Janet H Walker on tue 14 may 96
I had a nice glaze, described in the glazebase as "cone 5 - 6". It was
nice and shiny at about cone five and a half but I recalibrated my kiln
sitter ("If it ain't broke, don't fix it.") and now the glaze comes out
with lots of big pinholes at a real cone 6. So, sounds like classic
overfiring. (The glaze recipe reveals that the gloss probably comes
via grossly overfiring what is really a mat glaze.) So I'm going to try
adjusting the glaze by adding some silica and seeing if that extends the
range upwards. In the meantime, I lost some really quite charming pots
and I was wondering whether they are really headed for the bone pile or
whether you can "heal 'em" by refiring, e.g. with some of the adjusted
glaze on top of the whole mess.
Yeah, I realize that this isn't worth the effort BUT on the off chance
that I'll learn something I thought I'd try asking! Thanks!
Jan Walker
Fun & Functional Arts
Cambridge MA USA
F.Forsythe on wed 15 may 96
Janet H Walker wrote:
>
> ----------------------------Original message----------------------------
> I had a nice glaze, described in the glazebase as "cone 5 - 6". It was
> nice and shiny at about cone five and a half but I recalibrated my kiln
> sitter ("If it ain't broke, don't fix it.") and now the glaze comes out
> with lots of big pinholes at a real cone 6. So, sounds like classic
> overfiring. (The glaze recipe reveals that the gloss probably comes
> via grossly overfiring what is really a mat glaze.) So I'm going to try
> adjusting the glaze by adding some silica and seeing if that extends the
> range upwards. In the meantime, I lost some really quite charming pots
> and I was wondering whether they are really headed for the bone pile or
> whether you can "heal 'em" by refiring, e.g. with some of the adjusted
> glaze on top of the whole mess.
>
> Yeah, I realize that this isn't worth the effort BUT on the off chance
> that I'll learn something I thought I'd try asking! Thanks!
>
> Jan Walker
> Fun & Functional Arts
> Cambridge MA USA
Dear Jan:
I've only been doing this for about 4 years but bubbles/pinholing/
boiling -whatever you call it- is the curse of my life. I fire my pots
at cone 6. My theory at this point is as follows:
Glazes like water have a boiling point, but unlike water the boiling
point is very close to the melting point. Now great glazes are those
glazes where there is some room between the melt temp and the boiling
temp. Problem glazes are those where the melt temp and the boiling temp
are about the same. I have found that almost all my nicest glazes fall
into the second category.
Go figure!
The only way to get around this problem is "Soak Down". For me that
means dropping down 50-100 from peak Very Slowly! I think the best rate
is the rate your kiln took to climb up that last 50-100 rise.
{I was talking to someone today who suggested that as the clay body
shrinks during firing gas trapped in tiny air pockets etc. get squeezed
out forming bubbles which have to pass through the molten glass.}
Anyway I must say I am not an authority. I'm sure there are lots of
gurus on this server that will give you a more exhaustive theory. I've
heard lots but so far this is the only approach that has worked for me.
Fraser
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