Heidi Haugen on tue 10 oct 00
I'm having a crazing issue with a glaze combo I enjoy. Just delving
into glaze chemistry as questions/situations arise and this particular
one has sparked a myriad of what ifs and whys.
This combo is a white liner from Val Cushing's handbook under a revised
(RR) Val Cushing recipe. I believe that it is the white liner that has
the fit problem.
1. when layering glazes, is it vital that their Si:Al ratios be close?
2. if the bottom layer of glaze crazes, is it possible that the top
glaze would cover up the
cracks so that they were still visible but not crazed on the
surface?
3. what (numbers please) would be considered a high/low silica glaze-
min and max in the RO2 column of the unity formula for a cone 6
glaze. This white liner
is 2.295.
(I have the batch limits from Val Cushing's handbook)
4. the white liner I'm using underneath has a Si:Al ratio of 6.0:1 is
that low?
5. here's the recipes of anyone's interested:
v.c. satin white liner #1 (IMHO really ugly by itself!)
Custer Feldspar 52
Whiting 18
Zinc Oxide 8
EPK 14
flint 8
add:
Tin Oxide 4
Frit 3134 10
RR Revision of V.C.71 Base
Custer 24.0
Frit 3134 16.0
Whiting 13.0
Talc 8.0
EPK 19.0
Silica 20.0
Add:
Cobalt Carb .75
Copper Carb 2.0
(had this glaze tested at Alfred, very stable-hooray again for RR)
Thanks if advance,
Heidi Haugen in Montana
Dave Finkelnburg on tue 10 oct 00
Hi Heidi!
Great questions! The answers should be of interest to some others on
the list!
"1. when layering glazes, is it vital that their Si:Al ratios be close?"
On the contrary, according to what I learned from Paul Lewing at his glaze
workshop this spring, wide differences in the Si:Al ratio between two
layered glazes can cause very interesting glaze interations.
"2. if the bottom layer of glaze crazes, is it possible that the top
glaze would cover up the
cracks so that they were still visible but not crazed on the
surface?" Sorry, Heidi, but it depends...on glaze thickness, how heavy the
crazing is, how much the two glaze layers interact. In general a heavily
crazing glaze doesn't make a very solid foundation for your glaze surface.
:-)
"3. what (numbers please) would be considered a high/low silica glaze-
min and max in the RO2 column of the unity formula for a cone 6
glaze?" My copy of Insight gives the Green and Cooper C6 silica limits as
2.4 to 4.7. Your glaze is a little low on silica according to that, which
is why it has a satin surface.
"4. the white liner I'm using underneath has a Si:Al ratio of 6.0:1 is
that low?" It's on the lower end, but very reasonable. Most glazes range
somewhere between 5 and 15:1.
"here's the recipes of anyone's interested:...." Heidi, your Satin White
Liner has quite high expansion and will craze on practically any stoneware
clay. I'd look for another liner glaze, or do a major revision. Ideally
your liner glaze (my personal opinion here) shouldn't craze.
Ron's revision of Val Cushing's 71 Base is a good glaze, has a Si:Al
ratio of about 8.5, which isn't significantly different than your liner, not
different enough to have much effect.
I hope this helps!
Dave Finkelnburg in southern Idaho where it actually rained
today!!!!! :-)
Ron Roy on wed 18 oct 00
Hi Heidi,
Well I know which glaze is crazing - and one of the reasons is - it's short
of silica. I think it would be easier to make the base glaze into a liner
glaze. The first step would be to run in tin - 4,6 and 8 or if thats too
expensive run in zircopax (superpax or ultrox would be OK) at 6, 9 and 12.
No the ratios do not need to be the same - in fact you will get more
reaction between the two if they are not. More important for intereactions
would be differences in viscosity.
It is possible for the crazing glaze to raise the expansion of the non
crazing glaze so they both will craze. You can mix equal amountds of those
two glazes and test it that way. My calculations would put that mixture at
about 524 but I don't know if that will craze on the clay you are using.
Remember, crazing happens on the way down - after the glaze has set - say
at 600C so you question about the crackes showing underneath does not make
sense. The twp glazes are bonded together and to the body at that point -
if the either glaze crazes they both will because they have melted
together.
2.29 is low for silica for a cone 6 glaze - Hesselberths experiments are
indicating you need a minimum of 2.4 and 2.6 is much better. The glazes
that craze are often those that are short of silica - because silica helps
to lower the expansion of a glaze.
Ratio is a way of forecasting surface - below 5 probably means a matte
glaze - above 10 shiny. This is not so useful with boron glazes because it
promotes melting and does not recrystalize.
Let me know if this raises more questions - RR
>I'm having a crazing issue with a glaze combo I enjoy. Just delving
>into glaze chemistry as questions/situations arise and this particular
>one has sparked a myriad of what ifs and whys.
>This combo is a white liner from Val Cushing's handbook under a revised
>(RR) Val Cushing recipe. I believe that it is the white liner that has
>the fit problem.
>
>1. when layering glazes, is it vital that their Si:Al ratios be close?
>
>2. if the bottom layer of glaze crazes, is it possible that the top
>glaze would cover up the
> cracks so that they were still visible but not crazed on the
>surface?
>
>3. what (numbers please) would be considered a high/low silica glaze-
> min and max in the RO2 column of the unity formula for a cone 6
>glaze. This white liner
> is 2.295.
> (I have the batch limits from Val Cushing's handbook)
>
>4. the white liner I'm using underneath has a Si:Al ratio of 6.0:1 is
>that low?
>
>5. here's the recipes of anyone's interested:
>
> v.c. satin white liner #1 (IMHO really ugly by itself!)
>Custer Feldspar 52
>Whiting 18
>Zinc Oxide 8
>EPK 14
>flint 8
>add:
>Tin Oxide 4
>Frit 3134 10
>
>RR Revision of V.C.71 Base
>Custer 24.0
>Frit 3134 16.0
>Whiting 13.0
>Talc 8.0
>EPK 19.0
>Silica 20.0
>Add:
>Cobalt Carb .75
>Copper Carb 2.0
>(had this glaze tested at Alfred, very stable-hooray again for RR)
>
>Thanks if advance,
>Heidi Haugen in Montana
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
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