Craig Martell on mon 18 dec 00
Heidi wanted to know:
>If two glazes have very similar si:al ratios (8.49-8.56)shouldn't their
>surface textures be similar? The analysis is also close, the most
>noticable difference-B203: .05-.16 and SiO2: 2.82-2.9 can someone
>help me to understand why one is quite glossy and one much more of a
>satin matt?
Hi:
The glaze with more of B2O3 will flux more and boron is also a glass former.
later, Craig Martell in Oregon
Heidi Haugen on mon 18 dec 00
If two glazes have very similar si:al ratios (8.49-8.56)shouldn't their
surface textures be similar? The analysis is also close, the most
noticable difference-B203: .05-.16 and SiO2: 2.82-2.9 can someone
help me to understand why one is quite glossy and one much more of a
satin matt? The only conclusion I can come to is that Boron is fluxing
it more? (I realize that a glaze with si:al of 8.49 "should" be glossy
but it's not, it's colorants are copper and rutile. It could be related
to thickness but I'm definitely not halluncinating)
More details-I'm firing cone 6 ox and trying to reduce expansion while
retaining virtually all the characteristics of the original glaze. RRoy
revised the original for me and I use that as my primary base glaze,
but it's too glossy for my decorating style on green. I'm trying to
work through it myself this time.
Question 2: What are the "stable" ranges for Aluminum and Silica at
cone 6. (I've got Cushings numbers but didn't know if John H. and R Roy
had revised them)
Completely unrelated Question 3, but since i'm on a roll I'll throw it
out.
Talked to Skutt today and he told me that I could be getting a 240v read
from the outlet on my ohm-meter but that possibly when the kiln is
drawing power while on high the voltage could be reduced --why would
that be? If I'm getting 240 to the outlet, why would the draw change
that?
Thanks to all in advance,
Heidi Haugen
In Western Montana, digging out from this weeks snow surge and ready to
head to Minnesota for the holidays (with permission from the Mayor to
come over and peruse the leftover gems from his show (big smile) )
iandol on wed 20 dec 00
Dear Heidi Haugen,
At the risk of offending those who are more knowledgable about glazing =
and have at their fingertips computer programs with all the answers I =
will throw in my idea about the alteration in texture of your glaze.
Without seeing the actual recipes I would hazard that the increase in =
the Boron Oxide content, regardless of its origin, is the culprit here.
As I said in a previous post, in making a substitution or change to =
alter a firing range it is important to know as much information as =
possible about every ingredient, both as a compound and in the abstract =
senses of a unity formula or list of oxides. This is why books like =
those of Rhodes, Cooper and Royle, Hopper and Janet and Frank Hamer need =
augmenting with some well based physical chemistry and the precision of =
scientific thought processes. I recommend Kingery et al, Introduction to =
Ceramics.
My opinion is that the additional Boric compound is functioning as =
melter and glass former at a much lower temperature than you anticipate. =
Boric oxide melts at about 450 deg Celsius and sinters as low as 160 deg =
Celsius. These properties give the glaze with the higher concentration =
of B203 a longer time to dissolve all of the refractory substances(Ca, =
Sr, Mg, Zn, Al, Si)and to hold them in solution when the glaze congeals. =
To get a satin matt effect with a higher boron content you would need =
to retard the cooling rate to somewhere between three quarters and half =
the natural cooling rate of the kiln, that is, extend the cooling time =
by twenty five to fifty percent. This will allow calcium and magnesium =
silicate minerals to nucleate and crystallise if compounds of those =
elements are in the original recipes. The alternative is to reduce the =
firing temperature.
Should you wish for further information please contact me off list.
Best regards,
Ivor Lewis
Jeff Lawrence on tue 26 dec 00
Hello Heidi,
I've found in my tinkering with a cone 04 calcium mat glaze that Craig and
Ivor are correct in predicting boron will shine up a mat glaze.
When I tried to make my glaze melt a bit more by adding Cadycal (calcium
borate) I got both a better melt and an unwanted shine. Since the calcium
introduced by the Cadycal should, if anything, have increased the tendency
for the calcium to crystallize and make the glaze mat, the accusing finger
points directly at the boron component.
For what it's worth, I found that 2-3% lithium carbonate (as opposed to 5-6%
Cadycal) gave me the melt without impacting the calcium mat surface. At a
shindig in Florida a year or so back, Ron Roy also justified a tiny amount
of strontium in a cone 6 glaze because he found that a greater variety of
fluxes promotes a better melting.
Have fun!
Jeff Lawrence ph. 505-753-5913
Sun Dagger Design fx. 505-753-8074
18496 US HWY 285/84 jml@sundagger.com
Espanola, NM 87532 www.sundagger.com
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