William & Susan Schran User on thu 6 jul 06
On 7/5/06 10:25 PM, "Jenni Meredith" wrote:
> I was wondering if anyone could supply a recipe for a matt or satin
> transparent glaze at earthenware temperatures
I would think it would be difficult to develop a truly transparent mat
glaze.
The firing down and crystallization of the glaze forming the mat surface,
would naturally create some opacity.
Don't know why you would not want to use a frit "because they they are
difficult to calculate." Frit formulas are available and they are
consistent, so the formula will remain the same batch after batch.
Here's a link to a low fire glaze base on the Digitalfire web site where you
can read about the whys and wherefores of materials used to create the
glaze:
http://www.ceramic-materials.com/cermat/education/226.html
-- William "Bill" Schran
Fredericksburg, Virginia
wschran@cox.net
wschran@nvcc.edu
John and Judy Hesselberth on thu 6 jul 06
On Jul 5, 2006, at 10:25 PM, Jenni Meredith wrote:
> Hi
> I was wondering if anyone could supply a recipe for a matt or
> satin
> transparent glaze at earthenware temperatures (about 1060c)?
>
> And preferably without zinc so it will not interfere with any
> colours
> and preferabbly without a frit because they can be difficult to
> calculate.
Hi Jenni,
I am afraid you will not find a transparent matte. What makes a glaze
matte is the presence of tiny crystals. They also reflect light and
interfere with transparency.
Slow cooling is beneficial because it allows time for those tiny
crystals to form. I can't guess whether a commercial matte glaze will
benefit from slow cooling or not without knowing how it was
formulated. Many commercial matte glazes are matte whether they are
slow or fast cooled. This is because they are in the class of glazes
I call "unmelted mattes", that is they contain crystalline material
because they were never fully melted or have an abundance of one of
the materials we use as opacifiers. They are generally not very
stable glazes and very suspect for use on functional work..
Sorry I can't be of more help, but a transparent matte might best be
achieved by taking a transparent glossy and doing something like acid
etching or sand blasting after firing. Even then the process of
etching or sand blasting will quickly change the way it reflects
light and the transparency will quickly diminish.
Regards,
John
Bruce Girrell on thu 6 jul 06
John Hesselberth wrote:
>I am afraid you will not find a transparent matte. What makes
>a glaze matte is the presence of tiny crystals.
John,
Wouldn't bumping up the silica work? As you move right from the eutectic
trough on a Currie tile, the glaze appearance changes to more of a matte,
though probably "satin" would be more a closer descriptor. Whatever you call
it, it's not as shiny as the glazes in the eutectic trough.
I assume that the change in surface appearance is due to the incomplete melt
of silica. Eventually the glazes go opaque, but I would think that there
might be a middle ground that provides acceptable texture and transparency.
Of course, that's where the opalescent glazes lie too, so that presents
another problem.
Bruce "just thinking out loud" Girrell
Dave Finkelnburg on thu 6 jul 06
Bruce,
What you suggest is totally subjective. Sure, a
glaze that is "semi-matte," somewhere between glossy
and matte, may have some qualities of transparency and
mattness that make it acceptable to a given artist.
You are talking a matter of degree, though. John
apparently defines a matte glaze as one with a
sufficiently crystalline surface to disrupt light and
lose any glossy appearance. That is going to make it
opaque.
You are right, adding silica can move you from a
truly matte region into a semi-matte area. If you
keep adding silica eventually you will have a glossy
glaze, and if you add even more silica it won't melt
and you will have a "matte" appearance from lack of
glaze melt...also known as an unstable glaze... :-(
Good potting!
Dave Finkelnburg
--- Bruce Girrell wrote:
> Wouldn't bumping up the silica work? As you move
> right from the eutectic
> trough on a Currie tile, the glaze appearance
> changes to more of a matte,
> though probably "satin" would be more a closer
> descriptor. Whatever you call
> it, it's not as shiny as the glazes in the eutectic
> trough.
>
> I assume that the change in surface appearance is
> due to the incomplete melt
> of silica. Eventually the glazes go opaque, but I
> would think that there
> might be a middle ground that provides acceptable
> texture and transparency.
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John and Judy Hesselberth on thu 6 jul 06
Hi Bruce,
Of course there is a continuum. I interpreted Jenni's request to be
one for "window glass" transparency and a matte surface. When
crystals start to be present--whether reprecipitated or never
dissolved--pure transparency is lost. I refer to those glazes as
translucent rather than transparent. So you could probably develop a
semiglossy glaze that was very translucent approaching transparency
or a semimatte where you could still see the clay color through the
glaze.
Of course the reality is that window glass transparency is very
difficult to achieve in any glaze. Even the best glossy glazes are
often populated with a significant number of tiny bubbles which also
affect transparency or give opalescence.
John
On Jul 6, 2006, at 12:20 PM, Bruce Girrell wrote:
> John Hesselberth wrote:
>
>> I am afraid you will not find a transparent matte. What makes
>> a glaze matte is the presence of tiny crystals.
>
> John,
>
> Wouldn't bumping up the silica work? As you move right from the
> eutectic
> trough on a Currie tile, the glaze appearance changes to more of a
> matte,
> though probably "satin" would be more a closer descriptor. Whatever
> you call
> it, it's not as shiny as the glazes in the eutectic trough.
Jenni Meredith on thu 6 jul 06
Hi
I was wondering if anyone could supply a recipe for a matt or satin
transparent glaze at earthenware temperatures (about 1060c)?
And preferably without zinc so it will not interfere with any colours
and preferabbly without a frit because they can be difficult to calculate.
I have some CMC glaze binder which the suppliers tell me I can add to a
slop glaze to make it into a brush-on but I don't know what is in it so not
sure what I would be doing to the glaze balance if I did that.
Also I have read about fring down to get a good matt, and wondered if I
would need to do that if I bought a mtt glaze?
Any help much appreciated.
Thanks
Jenni
Snail Scott on fri 7 jul 06
At 03:25 AM 7/6/2006 +0100, you wrote:
>...I have some CMC glaze binder which the suppliers tell me I can add to a
>slop glaze to make it into a brush-on but I don't know what is in it so not
>sure what I would be doing to the glaze balance if I did that...
It's an organic gum, and will burn out in firing.
It has no effect on the melt chemistry. It only
affects the unfired glaze's handling properties.
> Also I have read about fring down to get a good matt, and wondered if I
>would need to do that if I bought a mtt glaze?
Most matt glazes don't need specially slowed
cooling to be an OK matt, but they'll often be
better and more matted if you do. It doesn't
matter whether you mixed it or someone else did.
Microcrystalline matts need some time to develop
those little crystals that make it look matt.
If a matt glaze is not of this type, then it's
simply a gloss glaze designed for higher
temperatures but underfired. This ought never
to be the case with a glaze marketed as a true
matt.
-Snail
Ivor and Olive Lewis on fri 7 jul 06
I agree with John. Transparency and Matness oppose each other. If it is =
essential to have a transparent surface to reveal underglaze decoration =
then the best way is to apply a transparent glaze and, after the pot has =
cooled, to lightly sandblast with fine Garnet sand to dull the surface.
Ivor Lewis.
Redhill,
South Australia.
John and Judy Hesselberth on sat 8 jul 06
On Jul 7, 2006, at 10:02 AM, Snail Scott wrote:
> This ought never
> to be the case with a glaze marketed as a true
> matt.
Hi Snail,
I totally agree with your statement above but, unfortunately, it is
not the case. Quite a number of commercial matte glazes appear to be
designed to be matte even in our rapid-cooling, underinsulated
electric kilns. That means they are almost certainly unmelted mattes.
Controlled cooling of electric kilns was not in common practice at
all until about 4 or 5 years ago. Yes, macrocrystalline glazes have
always been control cooled and an occasional potter has discovered
the benefits herself, but there was nothing in the kiln manufacturers
literature or the open literature any longer than about one sentence
buried in the middle of a 200 page book that suggested slow cooling
was necessary to get good stable mattes in an electric kiln.
For that matter, it is still not common knowledge. I am certain it is
not being taught in very many classes or workshops yet. I spend
considerable time in the workshops I teach talking about the benefits
of paying attention to the cooling rate and I would guess 2/3 of the
potters I interact with are absolutely amazed that it has any effect
on anything. Even some of them decide to forget about taking
advantage of controlled cooling because it is too much trouble--now
that I find to be really sad. Therefore, I think it is certain that
most commercial matte glazes were developed to be matte in fairly
rapid, uncontrolled cooling. I have tested a number and found them to
be unstable--and an occasional stable one also--so I am pretty
confident this is the case.
It is unfortunate that not all developers of commercial glazes know
what they are doing. It is fortunate that some do know what they are
doing, but I have yet to see any of them recommend controlled cooling
or give a recommended cooling rate for their glazes--though someone,
somewhere probably does. The problem for potters using commercial
glazes is figuring out which of those suppliers know what they are
doing and are providing a reliable product. And that is one of the
major reasons I encourage potters to mix their own--then they can
know what they have.
Regards,
John
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