Pfeiffer, Dan R (Dan) on fri 15 apr 05
I have been looking at glaze cal programs to see if I can find one that
would help make sense of the mod's we are making to stop the crazing.
So far GlazeBase I like the best with matrix a very close second. My guess
is matrix is better but I am not sure that we will use most of the good
thing it has until we get better at the chemistry. The question I have is
how the COE is calculated when color oxides are added. In both programs the
CEO goes up as I add cobalt but the glaze crazes less on the pot.?? It would
seem crazing is more than just the CEO :-( I was hoping to find one magic
number that would say at this point the glaze will not craze.
One other thing I noted was that in matrix when I added the cobalt the
AL/Si graph moved around as the account of cobalt when up. Why is this?
Hear is the recipe (end result of mixing old plus new) I came up with to fix
the current pot of glaze.
Custer feldspar 50
Flint 44
EPK 20
Whiting 20
Talc 10
We hope to test this Monday.
Dan & Laurel in Elkmont Al
Potters Council Members
David Hewitt on sat 16 apr 05
This is an interesting question to me as I admit that I do not know the
answer.
What I do know is that most of the sets of Ceramists coefficients of
expansion for different oxides do not actually include the colouring
oxides.
I think I am correct in saying that most glaze programs which do include
a coefficient for the colouring oxides have 'assessed' what that they
think is the likely order for such a coefficient and attached these to
the coefficients produced by, say English & Turner. To my knowledge the
only Ceramists to produce coefficients for colouring oxides are Mayer &
Havas and Appen. All the data on this that I have been able to collect
is shown on my web site under 'Calculating Crazing'.
Web:- http://www.dhpot.demon.co.uk
In my CeramDat program I give three coefficients, English & Turner,
McLinden and Appen. If, therefore, you enter a recipe which includes a
colouring oxide, only the Appen figure includes this in its calculation
of a glaze Coefficient of expansion.
Unless I learn otherwise I tend to ignore the colouring oxides when
analysing a recipe for expansion problems because of the type of
practical results which you mention. I would be pleased to hear what
others think on this.
David
In message , "Pfeiffer, Dan R (Dan)"
writes
>I have been looking at glaze cal programs to see if I can find one that
>would help make sense of the mod's we are making to stop the crazing.
>So far GlazeBase I like the best with matrix a very close second. My guess
>is matrix is better but I am not sure that we will use most of the good
>thing it has until we get better at the chemistry. The question I have is
>how the COE is calculated when color oxides are added. In both programs the
>CEO goes up as I add cobalt but the glaze crazes less on the pot.?? It would
>seem crazing is more than just the CEO :-( I was hoping to find one magic
>number that would say at this point the glaze will not craze.
>One other thing I noted was that in matrix when I added the cobalt the
>AL/Si graph moved around as the account of cobalt when up. Why is this?
>
>Hear is the recipe (end result of mixing old plus new) I came up with to fix
>the current pot of glaze.
>
>Custer feldspar 50
>Flint 44
>EPK 20
>Whiting 20
>Talc 10
>
>
>
>We hope to test this Monday.
>
>
>
>Dan & Laurel in Elkmont Al
>Potters Council Members
--
David Hewitt
Web:- http://www.dhpot.demon.co.uk
John Hesselberth on sat 16 apr 05
On Friday, April 15, 2005, at 04:45 PM, Pfeiffer, Dan R (Dan) wrote:
> The question I have is
> how the COE is calculated when color oxides are added. In both
> programs the
> CEO goes up as I add cobalt but the glaze crazes less on the pot.??
Hi Dan,
I suspect the various software authors have each treated this question
a little differently because there is no "right" way to handle it.
First, COEs for a glaze are calculated by the "rule of mixtures". Like
the perfect gas law it is an approximation of how the real world
works--a very useful approximation if its limitations are understood.
But it does have limitations. Then there is the problem that there is
not a single agreed on set of expansion coefficients that cover every
oxide we use. Every set has a hole or two in just the oxides are in
"base" glazes. If memory serves me, among the colorants, only iron
oxide has measured numbers that could be used in a consistent set with
the "base" glaze oxides. Any calculation you see that includes such
things as cobalt or copper has been estimated from other workers and
may or may not be at all accurate. My assessment of the literature is
that their is significant differences between the workers in the field
that have never been fully resolved to get a single comprehensive set
of expansion coefficients for all the oxides of interest to us.
So we have each taken our own approach. I had though most of us ignored
the colorants in the calculations because any inclusion would be as
likely to be misleading as it would be helpful. That is what I have
done with GlazeMaster for the things shown as an Additive. They are
simply not included in the calculations and the numbers don't change as
you change the additive levels. If, however, you mistakenly put a
colorant in the Ingredient section, GlazeMaster will include it in the
calculations in a way that confuses. For most colorants I have no
expansion data entered. Hence it assumes it is zero and the calculated
expansion for the glaze will go down as you increase the colorant
level. I do have an entry for iron and so if you enter it as an
ingredient the calculated expansion will probably go up (iron oxide has
a fairly high expansion coefficient)
But the bottom line in my opinion is that expansion calculation are
nowhere sophisticated or accurate enough to include colorants. I
recommend you list colorants as additives and not try to calculate
their effect on expansion. We just don't know enough to do it well.
Other software authors may have a different view.
Regards,
John
Ron Roy on mon 18 apr 05
Hi Dan,
just a few comments - if you are working at cone 6 your understanding of
all this will improve if you read chapter 5 in Mastering cone 6 glazes -
even if you are working at other temperatures.
As you are successful at eliminating crazing in a glaze you will gradually
become aware of what the magic expansion number is for that specific clay
body. As you work on other glazes you will have a much better idea of what
is need to eliminate crazing on any specific clay body.
In other words - the more you do it the better you get at it - because you
know what calculated expansion worked for other glazes.
Unless there is a lot of coloring oxide - it will not do much to the
overall expansion rate - 1% cobalt or copper is not worth worrying about -
10 iron oxide or rutile will have some effect.
When I am making adjustments to a glaze I often don't even count in the
oxides - unless there is a lot.
The glaze below would not craze on the porcelain body I use for instance -
but might dunt a body with an excess of cristobalite. The lesson is always
the same - each clay body has a different expansion/contraction rate - so
it pays to do your glaze testing on all the clays you use.
RR
>I have been looking at glaze cal programs to see if I can find one that
>would help make sense of the mod's we are making to stop the crazing.
>So far GlazeBase I like the best with matrix a very close second. My guess
>is matrix is better but I am not sure that we will use most of the good
>thing it has until we get better at the chemistry. The question I have is
>how the COE is calculated when color oxides are added. In both programs the
>CEO goes up as I add cobalt but the glaze crazes less on the pot.?? It would
>seem crazing is more than just the CEO :-( I was hoping to find one magic
>number that would say at this point the glaze will not craze.
>One other thing I noted was that in matrix when I added the cobalt the
>AL/Si graph moved around as the account of cobalt when up. Why is this?
>
>Hear is the recipe (end result of mixing old plus new) I came up with to fix
>the current pot of glaze.
>
>Custer feldspar 50
>Flint 44
>EPK 20
>Whiting 20
>Talc 10
>
>
>
>We hope to test this Monday.
>
>
>
>Dan & Laurel in Elkmont Al
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on tue 19 apr 05
I have expansion numbers for iron, titanium, tin, zirconium and manganese
in the set I use and do find them useful.
On the other hand I often work on glazes using only the base ingredients -
if I am lowering expansion - looking at only the base materials - I am
still lowering the over all expansion of the glaze - I simply add the
additional modifiers back in at the end.
There are even situations when I am working with a glaze that has a mystery
material in it - say a local clay. Mystery material means I don't have an
analysis for it.
I can still lower the expansion using the other materials - make em add up
to what I started with and then add the mystery material back in - the
expansion will be lowered for the whole glaze.
If it is a local clay I can always sub in Albany or Red Art for the mystery
clay - keeping the amount the same of course - and sub it back in at the
end.
We need to look at calculation software as simply a tool - a dumb tool -
that cannot think for itself - then we can trick it into giving us results
in spite of the inherent limitations.
If you don't think tricking a computer program into giving up stuff it was
not designed to give up fun - well - that probably means you haven't done
it yet.
RR
>But the bottom line in my opinion is that expansion calculation are
>nowhere sophisticated or accurate enough to include colorants. I
>recommend you list colorants as additives and not try to calculate
>their effect on expansion. We just don't know enough to do it well.
>Other software authors may have a different view.
>
>Regards,
>
>John
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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