Fabienne Cassman on sat 12 sep 98
------------------
Hello everyone :)
I was wondering if anyone could help me further my understanding of glazes.
I feel overwhelmed by it all and the more I read on the more questions I
have (even confusion sometimes)
Q1: Is there really a correlation between the Si:Al ratio and matt, satin
and glossy glazes? Can it be broken down? (approximately?)
Assuming we are talking about one and only one recipe, not comparing 2 or
more of them:
Q2: How does it affect a glaze if it is designed with SiO2 and/or Al2O3
hovering on the upper or lower limit? (many possible combos while still
maintaining a fairly constant COE) This raises a lot of questions about
each flux as well, what if my primary flux if KNaO or B2O3 or else=3B what's
the general tendencies of the glaze appearance, durability, or other
quality in the end?
TIA for your time :)
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Fabienne
Yes, I have learned from my mistakes...
I can reproduce them exactly.
Paul Lewing on sun 13 sep 98
Fabienne Cassman wrote:
>
> ----------------------------Original message----------------------------
> ------------------
> Hello everyone :)
> I feel overwhelmed by it all and the more I read on the more questions I have
Fabiene,
Me, too, but I'll try to answer your question anyway.
>
> Q1: Is there really a correlation between the Si:Al ratio and matt, satin
> and glossy glazes? Can it be broken down? (approximately?)
Sort of. As usual, the answer to any ceramic question is: It depends.
Virtually all formulas that actually flux out to a usable glaze,
whether matt or glossy, have a Si:Al ratio of between about 5 and 10.
Below that and they will almost (note I said almost) always be very
matt, above that they will be runny.
I always look at two other numbers as well as this ratio and the limit
formulas in evaluating a recipe. One is total percentage by weight of
silica. For a cone 5 glaze this needs to be between 52% and 64%, and
for cone 10, between 57% and 70%. If there's more than that, you end
up with unmelted refractory silica, hence a dry surface. If less, you
end up with leftover fluxes that have nothing to melt, so it's still
dry.
The other number to look for is the amount of boron, especially if
you're working at cone 5 (I have no experience with low-fire, so I'm
only telling you about what I know about here. I assume boron is also
important at cone 04, but I just don't know how). Boron is a real
ringer in the system, a complete anomoly. It fits into the column for
the amphoterics, but sometimes, depending on amount and temperature,
it acts like a glass-former, and sometimes like a flux, or maybe like
both. You can get an idea of how wierd boron is just by the fact that
many glaze programs let you include it with the flux or not, and the
new version of Insight will give you not just a Si:Al ratio, but a
SiB:Al ratio. One is considering it as a flux, the other as a
glass-former. You have to be leery of the little graph in Hyperglaze
that graphs the surface quality of your glaze because it charts Si:AL
ratio only.
Another thing that complicates this is the relative fluxing power of
various oxides. Obviously, a combination of K, Na, Pb,and Li that
adds up to 1 in the RO column will be quite a bit more active a melt
(consequently glossier) than a mixture of Ca, Mg, and Ba, even if the
Si:AL ratio is the same. I use a glaze with a ratio of 4.4 which is
very glossy, but it has a lot of boron and a lot of lithium, so you
see none of this is hard and fast.
> Q2: How does it affect a glaze if it is designed with SiO2 and/or Al2O3 hoveri
The total amount of the Alumina, regardless of the ratio to the
silica, tells you a couple of things. Alumina is the hardest of the
glaze oxides, so more alumina makes for harder, more durable, glazes,
assuming it's in balance with the other ingredients. The other thing
alumina does is limit crystal development. So high-alumina glazes
tend to be flatter colors, with not much internal visual texture.
This is why floor tile glazes tend to be flat colors, rather than
crystally effects.
Anyway, keep on observing, keep on testing. Remember. it's always the
NEXT batch of tests that will make it all become clear.
Paul Lewing, Seattle
Tom Buck on sun 13 sep 98
Fabienne:
I am always snagged by your earnest appeals for help. So, See
below:
Tom Buck ) tel: 905-389-2339
& snailmail: 373 East 43rd St. Hamilton ON L8T 3E1 Canada
(westend Lake Ontario, province of Ontario, Canada).
On Sat, 12 Sep 1998, Fabienne Cassman wrote:
> ----------------------------Original message----------------------------
> ------------------
> Hello everyone :)
>
> I was wondering if anyone could help me further my understanding of glazes.
> I feel overwhelmed by it all and the more I read on the more questions I
> have (even confusion sometimes)
>
> Q1: Is there really a correlation between the Si:Al ratio and matt, satin
> and glossy glazes? Can it be broken down? (approximately?)
Yes, there is a correlation but it assumes some constants. 1) Your glaze
mix will melt at the appropriate cone, since if the glaze is underfired it
will be immature and may exhibit a "matt-like" look but the pot will not
have the physical strength and durability of a properly fired piece. 2)
The glaze will be largely silica (typically, a glaze has 60% by weight of
silica, or higher). 3) The alumina content stays within a specified range
so that the glaze does actually melt at the maturing cone. 4) The glaze
ingredients will be fine powders passing through a 100-mesh screen (if
some small percentage, say 2-3%, stays on the screen, just throw them
away). There are some other minor factors that might have an effect on a
teeter-totter mix.
Now, having written these caveats, here are the usual sign-posts:
If the SiO2/Al2O3 ratio is 6 or below, one can expect the glaze
to be matt, and the actual feel will depend on the ingredients (eg, did
any tiny crystals form on cooling?)
If the Ratio is 6 to 7+ the glaze often will be a satin matt but
take this guide with a big grain of salt.
If the Ratio is above 7, going on 12, the glaze will be glossy.
This usually happens but there are exceptions here as well. (eg,
Crystalline glazes).
Please note that these guidelines only apply if one stays within
the accepted Limits for a given Cone Range. It's a crapshoot once you
stray from the published Limit Forumlas.
> Assuming we are talking about one and only one recipe, not comparing 2 or
> more of them:
>
> Q2: How does it affect a glaze if it is designed with SiO2 and/or Al2O3
> hovering on the upper or lower limit? (many possible combos while still
> maintaining a fairly constant COE) This raises a lot of questions about
> each flux as well, what if my primary flux is KNaO or B2O3 or else;
> what's the general tendencies of the glaze appearance, durability, or
> other quality in the end?
The principal effect of high silica/low alumina, say for a C6 glaze
(C3-C7 range), will be a more fluid glaze at C6; it may run off your pot.
If the glaze is low silica/high alumina, you may achieve an incomplete
melt, ie, an immature-look glaze, and this effect may be spotty, so some
mottling may result, sometimes good, sometimes awful (eg, a rough dry spot
in an otherwise high-sheen glossy glaze, and further, this result may be
random and not reproducible).
As for the consideration of the flux oxide mix in the overall
glaze, this is far too complex to simplify without seriously getting it
wrong by too much oversimplifaction. In this area, testing is the only
sure way to achieve reproducible results.
Only some broad generalizations can be made, eg, the alkali oxides
(K2O, Na2O, and Li2O) will impart a high coefficient of linear expansion,
and too high a content of the alkali oxides will lead to a crazed or
cracked glaze. And too much B2O3 (from a frit, not Gerstley Borate please)
will yield a glass that will mark easily and not be as durable as a good
Si/Al glass. CaO is a good general-purpose flux for most cone ranges. MgO
is best when used at C10. To get some idea of how other glaze components
affect a silica-alumina melt one should read Parmelee/Harmon, or
Taylor/Bull, or ???? There's lots of data out there.
Tom Buck
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