Wes Rolley on sat 24 may 03
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At 04:59 PM 5/23/03 +0930, iandol wrote:
>Would it not be better teaching practice to suggest that the intended=20
>outcomes should be specified and then a suitable solution sought either by=
=20
>calculation or empirical testing? In other words.... Design the glaze.
Ivor,
There must be some point of knowledge that must be attained before one can=
=20
do this and I fear that I am not there yet. I really love the appearance=20
of the soft green wares of Michael Dodd that are pictured on page 102 of=20
Phil Rogers's book Ash Glazes (First edition). This is reported as a very=
=20
simple glaze with only three components, potash feldspar, ball clay and=20
willow ash. So far, I have not been able to achieve anything near that=20
given that I fire in an electric kiln. Even introducing the gas=20
modifications that Nil and Mel have written about would (1) violate=20
building codes and (2) invalidate my home owners insurance. Also, I do not=
=20
have a source of willows that could be made into enough ash to use ... I do=
=20
make good use of oak ash for other glazes. I also do not have an analysis=20
for willow ash that could be plugged into a calculation program, and the=20
ash content is 40% by weight.
So where to start. It would seem to me that this is exactly where the use=
=20
of a grid technique is must useful with carefully designed experiments. I=
=20
am afraid that I do not know enough about where else to start. At least,=20
if one were to stay within the range of materials used in the glaze that=20
you were trying to imitate.
I, for one, would love to find a reference source that truly deals with the=
=20
process of designing glazes. Clayart provides a lot of information on=20
color response and glaze fit (adjusting COE), but there are many more=20
considerations for a good glaze including adhesion, surface (gloss, matte,=
=20
depth), durability, safety, variegated or not, smoothness of application,=20
etc.
"I find I have a great lot to learn =96 or unlearn. I seem to know far too=
=20
much and this knowledge obscures the really significant facts, but I am=20
getting on." -- Charles Rennie Mackintosh
Wesley C. Rolley
17211 Quail Court
Morgan Hill, CA 95037
wrolley@charter.net
(408)778-3024
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Ron Roy on fri 30 may 03
Hi Wes,
There are many techniques for designing glazes - as many as there are
individuals I suspect. You are right - time makes you better at it but the
sooner you start the sooner you begin to get better at it.
I usually start with a set of preferences and apply what I know to a
theoretical Seger molecular formula. Understanding the role of each oxide
in a glaze is required knowledge of course but there is lots of that
around.
If I were trying to design a durable transparent glossy glaze I would list
the requirements - make sure I understood the role of the oxides and
incorporate those parameters into the Seger formula.
Enough silica - say (4.0) for cone 10
Enough alumina (keeping the ratio of SiO2 to Al2O3) at 10 or above (0.4)
KNaO at about (0.25)
Plenty of CaO - a well recognized glaze "hardener" (0.65)
Some MgO to keep the expansion low enough to prevent crazing - 0.10
I would then make another - with a lower expansion and do a line blend
between the two. If it was not transparent enough I would do another with a
little less alumina and more silica or maybe just add more flux.
If I were trying to duplicate a glaze I would calculate a number of glazes
that were similar - like a shino glaze - and find out what makes them tick
- high alumina, KNaO and maybe Li2O - low ratio and low silica. In fact I
did that and came up with my own and it acts like I want it to.
What you learn along the way is all usable - and the best combination is
calculation and line blending.
RR
>There must be some point of knowledge that must be attained before one can
>do this and I fear that I am not there yet. I really love the appearance
>of the soft green wares of Michael Dodd that are pictured on page 102 of
>Phil Rogers's book Ash Glazes (First edition). This is reported as a very
>simple glaze with only three components, potash feldspar, ball clay and
>willow ash. So far, I have not been able to achieve anything near that
>given that I fire in an electric kiln. Even introducing the gas
>modifications that Nil and Mel have written about would (1) violate
>building codes and (2) invalidate my home owners insurance. Also, I do not
>have a source of willows that could be made into enough ash to use ... I do
>make good use of oak ash for other glazes. I also do not have an analysis
>for willow ash that could be plugged into a calculation program, and the
>ash content is 40% by weight.
>
>So where to start. It would seem to me that this is exactly where the use
>of a grid technique is must useful with carefully designed experiments. I
>am afraid that I do not know enough about where else to start. At least,
>if one were to stay within the range of materials used in the glaze that
>you were trying to imitate.
>
>I, for one, would love to find a reference source that truly deals with the
>process of designing glazes. Clayart provides a lot of information on
>color response and glaze fit (adjusting COE), but there are many more
>considerations for a good glaze including adhesion, surface (gloss, matte,
>depth), durability, safety, variegated or not, smoothness of application,
>etc.
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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