David Hendley on tue 15 jun 99
Back in March, there was an interesting discussion about predicting
glaze durability from limit formulas.
Marcia Kindlmann reported that she had dishes glazed with 'Tallow Matt'
that were still in good condition after 20 years of daily use.
Her dishes glazed with 'Seacrest', however, were badly worn and
scratched.
The interesting part is that 'Tallow Matt' is deficient in silica, according
to accepted limit formulas, and should not produce a durable glaze,
while 'Seacrest' is well within the limits, yet it turned out to be not
very durable.
>About the Tallow Matt, I wrote:
>
> I would think, by looking at either the recipe or the formula, that this
> glaze would have a rough surface, would make an annoying sound when
> scraped with a fork, and would be marked by a knife cutting on it. It
> just doesn't have enough silica to make 'good glass'. I might just mix
> up a batch of your 'Tallow Matt' and try it out.
(Recipe below)
To which Marcia Kindlmann replied:
>
>Yes, it would be good to see if this glaze works for you, because it
>certainly was successful for me when fired in a gas kiln (not mine) 20
>years ago on a brown stoneware from Cutter. Didn't have a rough
>surface, hasn't marked with cutlery, doesn't make those annoying noises,
>feels to the touch as buttery smooth as can be. (on those particular pots of
>mine).
>
Well, I did mix up a batch, and glazed a plate and a mug with it.
They were fired in my wood kiln, cone 10 reduction.
I brought the pieces home and put them in my cabinet for
random regular use by my family.
The glaze does have a nice surface, for a matt glaze. Very handsome,
not as smooth as a glossy glaze, but acceptably smooth for functional
ware.
I wanted to put it through the test of daily use.
Sure enough, after a couple of months of use, the plate IS marked
from tableware being dragged across it. It is not showing other
signs of wear, however.
And, I noticed, it is right on the verge of making that annoying sound
when scraped with a fork.
My conclusion is that, as I expected, this glaze has the same problems
as all silica-deficient matt glazes when used on functional pottery
surfaces -
only not nearly as bad as most.
If I really wanted that stoney look, this is the glaze I would use, because
it is functionally better than most low-silica high-magnesium matts.
This is not readily discernable from limit formulas.
Compared to, say, Rhodes #32, Tallow Matt produces a much nicer
functional surface, yet if you look at the molecular formulas, they
are very similar.
One difference is phosperous - Tallow Matt has it and Rhodes #32
doesn't.
I hope some of you will respond with any thoughts you have about this.
Some things I wonder about:
Does anyone have any ideas or references about the effect of
phosperous in glazes, relating to surface texture or durability?
I used synthetic bone ash (tri-calcium phosphate) in my recipe.
(I also used EPK instead of Ga. kaolin, but I think that is insignificant).
Has anyone noticed a difference in substituting tri-cal p. for bone ash?
Any ideas as to why Marcia's dishes were not marked by utensils
and mine was?
If anyone else has used this glaze, what have been your experiences?
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com
>About the Tallow Matt, I wrote:
>
> I would think, by looking at either the recipe or the formula, that this
> glaze would have a rough surface, would make an annoying sound when
> scraped with a fork, and would be marked by a knife cutting on it. It
> just doesn't have enough silica to make 'good glass'. I might just mix
> up a batch of your 'Tallow Matt' and try it out.
To which Marcia Kindlmann replied:
>
>Yes, it would be good to see if this glaze works for you, because it
>certainly was successful for me when fired in a gas kiln (not mine) 20
>years ago on a brown stoneware from Cutter. Didn't have a rough
>surface, hasn't marked with cutlery, doesn't make those annoying noises,
>feels to the touch as buttery smooth as can be. (on those particular pots of
>mine).
>
>Which brings me to a question about the relationship between field
>experience and limit formulas: were those formulas originally obtained
>from observing glazes fired in oxidation, not reduction?
>
>Could it be that reduction has had a crucial influence in making this
>Tallow Matt behave differently than would be predicted from glaze
>analysis? There is no iron added in the recipe, but clearly the clay
>body has had an influence on this glaze -- iron spots are lusciously
>melting into the glaze from the clay, and the tawny color moves toward
>brown where the glaze is thinner. To get the ivory color we were
>double-dipping it. And I'm told that this particular gas kiln was
>mildly reducing all the way up (something about the way the burners
>didn't fully combust the gas), so perhaps there'd be lots of opportunity
>for the iron in the clay to influence the glaze, make it melt from a dry
>(noisy-with-cutlery) matt to a buttery matt?
>
>Here's the recipe again just for handier reference: (don't know its original
>source)
>
>Tallow Matt cone 9 (but probably fired closer to cone 10)
>color: creamy ivory-tan on brown stoneware, buttery surface
> (in those firings)
>
>Custer Spar 44
>Dolomite 15.5
>Ga. Kaolin 26.4
>Bone Ash 7.6
>Talc 6.5
> -----------
> 100
>
>This could be a fruitful & highly interesting line of inquiry, I think --
>again, one where hearing field experience from various sources would be
really
>helpful in getting some ideas for further research -- ah, more variables to
>consider....
>
>Marcia in CT
>
>
Craig Martell on wed 16 jun 99
-----Original Message-----
From: David Hendley
>One difference is phosperous - Tallow Matt has it and Rhodes #32
>doesn't.
>I hope some of you will respond with any thoughts you have about this.
>Some things I wonder about:
>Does anyone have any ideas or references about the effect of
>phosperous in glazes, relating to surface texture or durability?
Hi:
I've read that P2O5 will form a secondary glass phase in glazes along with
silica. Phosphorous doesn't enter into the silica chains though and makes a
fairly soft glass. It may however contribute something to the durability
depending on how much is in the glaze.
regards, Craig Martell in Oregon
Lynne Antone on wed 16 jun 99
<Has anyone noticed a difference in substituting tri-cal p. for bone ash?
Any ideas as to why Marcia's dishes were not marked by utensils
and mine was?
If anyone else has used this glaze, what have been your experiences?>>
David and all,
I am only beginning to scratch (pun intended) the surface of glazes. I
recently took a Paul Lewing workshop (out at Barb Kates studio here in
Washington state) on glaze calculations and comparisons of glaze software.
One interesting subject we touched on was how the clay body itself can have
quite an effect on the glaze. I had always thought in terms of the effect of
the color of the clay body, but now am seeing how different clays can make
subtle to horrific changes in the glaze. Could this possibly be what is
making the difference? Marcia's quote from your post says she used a brown
stoneware.
Just trying to get some inkling of this aspect of potting; I think I'm too
old for this, right Joyce?
Lynne Antone
Olympia WA
Wishing for more beautiful weather like yesterday instead of the clouds again.
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