Vince Pitelka on mon 22 apr 96
Bill -
Thanks for your kind words. Volatization doesn't just occur during reduction.
As a firing proceeds towards red heat and on through the melting cycle, as long
as things are heating up, something is usually dissociating. The early
dissociation - chemically combined water, sulfates, carbonates, etc., occurs
before and during the sintering stage, while the clay is still porous (except
in the case of excessive reduction before body-reduction temperatures) and the
gases escape easily. When a glassy-phase begins to form, both in the clay and
the glaze, things become more complex, especially with complete melting and
fusion. Depending on the viscosity and thickness (thus the problem where
glazes overlap) of the glaze, materials which dissociate will form bubbles,
which may or may not heal completely as the glaze matures and cools. Excessive
content of powerful fluxes, fluorine compounds, manganese, and other materials
will cause more volatization, bubbling, pinholing, and cratering than normal.
Also, we are, of course, dealing with stuff that's mined out of the earth, and
the contents of these minerals varies, with numerous trace ingredients often
present. Dissociation/volatization are hard to predict - the only way to avoid
the pitfalls (pinholes?) of this inevitable part of the firing process is to
fire slow at the end and/or soak the kiln. Remember, as long as the temperature
is increasing SOMETHING will be volatizing at a greater or lesser degree, while
if we stabilize the temperature the volatization will cease and the glaze
should "heal," assuming it is not excessively viscous.
In terms of accelerated volatization during reduction, Hell, I dunno. Logic
tells me that due to the natural dispersion of materials in a fluid from an
area of high concentration to an area of lower concentration, expecially
with the high atomic/thermal movement of glaze-maturing temperatures, the
free carbons removing oxygen from the surface of the glaze will naturally cause
complex oxides deeper within the glaze to relinquish oxygen atoms to those
oxides closer to the surface, creating increased turbulence and increased
volatization. OK you glaze technogeeks, does this make any sense?
- Vince
Vince Pitelka - wkp0067@tntech.edu
Appalachian Center for Crafts - Tennessee Technological University
Smithville, TN
Ron Roy on tue 23 apr 96
Some additions to Vince's post.
Zinc oxide can be problematic - especially in a reducing atmosphere. It is
very easily reduced to the metal even at low temperatures and I believe
it's reputation for causing pin holing is due, at least in part, to boiling
metalic zinc. Same deal for lead by the way.
The other material which can lead to blisters is Fe2O3 (a minor flux) -
when reduced it becomes FeO a very strong flux. If there are bits of iron
in a body (fireclay) and there has not been sufficient oxygen in the bique
during carbon burn off (700C to 900C) then these bits will be reduced to
FeO which will remain in the body waiting to overfire the clay around them.
The locally overfired clay releases gas as it breaks down and causes the
blisters. The symptom which confirms this - refiring (acts some what like a
soak) does NOT cure the problem. Stiffer glazes will suffer more than the
more fluid kind. Change firing cycle of bisque.
Glazes sometimes go through a boiling stage - if that happens to coincide
with shutoff temperature and the glaze is stiff and/or temperature falls
too fast the blisters get frozen - a pinhole is a blister which almost
heals over. This type of blister can be cured in most glazes by soaking,
lowering or raising the maturung tempature of the glaze and /or slower
cooling.
To the person who asked the original question: I would be willing to look
at the glazes to see if there is a common factor between the glazes which
blister and those which do not and make suggestions.
Send me the recipes divided into the two groups. I need to have a better
description of the blister - all broken or some still a bubble, raised
edges, begining to heal over, almost healed over? More blisters on
horizontal surface (less on virtical) evenly distributed. The bubbles only
on the inside - I need to know what type of clay, if there is a change of
colour inside the bisque (break some bisqued clay).
Does the blistering happen in a hotter part of the kiln. Well it seems I
have to know everything - as much as possible anyway.
I just got to all the raku questions - I am talking about hi fire reduction
glazes. The comments about firing through the bubble stage do apply to the
raku problem - unless the bubbles are happening in the reduction - if that
is the case try taking any zinc or lead out of the glaze.
Ron Roy, Toronto, Canada
| |
|
