Jonathan Kirkendall on wed 18 mar 98
Hi,
I have spent the past several weeks delving into glaze chemistry as I
seen to do every year or so when sales are slow. I have been playing
with my Hyperglaze software, and have some questions about the Si:Al
ratio in glazes.
1) What are recommended ratios for mat, satin, and gloss glazes?
2) Does the recommended ratio change depending of temperature of firing?
3) At what point is there too little silica to prevent leaching of oxides?
4) Does working with this ratio help when trying to develop a glaze that
melts fine on porcelain but not on stoneware?
Thanks!
Jonathan in DC
PS Just quit my fulltime job and took on a part time job so I can be in
my studio more. Yikes!
Ron Roy on thu 19 mar 98
Hi Jonathan,
1) This is not always true - there are other factors involved like over
supply of certain oxides, opacifiers etc. If the ration of SiO2 to Al2O3 is
5 or less then you can usually expect a matte glaze. From 5 to 10 usually
results in a semi matte or semi gloss. Over 10 is usually shiny. The amount
of B2O3 has the affect of moving this scale lower - many of the boron
clears have a ratio well under 10.
2) No. This is not so simple at lower temperatures however because the mid
range fluxes - CaO, MgO, BaO, SrO and ZnO will not do much melting - the
result can be opacity and mattness instead of melting especially if there
is a substantial amount of them.
3) Depends on the temperature fired to - the right amount of silica
protects against acid attack and the alumina content determines how well
the glaze will resist alkali (soap) attack. We use limit formulas to try
and estimate the chemical resistance of glazes.
4) I don't think so - Glazes melt better on porcelain period - To get a
glaze that works well on porcelain but not on stoneware I would keep the
ratio the same and increase the both SiO2 and Al2O3 - making the glaze more
refractory. If the glaze was too stiff on porcelain I would drop both -
keeping the ratio the same.
>----------------------------Original message----------------------------
>Hi,
>I have spent the past several weeks delving into glaze chemistry as I
>seen to do every year or so when sales are slow. I have been playing
>with my Hyperglaze software, and have some questions about the Si:Al
>ratio in glazes.
>
>1) What are recommended ratios for mat, satin, and gloss glazes?
>2) Does the recommended ratio change depending of temperature of firing?
>3) At what point is there too little silica to prevent leaching of oxides?
>4) Does working with this ratio help when trying to develop a glaze that
>melts fine on porcelain but not on stoneware?
>
>Thanks!
>Jonathan in DC
>
>PS Just quit my fulltime job and took on a part time job so I can be in
>my studio more. Yikes!
Ron Roy
93 Pegasus trail
Scarborough Otario
Canada M1G 3N8
Phone: 416-439-2621
Fax: 416-438-7849
Web page: Home page http://digitalfire.com/education/people/ronroy.htm
Paul Lewing on thu 19 mar 98
Hi, Jonathon,
I'm sure you will be getting much better replies to your questions
about Si:Al ratio than I can provide, but I did want to weigh in with
some things that you should look at in conjunction with the ratio
number.
One is the amounts of the Al and Si, especially the Si. It's very
possible to have both of these be high or low, and have the ratio of
the two be in an acceptable range. This is what limit formulas tell
you. I find it useful to check the percent of toatal silica. For a
cone 5 glaze, it needs to be between 52% and 65%. For a cone 10
glaze, between 55% and 70%. Either more or less than that will result
in a dry immature looking glaze.
The other number you always have to look at is the amount of Boron.
Boron is a real ringer in glaze calculation, especially at cone 5-6.
Different calculation programs handle this in different ways, if they
handle it at all. On HyperGlaze, when you ask it to graph the surface
characteristic of your recipe, it graphs only the ratio. Sometime put
in a glossy glaze with lots of Boron, and a low ratio, and it will
tell you that it will be a matte glaze. Insight 4 didn't give you any
info about the efect of B at all, but the new version 5 gives you both
a Si:Al ratio and a SiB:Al ratio. And both versions gave you the
option of including B with the fluxes in unity or not. I think HG
does, too. I understand GlazeChem gives you a ratio of total RO2 to
R2O3, which might be helpful.
Happy calculating!
Paul Lewing, Seattle
http://digitalfire.com/education/people/lewing
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