Lili Krakowski on fri 30 apr 04
Dave answered the question about a non-crazing glaze perfectly.
But it has been my impression over the years that slip glazes, and high =
clay content glazes are less likely to craze than others. Or, if I had =
to get ahold of a glaze quickly to do a job, I would go to a book of =
glazes and pick serveral high clay content ones to try. =20
Is this an a lot of hoohah, or is this observation based on something =
akin to reality?
Lili Krakowski
Be of good courage
Paul Lewing on fri 30 apr 04
on 4/30/04 9:11 AM, Lili Krakowski at mlkrakowski@CITLINK.NET wrote:
> slip glazes, and high =
> clay content glazes are less likely to craze than others.
> Is this an a lot of hoohah, or is this observation based on something =
> akin to reality?
It is based on reality. The 3 oxides with the lowest Coefficients of
Expansion are boron, alumina and silica. Clay is about 1 part alumina and 6
parts silica. So glazes high in clay do tend to have low COE's, hence do
not craze.
Paul Lewing, Seattle
Ron Roy on sun 2 may 04
Hi Lili,
Interesting question - so I have been doing some calculations to try and
draw some conclusions.
Adding EPK brings down the expansion faster than OM#4 for instance in a
glaze in the normal range of expansion. If you add Red Art it comes down
even slower.
There are glazes with a low expansion rate - if you added Red Art you would
be increasing the expansion.
It does depend on the clays expansion rate and the glazes expansion rate of
course but normally - adding clay to a glaze will lower the expansion rate.
It is difficult to look at a recipe and come to the conclusion that it will
not craze however. I can make glazes with high amounts of raw clay in the
recipe that will craze - not hard to do - so looking at the recipe is not
always the best way to answer this question.
Many clay matte glazes have a lot of clay in the recipe - but because they
need to be short of silica to be matte - they still craze.
It is also true that if you have a lot of clay in a glaze (and it melts
properly) you can't have a lot of feldspar - because spar has a fair amount
of alumina in it - plus sodium and potassium. Sodium and potassium have the
highest expansion rates of all the oxides we use.
Perhaps what you are seeing is less high expansion fluxes and more low
expansion fluxes.
When I start up with a glaze now - to fix the expansion - I do start
drawing conclusions as soon as I see the recipe. Mainly it hinges on how
much spar - or how much Spodumene or Lithium Carb.
I will start keeping an eye on the clay content to see if it is an
indicator of expansion - but I have the feeling it will not be as good and
indicator as some other constituents.
RR
>Dave answered the question about a non-crazing glaze perfectly.
>
>But it has been my impression over the years that slip glazes, and high
>clay content glazes are less likely to craze than others. Or, if I had to
>get ahold of a glaze quickly to do a job, I would go to a book of glazes
>and pick serveral high clay content ones to try.
>
>Is this an a lot of hoohah, or is this observation based on something akin
>to reality?
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Joseph Coniglio on mon 3 may 04
Howdy all and
Morn'n MaryB,
Low coefficients materials one and all. I call these satins, (can be)
very smooth, still have a level of depth, very vitrious and not so glassy
at all. Real Safe. Can still be pretty. I'm going there.
I'm going to explore these. It's where my hearts been for a long time
and I have to learn to formulate them. This is where I want to be and I
think I won't have any problems with slip glazes, and probably won't be
bored with them.
Thanks!!
Joseph
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