98
A couple of weeks ago I asked for help with a glaze that I loved but
was causing problems with drips appearing after firing. Ron Roy re-
worked my glaze and gave me three possible, slightly different, glazes
to try. I wanted to share the results with the group.
Here is my original glaze that showed drips after firing:
Cone 5 ox Matt Base
74.7 Cornwall Stone
17.9 Whiting
5.1 Ger. Bor
2.4 Bentonite
Here were Ron's suggestions:
Revision #1
Cornwall stone 33
G200 20
Whiting 19
Gers. Bor 11
EPK 6
Silica 11
Bentonite 2
Revision #3
Cornwall Stone 34
G200 24
Whiting 17
Gers. Bor 9
EPK 5
Silica 11
Bentonite
2
Version #2 was a combination of the two which if you do the math
comes out as
Cornwall Stone 33.5
G200 22
Whiting 18
Ger. Bor 10
EPK 5.5
Silica 11
Bentonite 2
Here are the results. My favorite was #2 but all three were good! I put
the glaze on kind of thick with big drip marks and it all smoothed out--
YES!!! I use this glaze on Speckled brownstone from Highwater. It doesn't
seem to run much but does smooth out so drips don't show. Speckles show
through nicely. I think it would be a great glaze for slip work. And
if you want a glaze that works with crimsons and pinks from mason stains -
this is a keeper! I add 5% Superpax because I want the glaze to be white.
Ron, Thank you! Thank you! Sue.
Ron Roy on sun 15 nov 98
So now I find myself reviewing my own revisions - is my world right side up
or upside down - I can't tell anymore.
They all have an oversupply of CaO - the original has the most oversupply
and my #3 has the least - oversupply.
They all have more than enough silica and alumina to satisfy the limits I
use with boron in unity with the fluxes.
They are all going to craze on many stoneware bodies - definitely vitrified
cone 5 and 6 porcelains.
I often say this and the reason is - the more free quartz in a body the
more drop in volume of that free quartz at 573C. This is the quartz
inversion which helps to keep glazes from crazing - because it raises the
expansion/contraction of the body. The reason that vitrified porcelain (or
any other well vitrified clay) is harder to fit glazes to without them
crazing - More of the free quartz becomes part of the glassy phase of the
body - no longer a crystal and does not go through the crystal quartz
inversion - does not increase the expansion/contraction of the body and
does not help with the compression of the glaze which counteracts crazing.
Now if the glaze is still soft enough to adjust to the body when it drops
through 573C then all that is of no consequence.
RR
>----------------------------Original message----------------------------
>A couple of weeks ago I asked for help with a glaze that I loved but
>was causing problems with drips appearing after firing. Ron Roy re-
>worked my glaze and gave me three possible, slightly different, glazes
>to try. I wanted to share the results with the group.
>
>Here is my original glaze that showed drips after firing:
>Cone 5 ox Matt Base
>74.7 Cornwall Stone
>17.9 Whiting
>5.1 Ger. Bor
>2.4 Bentonite
>
>Here were Ron's suggestions:
>
>Revision #1
>Cornwall stone 33
>G200 20
>Whiting 19
>Gers. Bor 11
>EPK 6
>Silica 11
>Bentonite 2
>
>Revision #3
>Cornwall Stone 34
>G200 24
>Whiting 17
>Gers. Bor 9
>EPK 5
>Silica 11
>Bentonite
>2
>
>
>Version #2 was a combination of the two which if you do the math
>comes out as
>Cornwall Stone 33.5
>G200 22
>Whiting 18
>Ger. Bor 10
>EPK 5.5
>Silica 11
>Bentonite 2
>
>Here are the results. My favorite was #2 but all three were good! I put
>the glaze on kind of thick with big drip marks and it all smoothed out--
>YES!!! I use this glaze on Speckled brownstone from Highwater. It doesn't
>seem to run much but does smooth out so drips don't show. Speckles show
>through nicely. I think it would be a great glaze for slip work. And
>if you want a glaze that works with crimsons and pinks from mason stains -
>this is a keeper! I add 5% Superpax because I want the glaze to be white.
>
>Ron, Thank you! Thank you! Sue.
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
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