Karl P. Platt on mon 4 nov 96
This glaze, posted here yesterday, was noted to crawl:
Recipe: Percent Batch
Nepheline Syenite 47.30 2365
Gerstley borate 27.00 1350
flint (325m) 20.30 1015
EPK 5.40 270
Totals: 100.00 % 5000 gm
Also add:
cobalt oxide 1.00 50
red iron oxide 2.00 100
rutile 4.00 200
cmc (mix dry powders well)0.15 8
The composition above bears little relation to :
61.61 % SiO2
14.23 % Al2O3
8.49 % B2O3
2.37 % K2O
6.55 % Na2O
6.72 % CaO
0.05 % MgO
I get something that looks like this:
Wt% Mol%
KNa2O 7.37% 7.85%
CaO 7.62% 8.99%
CoO 1.04% 0.92%
Al2O3 14.50% 9.40%
B2O3 14.19% 13.40%
Fe2O3 2.08% 0.86%
SiO2 53.20% 58.59%
I get numbers that look a lot different. For several of the raw
materials actual analysis were used and for others, the table in
Parmelee's book, which has never really failed me, are applied. Rutile,
as it is a highly fugitive material was not included in the calculation.
This is all to say that there are some descrepancies somewhere that need
to be reconciled.
The analysis furnished in the original post does not account for the
coloring materials. This is not a good thing to do as these can and do
profoundly affect the character of the glaze -- especially here where
we're looking at 1% CoO and 2% Fe.
Anyway, none of this has much to do with the cause of the crawling and
that is the use of Ghastly Borate. Ditch the Ghastly Borate and your
problems will vanish. Why? Well, once again, Ghastly Borate has lots of
water soluble borate. When the glaze is applied to the ceramic, the
borates are carried into the pores of the clay and ultimately form a
film between the body and unfired glaze. This film, being rich in
pure-ish borate, melts a lot lower than the glaze sitting on it. It thus
melts well before the glaze and prevents adhesion of the glaze once it
does melt -- it's a surface tension thing I don't want to go into now.
The effect can be likened to water on a waxed car. There are also
reasons why this effect is concentrated on rims, etc., which has to do
with how the article dries after it is glazed -- and perhaps with how it
was dried in the first place, but the core problem lies in the Ghastly
Borate.
The reason anyone bothered to invent frit was to solve problems like
this a long time ago. Why on earth does anyone still use Ghastly Borate?
Study a bit on frit and one will see that a foremost reason for
employing frit is to render borates insoluble.
>>if fired higher turns greener<<
This is related to the presence of TiO2
KPP -- Who'll use Ghastly Borate only if civilization ends
--
Matar-se seu televisco
Akita-jin \"Lee Love\" on tue 5 nov 96
<...>
> KPP -- Who'll use Ghastly Borate only if civilization ends
<...>
Some might say it is time for you to start using it! ;^) Like Gandhi
said, when someone asked him what he thought about Western
Civilization: "I think it would be a good ideal!" (8^)
Lee
====================================================
/(o\ Lee Love In "St. Paul", MN ' Come see some pixs of my AkitaPup:
\o)/ mailto:LeeLove@millcomm.com ' http://www.millcomm.com/~leelove
Zatoichi@bruce-lee.com ' "It gets late early out there."
-Yogi Berra-
Fred Hayward on wed 6 nov 96
Karl P. Platt wrote:
>
> ----------------------------Original message----------------------------
> This glaze, posted here yesterday, was noted to crawl:
>
> Recipe: Percent Batch
> Nepheline Syenite 47.30 2365
> Gerstley borate 27.00 1350
> flint (325m) 20.30 1015
> EPK 5.40 270
> Totals: 100.00 % 5000 gm
>
> Also add:
> cobalt oxide 1.00 50
> red iron oxide 2.00 100
> rutile 4.00 200
> cmc (mix dry powders well)0.15 8
> snip, snip
>
> Anyway, none of this has much to do with the cause of the crawling and
> that is the use of Ghastly Borate. Ditch the Ghastly Borate and your
> problems will vanish.
What should be substituted for the "ghastly borate"??
Claudia
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