Michael Banks on wed 21 oct 98
----------------------------Original message----------------------------
At 09:46 AM 10/20/98 EDT, you wrote:
>I am certainly hoping that everyone who addresses your problem answers you
on
>the list. Ever since opening a new bag of Gerstley Borate I am having
these
>same difficulties with my favorite white glaze which uses zircopax or
superpax
>for the opacifier. I don't have any of the old Gerstley Borate left, so I
>cannot go back to using it to see if that is causing the problem for sure.
>
>Carolynn Palmer, Somerset Center, Michigan
Carolynn,
You should consider sulphate salt contamination, or other source of sulphur
contamination of the Gerstley Borate, or your glaze. Sulphur vapour exiting
the glaze is a common cause of unhealed pinholes/craters. Gypsum (CaSO4)
can occur in natural mined borates like Gerstley Borate and is a possible
contaminant. Prolonging the heating time between 950 degrees C and the
first fusion temp of your glaze in your kiln (bungs out) may clear the
problem.
Michael Banks
Nelson, NZ
mjbanks@clear.net.nz
Joy Holdread on sat 24 oct 98
In a message dated 10/21/98 10:05:46 AM US Mountain Standard Time,
mjbanks@clear.net.nz writes:
>
> Carolynn,
> You should consider sulphate salt contamination, or other source of sulphur
> contamination of the Gerstley Borate, or your glaze. Sulphur vapor exiting
> the glaze is a common cause of unhealed pinholes/craters. Gypsum (CaSO4)
> can occur in natural mined borates like Gerstley Borate and is a possible
> contaminant. Prolonging the heating time between 950 degrees C and the
> first fusion temp of your glaze in your kiln (bungs out) may clear the
> problem.
>
> Michael Banks
> Nelson, NZ
> mjbanks@clear.net.nz
>
>
How do you define prolonged heating? 200 degrees per hour is what I fire at &
I have one glaze wanting to pin hole.
Joy in Tucson
Michael Banks on mon 26 oct 98
-----Original Message-----
>How do you define prolonged heating? 200 degrees per hour is what I fire
at &
>I have one glaze wanting to pin hole.
>Joy in Tucson
>
Good question Joy!
I was hoping I wouldn't have to elaborate and descend (or ascend) into the
eye-glazing realm of chemistry, but here goes.
The short answer to your question is; It will vary with density of stacking,
electric or gas/oil/wood firing etc. Too many parameters... But the main
problem is to allow time for sulphates or other decomposing minerals (eg
organics, fluorides, carbonates, manganese dioxide etc) to de-gas fully
before the onset of glaze fusion.
Once a glaze is fully molten, evolved gases have to burst through the
surface tension. In many glazes this is not a problem and most glazes go
through a bubbling stage. But if slow trains of bubbles continue to emanate
from refractory mineral grains late in the firing, or even during the
cooling cycle, this can cause pinholes or craters.
Sulphates (gypsum, anhydrite, barite, epsomite etc) seem to give real
problems in glazes compared to the others. Possibly because they have high
decomposition temperatures and only reluctantly start to de-gas as a result
of eutectic reactions with the other glaze ingredients. This means that the
ingredients have to start sintering before the sulphates give up their
sulphur dioxide gas. So what happens is, the tiny grains of sulphate slowly
produce tiny plumes of gas (or trains of bubbles if the glaze melts first)
which have to diffuse out of the glaze coat (or burst through the surface,
if molten). Sulphur dioxide (SO2) gas bubbles also seem to have an
anti-wetting effect on molten glaze causing it to fail to heal pinholes.
Fluorine gas bubbles on the other hand (produced by fluorspar, cryolite,
lepidolite, some frits etc) seems to be less of a problem and is known to
fluidise silicate liquids.
Unfortunately (for the potter) the sintering temperature can be quite close
to the onset of full melting, but if we can prolong this period a bit before
a molten film seals the glaze, chances are that the bulk of the gases may
clear the surface before they cause unhealed holes. The trick is to slow the
kiln down sufficiently in this temperature window to allow sulphur dioxide
to clear the glaze, right across the kiln. Electric kilns have longer
thermal lag times and less ventilation than flame heated ones. Many people
slap the bungs in too early in electrics in my experience. This can entrap
late gases and can cause corrosion, even to stainless steel. In most kilns,
racing along at 200 degrees C is too fast to allow this (but I realise you
may be talking degrees F, Joy). Try 100 degrees per hour from 900 to 1050C.
Pinholing is a huge subject and too big to explore here, so I'd better not
start! (I haven't even touched on the effects of viscosity, surface tension
modifiers, feldspathic pinholing, sulphates in the pot body, soda
volatilisation etc). I refer everyone to Harry Frasers book first please.
Also I couldn't possibly comment on your glaze "wanting to pin hole"!!!
I hear they have animal psychiatrists in America, maybe glaze shrinks too??
Michael Banks
Nelson, NZ
mjbanks@clear.net.nz
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