Jeff Lawrence on wed 19 mar 97
Ron Roy observed on soaking:
>Crystals form on cooling - the faster you cool the less crystals you will
>have. Adding alumina to a glaze will slow down crystal formation and
>eventually eliminate it.
Hello Ron and other gurus,
I am puzzled by the phenomenon of crystalline mats. My understanding is that
a mature mat fluxes completely and then mats out during cooling as above.
Some people dismiss this with the "all mats are immature!" but as the weed
said, I seed em! THere are some luscious glazzes that scrutiny reveals
microcrystals in.
Is alumina from kaolin and from, say, AlO equally retardant?
Any alternates to titanium dioxide as seeding agents?
Any experience on encouraging microcrystalline growth for mat textures?
Thanks in advance!
Jeff Lawrence
Sun Dagger Design
ph/fax 505-753-5913
Tony Hansen on thu 20 mar 97
Jeff Lawrence wrote:
> I am puzzled by the phenomenon of crystalline mats. My understanding is that
> a mature mat fluxes completely and then mats out during cooling as above.
> Some people dismiss this with the "all mats are immature!"
If you fire a glaze higher and it still fires matte this is a good
indicator
that it is a true crystalline matte.
> Is alumina from kaolin and from, say, AlO equally retardant?
In y experience adding alumina calcined or hydrate doesn't work. In
theory
if it was finer it would have a similar effect to alumina sourced from
kaolin or
feldspar, but I have some 800 mesh material and its no go.
> Any experience on encouraging microcrystalline growth for mat textures?
Wollastonite seeds crystals. If calcia is high you'll get a matte.
Strontium, Titanium, and Magnesia all have mechanisms that produce true
mattes.
--
Tony Hansen, IMC
Ron Roy on thu 20 mar 97
>Ron Roy observed on soaking:
>>Crystals form on cooling - the faster you cool the less crystals you will
>>have. Adding alumina to a glaze will slow down crystal formation and
>>eventually eliminate it.
>Hello Ron and other gurus,
>I am puzzled by the phenomenon of crystalline mats. My understanding is that
>a mature mat fluxes completely and then mats out during cooling as above.
>Some people dismiss this with the "all mats are immature!" but as the weed
>said, I seed em! THere are some luscious glazzes that scrutiny reveals
>microcrystals in.
Hi Jeff
All or glazes would be mat if cooled slow enough. Our shiny glazes are
super cooled liquids. In crystalline glazes alumina must be kept to a
minimum so the crystals can grow. In a clay mat the alumina is oversupplied
and crystallizes on cooling because there is so much of it compared to the
silica. If you cooled these glazes fast enough they would be shiny.
Immature glaze are different in that they never melt properly
>Is alumina from kaolin and from, say, AlO equally retardant?
The preferred source of Al2O3 for us is feldspar and clay because it is
partially "cooked" - I assume that AlO is either Alumina Hydrate or
calcined alumina - Yes - if you can't get enough alumina from spar of clay.
It's not "cooked" (combined) so you will need more flux and/or better
mixing to help it melt. I use it in my C10R terra sig - which is based on
shino glaze knowledge - to get a rich iron red.
>
>Any alternates to titanium dioxide as seeding agents?
Any material which is hard to melt will give crystals a place to start. Any
of the Zirconium silicates (zircopax, super pax etc.), Tin will work. Zinc
- if over supplied.
Rutile - which is mostly titanium will do fine as will illmanite. Even
Magnesia and Calcium will produce crystals if present in high enough
concentrations in the right kind of glaze.
>Any experience on encouraging microcrystalline growth for mat textures?
I use a mat glaze I call snow white on my snow plates - it has 20%
zircopax. It will run if I put it on thick enough but will crystallize on
cooling. I don't think it is hard to design these kinds of glazes - some
seeding material really helps the process and if you can control the
cooling you have an additional advantage.
All crystals can be "helped to grow" by finding the temperature at which
they grow well. You can even get then to continue growing by reheating -
say to a soft bisque like cone 08. The trick is not to start melting them
again because they would have to start growing again. The reheating is
interesting - the crystals grow bigger on the way up and on the way down.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
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