Marcia Kindlmann on sun 7 mar 99
Subject: limits and glaze durability
I'm thankful to Ruth Ballou, John Hesselberth, David Hendley, Ron Roy
and others who've been posting on this subject, giving us perspective
with their understanding that limits are not sure-fire rules for
avoiding leaching or durabilty problems with glazes -- but rather
guidelines that still need to be tested with specific glazes on specific
pots -- whether that be with a laboratory test to detect metals leaching
or an at-home test such as putting a piece in the dishwasher to see
whether the glaze surface gets dulled after a month or so in the strong
alkali environment. (was that sentence long enough?)
In this vein here are three glazes that have seen long-term use in our
household; two of these have held up to repeated dishwasherings and one
has not. I am curious how their recipes stack up to the limit formulas,
whether the difference in durability could have been predicted. I'm
offering these just as data-points, thinking that perhaps someone else
might have had different results with these glazes on a different clay
body, different firing.
These are cone 10 glazes on stoneware (a typical commercial body
described as "production toast" - not a white stoneware) fired years ago
in moderate reduction in a gas kiln -- can't report the firing profile
because it wasn't me firing this kiln. But the glazes all appeared
equally "mature" & properly-fired -- not under- or over-fired, when
they came out of the kiln.
Rhodes #23, on p. 322 in Daniel Rhodes' _Clay and Glazes for the Potter_
cone 9 - 10
Albany Slip 60
Cornwall Stone 25
Iron oxide 5 -- I used the "pure" Fe2O3 for a livlier black
Whiting 10 than obtained with "Spanish Red"
--------
100
This Rhodes #23 glaze has kept its shiny black surface through
long-term kitchen use & repeated dishwashering.
But, below, "Secrest Black" does not -- it has become dull and
pitted in use. Originally gloss black.
Secrest Black cone 10
Custer Spar 54
Flint 22.5
Whiting 13
Kaolin (Georgia) 9
BaCO3 2.5
Zinc Oxide 2.5
---------
103.5
ADD:
Fe2O3 (Spanish) 7.5
Now here's a matt glaze that has kept its original satin-smooth buttery
surface throughout years of use/dishwashering:
Tallow Matt (a creamy ivory-tan color on dark stoneware)
Custer Spar 44
Dolomite 15.5
Ga. Kaolin 26.4
Bone Ash 7.6
Talc 6.5
-----------
100
So -- for what it's worth --this is a tiny bit of field-tested info.
Since these are "old standby" glaze recipes in use in many studios I'd
be curious what results others have had -- with durability of surface --
in long-term functional use of pottery with these & other popular glazes.
I'm grateful for those on Clayart who're researching these matters,
hoping to develop more predictive guidelines for functional glazes --
for this work does take a lot of time and patience. Would it be
helpful for those who're studying limit formulas if others would
post long-term results with glazes in functional use??
Marcia in CT
Gavin Stairs on mon 8 mar 99
Hi All,
I guess I'm going to put my oar in, too.
This is a complex subject. Seger et al. tried to simplify it, and
succeeded pretty well, at least for their time. The problem is a bit
analogous to lens design. Before computers, all calculations to design
lenses were done by "calculators": rooms full of people, mostly young
ladies, with paper and pen and tables of mostly correct function values. I
don't imagine glaze "calculators" ever got that organized, but in Seger's
time, not to mention in the memory of several of us here on this list,
calculations that now happen in seconds or less took minutes or more. So,
the order of the day was to simplify. Lens design took a marvelous stride
forward when computerized calculations became common. Our wonderful 35mm
camera optics are the result of that: the 10:1 zooms weren't possible by
the old methods. Well, we haven't quite caught up in glaze calculation.
All we are doing so far is speeding up the Seger method. Even that is a
great stride, since people who can't quite grasp the old method CAN do it
with the aid of a glaze calculating program on a personal computer.
The next step is a bit harder. Here's a list of what complicates life in
"exact" glaze calculation: ionic radii, valences, bond angles, redox
potentials, which are chemistry simplifications for quantum dynamics of
bond formation, and which go together to produce ... structure! No-one yet
knows how to do exact calculations in these domains, although there are
very nice programs available to do some close approximations. But these
are research tools, not glaze calculators. They are used to design drugs
and model enzymes and all that stuff that biochemists are fond of these
days. The next advance on Seger will come, I suspect, from including some
of these effects in a glaze calculator, so that we can understand by
calculation how the glaze structure changes when we add a bit more sodium.
Here's why this is important for production potters. Glazes are glasses
with crystalline components. Most glazes are approximately the composition
of some naturally occurring minerals, fused. When a glass is close to a
true mineral composition, some of it will try to become that mineral, and
the rest will fail. In other words, the glaze will become two different
substances, rather than only one. Like a hare's fur glaze. Those two
parts will have different chemistries, neither of which is quite like the
whole glaze together. That, simplified, is why it is difficult to predict
the chemical behavior of a mature glaze using only Seger's approximations.
I believe that we will eventually be able to include some of that
complexity in glaze calculators, and when we do, they will become much
better at prediction. Right now, our tools, though marvelous and very
useful, are not at that level.
So, what do we do? Throw up our hands and reach for the recipe book? No,
we test for properties that are important, like leach behavior of toxic
ions, and we use what we have to guide us to better glazes. The programs
don't tell us what we should have, but they are very good for telling us
what we DO have, and what we will have if we change the recipe slightly.
And that is a whole lot better than nothing.
Of course, if the whole thing leaves you cold, then I suggest you take up
sculpture.
Cheers, Gavin
===============================================
Gavin Stairs
Stairs Small Systems (S3)
921 College Street, # 1-A
Toronto, Ontario, Canada M6H 1A1
(416)530-0419
Barney Adams on tue 9 mar 99
Hi,
I think the main point here is testing. The limits give you some
where to start, but Ron and Tony admit that any glaze still needs
to be tested. The variables are many with or without the limits.
They work out nice for me because they give me a spec to follow.
I imagine once I feel more comfortable I will push the limits in
some areas to see what happens. I think we all agree that the glazes
need to be durable and safe as possible. The only for sure thing for
this is to test.
Barney
John Hesselberth on tue 9 mar 99
Hi Marcia,
I find the kind of information you posted extremely interesting and would
like to encourage others to post similar information or the results of
formal leach testing. Data like these cannot be used to prove anything
because they are relatively uncontrolled; however they can be extremely
helpful in developing hypotheses about what is going on. These
hypotheses can later be tested in controlled experiments. That's what
the so-called scientific method is all about.
With respect to the specific recipes you posted, my analysis shows the
one with highest silica (Secrest Black @ 4.1) is the least durable. That
would not be predicted by limit formulas. Tallow Matt is very low in
silica (2.1) for a Cone 10 glaze and Rhodes #23 is intermediate (3.3).
Tallow Matt would be considered out of limits, yet has been durable in
your use. Rhodes #23 is within limits.
All 3 are virtually the same in alumina content ranging from 0.46-0.48.
These numbers are well within alumina limits.
The flux contents, by my calculations are all within normal ranges except
Tallow Matt is a little out of limits on the high side for MgO. However,
I find it interesting that the non durable glaze is the only one with
zinc and it also has the highest KNa level (0.32--right at the upper end
of some limit numbers-- vs 0.19 and 0.21 for the other 2). You may
recall from previous Clayart posts, that there is considerable debate on
whether or not any zinc remains in a glaze fired to cone 10. I don't
have a position on this; however with the complexity of mixtures of this
type and the possibility of forming eutectics, I would think it is
possible. Could it be that zinc is the culprit here? Or the higher KNa
level? I, of course, don't know but it is this kind of clue that I will
keep in mind as I look at other data and begin to formulate hypotheses
about what is happening.
So the bottom line to your question about limit formulas predicting these
results--I would have to say no they don't. Limit formula hypothesis
would have predicted Secrest to be durable and Tallow Matt to be
non-durable.
I'll be interested to read how others view these glazes. With all of us
using (probably) slightly different material analyses and slightly
different limit numbers, there may be some differing conclusions.
Thanks for sharing this info and please, other Clayart members, help us
with whatever data you have. If you feel it will clutter up the list,
then just clutter up my mailbox instead. I still have received very,
very few (like about 2) sets of results from Alfred testing other than my
own in spite of my regular pleas for them. John
Marcia Kindlmann wrote:
>----------------------------Original message----------------------------
>Subject: limits and glaze durability
>
>I'm thankful to Ruth Ballou, John Hesselberth, David Hendley, Ron Roy
>and others who've been posting on this subject, giving us perspective
>with their understanding that limits are not sure-fire rules for
>avoiding leaching or durabilty problems with glazes -- but rather
>guidelines that still need to be tested with specific glazes on specific
>pots -- whether that be with a laboratory test to detect metals leaching
>or an at-home test such as putting a piece in the dishwasher to see
>whether the glaze surface gets dulled after a month or so in the strong
>alkali environment. (was that sentence long enough?)
>
>In this vein here are three glazes that have seen long-term use in our
>household; two of these have held up to repeated dishwasherings and one
>has not. I am curious how their recipes stack up to the limit formulas,
>whether the difference in durability could have been predicted. I'm
>offering these just as data-points, thinking that perhaps someone else
>might have had different results with these glazes on a different clay
>body, different firing.
>
>These are cone 10 glazes on stoneware (a typical commercial body
>described as "production toast" - not a white stoneware) fired years ago
>in moderate reduction in a gas kiln -- can't report the firing profile
>because it wasn't me firing this kiln. But the glazes all appeared
>equally "mature" & properly-fired -- not under- or over-fired, when
>they came out of the kiln.
>
>Rhodes #23, on p. 322 in Daniel Rhodes' _Clay and Glazes for the Potter_
>cone 9 - 10
>
>Albany Slip 60
>Cornwall Stone 25
>Iron oxide 5 -- I used the "pure" Fe2O3 for a livlier black
>Whiting 10 than obtained with "Spanish Red"
> --------
> 100
>
>This Rhodes #23 glaze has kept its shiny black surface through
>long-term kitchen use & repeated dishwashering.
>
>But, below, "Secrest Black" does not -- it has become dull and
>pitted in use. Originally gloss black.
>
>Secrest Black cone 10
>
>Custer Spar 54
>Flint 22.5
>Whiting 13
>Kaolin (Georgia) 9
>BaCO3 2.5
>Zinc Oxide 2.5
> ---------
> 103.5
>ADD:
>Fe2O3 (Spanish) 7.5
>
>
>
>Now here's a matt glaze that has kept its original satin-smooth buttery
>surface throughout years of use/dishwashering:
>
>Tallow Matt (a creamy ivory-tan color on dark stoneware)
>
>Custer Spar 44
>Dolomite 15.5
>Ga. Kaolin 26.4
>Bone Ash 7.6
>Talc 6.5
> -----------
> 100
>
>
>So -- for what it's worth --this is a tiny bit of field-tested info.
>Since these are "old standby" glaze recipes in use in many studios I'd
>be curious what results others have had -- with durability of surface --
>in long-term functional use of pottery with these & other popular glazes.
>
>I'm grateful for those on Clayart who're researching these matters,
>hoping to develop more predictive guidelines for functional glazes --
>for this work does take a lot of time and patience. Would it be
>helpful for those who're studying limit formulas if others would
>post long-term results with glazes in functional use??
>
>
>Marcia in CT
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is time for potters to claim their proper field. Pottery in its pure
form relies neither on sculptural additions nor on pictorial decorations.
but on the counterpoint of form, design, colour, texture and the quality
of the material, all directed to a function." Michael Cardew in "Pioneer
Pottery"
David Hendley on wed 10 mar 99
Marcia,
Wow, I'm really surprised by your experience with
the Seacrest black. I've never used it with as much iron
oxide as you are adding (I think I use about 5 or 6%
for a tenmoku), and I've used the base with other colors
for years and years. For a few years in the 70's this was the
only glaze I used, with 6 or 7 color variations, so I'm very well
familar with it. My recipe is slightly different. The barium carb.
is replaced with Gerstley Borate. I don't know why - that's
the recipe I got way back when. At 2 1/2 %, though, that
shouldn't make much difference.
I have 24 year old dishes made with this glaze, used almost
daily, put through a dishwasher, and they still look absolutely
bright and shiney.
On the other hand, I have some 6 year old ones that are a little
dull-looking after years of hard use.
The old ones have added zirconium opacifier (ultrox), which
is known to be hard, and the new ones have no opacifier and
either 1% cobalt oxide or 5% iron oxide.
Hmmm....maybe the colorants and additives are more important
than we might think in determining how hard a glaze is.
The glaze is WELL within the limit formulas, in fact, it has
a higher than average amount of silica, which is what makes
glazes shiney and hard. Also, plenty of alumina, and the fluxes
are well-balanced.
Your matt glaze, on the other hand, I can tell, just by looking
at the recipe, is way, way low in silica, according to limit
formulas.
(I neglected finishing this and sending it off yesterday. Thanks
for sending in the formulae, John).
Silica at 2.1 is VERY low. I would think, by looking at either
the recipe or the formula, that his 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.
Thanks for the report,
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com
At 11:49 PM 3/7/99 EST, you wrote:
>----------------------------Original message----------------------------
>Subject: limits and glaze durability
>
>I'm thankful to Ruth Ballou, John Hesselberth, David Hendley, Ron Roy
>and others who've been posting on this subject, giving us perspective
>with their understanding that limits are not sure-fire rules for
>avoiding leaching or durabilty problems with glazes -- but rather
>guidelines that still need to be tested with specific glazes on specific
>pots -- whether that be with a laboratory test to detect metals leaching
>or an at-home test such as putting a piece in the dishwasher to see
>whether the glaze surface gets dulled after a month or so in the strong
>alkali environment. (was that sentence long enough?)
>
>In this vein here are three glazes that have seen long-term use in our
>household; two of these have held up to repeated dishwasherings and one
>has not. I am curious how their recipes stack up to the limit formulas,
>whether the difference in durability could have been predicted. I'm
>offering these just as data-points, thinking that perhaps someone else
>might have had different results with these glazes on a different clay
>body, different firing.
>
>These are cone 10 glazes on stoneware (a typical commercial body
>described as "production toast" - not a white stoneware) fired years ago
>in moderate reduction in a gas kiln -- can't report the firing profile
>because it wasn't me firing this kiln. But the glazes all appeared
>equally "mature" & properly-fired -- not under- or over-fired, when
>they came out of the kiln.
>
>Rhodes #23, on p. 322 in Daniel Rhodes' _Clay and Glazes for the Potter_
>cone 9 - 10
>
>Albany Slip 60
>Cornwall Stone 25
>Iron oxide 5 -- I used the "pure" Fe2O3 for a livlier black
>Whiting 10 than obtained with "Spanish Red"
> --------
> 100
>
>This Rhodes #23 glaze has kept its shiny black surface through
>long-term kitchen use & repeated dishwashering.
>
>But, below, "Secrest Black" does not -- it has become dull and
>pitted in use. Originally gloss black.
>
>Secrest Black cone 10
>
>Custer Spar 54
>Flint 22.5
>Whiting 13
>Kaolin (Georgia) 9
>BaCO3 2.5
>Zinc Oxide 2.5
> ---------
> 103.5
>ADD:
>Fe2O3 (Spanish) 7.5
>
>
>
>Now here's a matt glaze that has kept its original satin-smooth buttery
>surface throughout years of use/dishwashering:
>
>Tallow Matt (a creamy ivory-tan color on dark stoneware)
>
>Custer Spar 44
>Dolomite 15.5
>Ga. Kaolin 26.4
>Bone Ash 7.6
>Talc 6.5
> -----------
> 100
>
>
>So -- for what it's worth --this is a tiny bit of field-tested info.
>Since these are "old standby" glaze recipes in use in many studios I'd
>be curious what results others have had -- with durability of surface --
>in long-term functional use of pottery with these & other popular glazes.
>
>I'm grateful for those on Clayart who're researching these matters,
>hoping to develop more predictive guidelines for functional glazes --
>for this work does take a lot of time and patience. Would it be
>helpful for those who're studying limit formulas if others would
>post long-term results with glazes in functional use??
>
>
>Marcia in CT
>
Michael Banks on thu 11 mar 99
Hi John,
First let me say that your posts on this topic have not been greeted with
"deafening silence and major yawns" here. I've been reading all your posts
with avid interest, especially the ones concerning copper.
Recently I took to task a pottery here, for their practice of glazing their
foodware with a soft, high soda, very crazed copper turquoise glaze. The
glaze also has an iridescent lustrous surface, which I know from my own
glaze experiments, to be exsolved copper metal. After reading a letter from
me regarding my concerns and after a bit of a debate, the pottery took the
atitude that because it was legally permitted, they would carry on doing it.
I guess that drinking wine from lead goblets was legal in 100BC Rome too,
but it didn't make it wise in the long run.
Your comments concerning Marcias stoneware glazes have raised some key
issues. The Secrest black may be the least durable, but it may be a mistake
to blame its zinc content. The softening effect of KNa on gloss glazes is
well established and, as you note, the flux unity mol proportion of 0.32 for
the alkalies in this glaze is very high. High KNa also reduces a glazes
ability to resist chemical dissolution, particularly from soda ash-based
dishwashing powders. All other oxide effects excluded, the effect of
alkalis on weakening glazes can be visualised by imagining a line extending
from sodium silicate (a very soft, soluble glass) at one end, to silica
glass (very hard, fairly insoluble) at the other. Useful glazes lie at
intermediate positions along this line and the closer a glaze is to the
silica end-point, then the harder and less soluble the glaze will be. As I
see it, one of the purposes of introducing the other fluxes (Li,Ca, Mg, Zn,
Ba etc) is to make use of powerful eutectic effects in facilitating a melt
and enabling a closer approach to the silica end. Other beneficial results
accrue almost as side effects, which include better filling of sites and
defects in the glass lattice by ions of a variety of diameters, producing
stronger chemical bonds ( I've ignored alumina on purpose, because although
it too contributes to stability, hardness etc, it's most important
contribution is to viscosity which allows the glaze to stay on the pot of
course, during melting).
The durability of Marcias Tallow matt glaze is very interesting, because it
illustrates the problem people have because of a mistaken belief. That
belief is, that limit formulae predict durability. I don't believe this
for one minute.
Limit formulae, as many contributers have noted in posts to Clayart, are
based on observations of what makes a good glass. Matt glazes are not 100%
glasses, they are composite mixtures of solid crystalline minerals and
glass. Applying unity formulae (and limits) to these composites runs into
the same problem that is met when someone tries to apply glaze calculation
regimes to ceramic bodies. Fired pottery is not a glass, but a composite: a
complex interlocking mosaic of crystalline minerals with a minor glass
component.
Therefore, limit formulae should only be taken as a guide to which mixtures
of ingredients will have a high probability of making a good glass. In
other words, I believe that they are useful mainly for formulating clear
gloss glazes. Limit formulae cannot predict glaze hardness, durability and
chemical stability for satin vellums, satins, semi-glosses or matt glazes,
because these are composite materials and often have mineral inclusions
harder and more chemically durable than any glass.
Magnesia matts for example, in my opinion one of the most desirable and
beautiful of glazes, can contain considerable cordierite (moh hardness:
7-7.5) and spinel (moh hardness 8.0) crystals. It is fine-grained crystals
of these minerals, both of which are harder than pure silica, which actually
matt the glaze surface. Hell on the cutlery though.
I'm beginning to feel the burden of being the only mineralogist opining on
Clayart. Come on you chemists, have a look at the importance of the
effects of minerals on glazes, not just the elemental analysis. Nearly all
the interesting glazes that get a mention here, from floating blue to copper
reds, rely on the separation of mineral inclusions. And minerals often have
surprising physical properties, nothing like a glass.
Michael Banks,
Nelson,
New Zealand
'Curiouser and curiouser!' cried Alice.
-----Original Message-----
(clip).......With respect to the specific recipes you posted, my analysis
shows the
one with highest silica (Secrest Black @ 4.1) is the least durable. That
would not be predicted by limit formulas. Tallow Matt is very low in
silica (2.1) for a Cone 10 glaze and Rhodes #23 is intermediate (3.3).
Tallow Matt would be considered out of limits, yet has been durable in
your use. Rhodes #23 is within limits.
All 3 are virtually the same in alumina content ranging from 0.46-0.48.
These numbers are well within alumina limits.
The flux contents, by my calculations are all within normal ranges except
Tallow Matt is a little out of limits on the high side for MgO. However,
I find it interesting that the non durable glaze is the only one with
zinc and it also has the highest KNa level (0.32--right at the upper end
of some limit numbers-- vs 0.19 and 0.21 for the other 2). You may
recall from previous Clayart posts, that there is considerable debate on
whether or not any zinc remains in a glaze fired to cone 10. I don't
have a position on this; however with the complexity of mixtures of this
type and the possibility of forming eutectics, I would think it is
possible. Could it be that zinc is the culprit here? Or the higher KNa
level? I, of course, don't know but it is this kind of clue that I will
keep in mind as I look at other data and begin to formulate hypotheses
about what is happening.
So the bottom line to your question about limit formulas predicting these
results--I would have to say no they don't. Limit formula hypothesis
would have predicted Secrest to be durable and Tallow Matt to be
non-durable.
I'll be interested to read how others view these glazes. With all of us
using (probably) slightly different material analyses and slightly
different limit numbers, there may be some differing conclusions.
Thanks for sharing this info and please, other Clayart members, help us
with whatever data you have. If you feel it will clutter up the list,
then just clutter up my mailbox instead. I still have received very,
very few (like about 2) sets of results from Alfred testing other than my
own in spite of my regular pleas for them. John
Ron Roy on thu 11 mar 99
Hi Marcia,
Thanks for this - we need more of this kind of reporting.
I cannot assume that the deterioration is from attack from alkaline
(washing) or acids (food) - you might want to tell us if the deterioration
is more evident on the inside as opposed to the outside of the Secrest
Black.
I have included the iron (when present) in unity because these glazes were
fired in reduction.
The Albany glaze - It has enough Alumina for a cone 10 glaze and is a
little short of Silica (58.79%) when compared to the Orton limits that come
with Insight.
The Secrest Black - I left out the Zinc oxide because it is no longer there
after a reduction firing. It has enough Alumina and Silica (63.45) - enough
of each.
The Tallow Matt - Has enough Alumina but is way low in Silica (52.32%.)
So in this case the limits are no use in predicting durability - they do
much better at predicting melt however.
>In this vein here are three glazes that have seen long-term use in our
>household; two of these have held up to repeated dishwasherings and one
>has not. I am curious how their recipes stack up to the limit formulas,
>whether the difference in durability could have been predicted. I'm
>offering these just as data-points, thinking that perhaps someone else
>might have had different results with these glazes on a different clay
>body, different firing.
>
>These are cone 10 glazes on stoneware (a typical commercial body
>described as "production toast" - not a white stoneware) fired years ago
>in moderate reduction in a gas kiln -- can't report the firing profile
>because it wasn't me firing this kiln. But the glazes all appeared
>equally "mature" & properly-fired -- not under- or over-fired, when
>they came out of the kiln.
>
>Rhodes #23, on p. 322 in Daniel Rhodes' _Clay and Glazes for the Potter_
>cone 9 - 10
>
>Albany Slip 60
>Cornwall Stone 25
>Iron oxide 5 -- I used the "pure" Fe2O3 for a livlier black
>Whiting 10 than obtained with "Spanish Red"
> --------
> 100
>
>This Rhodes #23 glaze has kept its shiny black surface through
>long-term kitchen use & repeated dishwashering.
>
>But, below, "Secrest Black" does not -- it has become dull and
>pitted in use. Originally gloss black.
>
>Secrest Black cone 10
>
>Custer Spar 54
>Flint 22.5
>Whiting 13
>Kaolin (Georgia) 9
>BaCO3 2.5
>Zinc Oxide 2.5
> ---------
> 103.5
>ADD:
>Fe2O3 (Spanish) 7.5
>
>
>
>Now here's a matt glaze that has kept its original satin-smooth buttery
>surface throughout years of use/dishwashering:
>
>Tallow Matt (a creamy ivory-tan color on dark stoneware)
>
>Custer Spar 44
>Dolomite 15.5
>Ga. Kaolin 26.4
>Bone Ash 7.6
>Talc 6.5
> -----------
> 100
> Would it be helpful for those who're studying limit formulas if others would
>post long-term results with glazes in functional use??
I for one would very much appreciate others posting this kind of data -
great stuff.
RR
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Marcia Kindlmann on fri 12 mar 99
Subject: limits and glaze durability
David Hendley's comments on Secrest Black and Tallow Matt -- comparing
my "field test" results with those that might be expected from glaze
calculation -- are most valuable and I hope that others can offer
experience on the long-term use of other glazes, how well they hold up
to repeated dishwasherings and other challenges of functional use (eg.
marking by silverware or not).
About the Tallow Matt, David 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.
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
And David commented about the Secrest Black -- since his version, with added
zirconium opacifier (ultrox) -- was durably shiny after 24 years --
> Hmmm....maybe the colorants and additives are more important than we
> might think in determining how hard a glaze is.
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
Gavin Stairs on fri 12 mar 99
At 04:45 PM 3/11/99 EST, Michael Banks wrote:
....
>I'm beginning to feel the burden of being the only mineralogist opining on
>Clayart. Come on you chemists, have a look at the importance of the
>effects of minerals on glazes, not just the elemental analysis. Nearly all
>the interesting glazes that get a mention here, from floating blue to copper
>reds, rely on the separation of mineral inclusions. And minerals often have
>surprising physical properties, nothing like a glass.
....
Hear, hear, Michael.
I am quite sure your hypotheses are correct, at least in general tenor. I
would go so far as to propose that it could be possible to plot some of
these relationships by a suitable experimental scheme. The only problem is
that it is a very complex system, and has many mineralological ins and
outs. The spinel family, to pick one at random, is not a single member,
but a class of semi-exact compositions. So the whole depends on many
variables, like firing history, granular size, starting materials and
viscocities (as in alumina). The literature is full of contributions to
our understanding of these systems, but relatively few monographs of
particular interest to studio potters.
I was told by one of the grey haired elders of the clay business that the
first order of business in bringing sense to this field is a paper hunt to
track and organize this stuff. Trouble is, this is very time consuming
work, and few of us can spare the time. Where is the latter day Seger to
come and undertake the task? And what department of ceramics is going to
hire him or her for the years necessary to do the work? And how to write
it and disseminate it to the people to whom it would be of use?
Anyway, we can carry on with the discussions in open fora like Clayart, and
see how far we can get. I'm not a mineralogist, my mineralogy was confined
to a few courses in first year, and I have learned more from the recent
literature on advanced ceramics. When I first read about a perovskite, I
had no idea what they were talking about.
Anyway, here's my idea for simplifying: First, plot the eutectics of the
multiaxis system of Si, Al, and the simple fluxes: Na, K, Ca, Mg. The
properties of interest are solidus, liquidus, and eutectoid viscocities and
solubilities of the melts, followed by hardness and chemical resistance of
the melt phases. That's almost a lifetime in itself, but it will give us a
playing field. The important thing to recognize is that the range of
compositions of glasses are limited by these properties, and that many of
the durability properties of real glazes will come from the interaction of
a near eutectic glass with a partially dissolved mineral (crystal) phase.
So the study of pure glasses and minerals in the first instance will
provide the basic knowledge with which to understand the far more complex
practical glazes. Much of this work has already been done: it can be
winkled out of the literature, if one knows what one is looking for. This
can then be used to inform a relatively small experimental program to yield
the information we want: how to predict the range of behaviors from a
practical glaze recipe, and what to do the change the behavior into what we
want.
Happy hunting.
Gavin
Gavin Stairs
Stairs Small Systems (S3)
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1
(416)530-0419 stairs@stairs.on.ca
John Hesselberth on fri 12 mar 99
Hi Michael,
You make some excellent points. Thanks. Treating mat glazes as a single
phase is a gross oversimplification and certainly not a satisfactory
thing to do. You may well be right on KNa being the culprit on Secrest
also. Good food for thought. John
Michael Banks wrote:
>
>The durability of Marcias Tallow matt glaze is very interesting, because it
>illustrates the problem people have because of a mistaken belief. That
>belief is, that limit formulae predict durability. I don't believe this
>for one minute.
>
>Limit formulae, as many contributers have noted in posts to Clayart, are
>based on observations of what makes a good glass. Matt glazes are not 100%
>glasses, they are composite mixtures of solid crystalline minerals and
>glass. Applying unity formulae (and limits) to these composites runs into
>the same problem that is met when someone tries to apply glaze calculation
>regimes to ceramic bodies. Fired pottery is not a glass, but a composite: a
>complex interlocking mosaic of crystalline minerals with a minor glass
>component.
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is time for potters to claim their proper field. Pottery in its pure
form relies neither on sculptural additions nor on pictorial decorations.
but on the counterpoint of form, design, colour, texture and the quality
of the material, all directed to a function." Michael Cardew in "Pioneer
Pottery"
Ruth Ballou on fri 12 mar 99
Michael,
Would the substitution of Gerstley Borate for the Barium Carbonate in David
Hendley's version of Seacrest Black account for its increased durability?
Ruth Ballou
>----------------------------Original message----------------------------
>
>Your comments concerning Marcias stoneware glazes have raised some key
>issues. The Secrest black may be the least durable, but it may be a mistake
>to blame its zinc content. The softening effect of KNa on gloss glazes is
>well established and, as you note, the flux unity mol proportion of 0.32 for
>the alkalies in this glaze is very high. High KNa also reduces a glazes
>ability to resist chemical dissolution, particularly from soda ash-based
>dishwashing powders. All other oxide effects excluded, the effect of
>alkalis on weakening glazes can be visualised by imagining a line extending
>from sodium silicate (a very soft, soluble glass) at one end, to silica
>glass (very hard, fairly insoluble) at the other. Useful glazes lie at
>intermediate positions along this line and the closer a glaze is to the
>silica end-point, then the harder and less soluble the glaze will be. As I
>see it, one of the purposes of introducing the other fluxes (Li,Ca, Mg, Zn,
>Ba etc) is to make use of powerful eutectic effects in facilitating a melt
>and enabling a closer approach to the silica end. Other beneficial results
>accrue almost as side effects, which include better filling of sites and
>defects in the glass lattice by ions of a variety of diameters, producing
>stronger chemical bonds ( I've ignored alumina on purpose, because although
>it too contributes to stability, hardness etc, it's most important
>contribution is to viscosity which allows the glaze to stay on the pot of
>course, during melting).
>
>
>Michael Banks,
>Nelson,
>New Zealand
>
>'Curiouser and curiouser!' cried Alice.
>
Marcia Kindlmann on fri 12 mar 99
Subject: limits and glaze durability
Ron, thanks for your comments and analysis. You noted,
> I cannot assume that the deterioration is from attack from alkaline
> (washing) or acids (food) - you might want to tell us if the
> deterioration is more evident on the inside as opposed to the outside of
> the Secrest Black.
Mmm, good point. Deterioration all over would indicate dishwasher
effect (alkaline) while if only on food surface that would mean
vulnerable to acids?
So I took a closer look and discovered --aha-- that in fact the Secrest
is shiny and bright on the underside of these dinnerplates & shallow
bowls, but dull & pitted on the inside where the food would sit, while
on the inside just under the rim the glaze is glossy again. But how
strange -- it's seldom that we have tomato sauce. How acidic can such
things as chicken, zucchini, garlic, olive oil, & rice be? A telling
instance would be the inside of a pitcher used to store orange juice,
but I have no Secrest pitcher.
Thanks for sharpening my observations. But now, what to make of this?
Marcia
Gavin Stairs on sat 13 mar 99
At 03:26 PM 3/12/99 EST, Marcia wrote:
....
>So I took a closer look and discovered --aha-- that in fact the Secrest
>is shiny and bright on the underside of these dinnerplates & shallow
>bowls, but dull & pitted on the inside where the food would sit, while
>on the inside just under the rim the glaze is glossy again. But how
>strange -- it's seldom that we have tomato sauce. How acidic can such
>things as chicken, zucchini, garlic, olive oil, & rice be? A telling
>instance would be the inside of a pitcher used to store orange juice,
>but I have no Secrest pitcher.
Hi Marcia (and Ron),
Clever detective work. You know that something in food is etching the
glaze, but what? As noted, it is most likely an acid, but most foods are
acidic. Not just the citrus juices and tomatoes. They are just some of
the most acid foods. Many fruits and vegetables are acid, as well as many
fermented foods, like wine, pickles, most sauces, mayonnaise, cheese,
yoghurt, sour cream, etc. Also, just plain water is neutral to acidic, and
pure water is a powerful solvent. But if your dishes are not exposed to
the stronger acids in foods, it would seem to indicate that the glaze is
not very resistant to acid attack.
Gavin
David Hendley on sat 13 mar 99
>So I took a closer look and discovered --aha-- that in fact the Secrest
>is shiny and bright on the underside of these dinnerplates & shallow
>bowls, but dull & pitted on the inside where the food would sit, while
>on the inside just under the rim the glaze is glossy again.
Marcia,
I have a THIRD theory for the deterioration of the glaze.
My black Seacrest plate, like yours, is also dull only on the top surface.
I don't think it is alkaline from washing or acid from food that
has dulled the surface, but rather 'mechanical wear'.
My plates are worn and dull-looking only on the surfaces that
are either cut and scraped with silverware, or that catch the
bottom of plates stacked on top of them. I think the bare clay
of a plate stacked on top of another plate is particularly damaging
to a soft glaze.
I also agree with Michael, that this glaze is pretty high in
KNaO, which are the 'softer' fluxes, which helps to explain the
glaze wearing with use.
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com
03:26 PM 3/12/99 EST, you wrote:
>----------------------------Original message----------------------------
>Subject: limits and glaze durability
>
>Ron, thanks for your comments and analysis. You noted,
>
>> I cannot assume that the deterioration is from attack from alkaline
>> (washing) or acids (food) - you might want to tell us if the
>> deterioration is more evident on the inside as opposed to the outside of
>> the Secrest Black.
>
>Mmm, good point. Deterioration all over would indicate dishwasher
>effect (alkaline) while if only on food surface that would mean
>vulnerable to acids?
>
>So I took a closer look and discovered --aha-- that in fact the Secrest
>is shiny and bright on the underside of these dinnerplates & shallow
>bowls, but dull & pitted on the inside where the food would sit, while
>on the inside just under the rim the glaze is glossy again. But how
>strange -- it's seldom that we have tomato sauce. How acidic can such
>things as chicken, zucchini, garlic, olive oil, & rice be? A telling
>instance would be the inside of a pitcher used to store orange juice,
>but I have no Secrest pitcher.
>
>Thanks for sharpening my observations. But now, what to make of this?
>
>Marcia
>
Michael Banks on sat 13 mar 99
Hi Ruth,
I've wouldn't have thought so on theoretical grounds, as the Gerstley borate
contains a lot of soda. However stranger things have happened and the
empirical evidence is more reliable than theory. If the glaze isn't durable
with barium in it, it would have been a good idea to remove it on safety
grounds. Otherwise it just reinforces my contention that unity (seger)
formulae cannot predict glaze durability, despite being the best system yet
devised to describe the actual molar composition of a glaze.
Speculating now, it is possible with this glaze, that the high ?iron content
(sorry - I've deleted the original recipe of Seacrest Black and can't
remember what's in it) in reduction has reacted with all the available
silica, thereby depriving barium of forming a stable silicate. Ideally,
barium normally reacts with excess alumina and silica to form celsian
(barium feldspar) in glazes, which is its most stable form. However, as
Janet de Boos has shown in leaching tests on barium-containing glazes, the
stability of this element in glazes is unpredictable and very complex. If
the barium was unable to form an alumino silicate, it could conceivably hang
about as a soluble oxide or salt, leading to loss by leaching.
Substituting gerstley borate for barium may well have led to a more stable
glaze in this instance. Whew!, finally convinced myself.
Michael
'Curiouser and curiouser!' cried Alice
Michael Banks
Nelson
New Zealand
----------------------------Original message----------------------------
Michael,
Would the substitution of Gerstley Borate for the Barium Carbonate in David
Hendley's version of Seacrest Black account for its increased durability?
Ruth Ballou
Ron Roy on sun 14 mar 99
Hi Marcia,
Interesting - the glaze has plenty of silica and alumina - and because
Custer is a potash spar it does not have very much sodium. The KNaO is high
however and I assume that would be the factor that makes this one not so
durable.
All the oxides are within the limit when I compare it to the Orton limits
(Seger I think)
As to which foods are acidic I have no reference data - I am sure when and
if you research this aspect you are going to find some surprising data.
I think I am going to do a little research on those three glazes - I find
the results so far from what I expected that I think we can learn more.
RR
>----------------------------Original message----------------------------
>Subject: limits and glaze durability
>
>Ron, thanks for your comments and analysis. You noted,
>
>> I cannot assume that the deterioration is from attack from alkaline
>> (washing) or acids (food) - you might want to tell us if the
>> deterioration is more evident on the inside as opposed to the outside of
>> the Secrest Black.
>
>Mmm, good point. Deterioration all over would indicate dishwasher
>effect (alkaline) while if only on food surface that would mean
>vulnerable to acids?
>
>So I took a closer look and discovered --aha-- that in fact the Secrest
>is shiny and bright on the underside of these dinnerplates & shallow
>bowls, but dull & pitted on the inside where the food would sit, while
>on the inside just under the rim the glaze is glossy again. But how
>strange -- it's seldom that we have tomato sauce. How acidic can such
>things as chicken, zucchini, garlic, olive oil, & rice be? A telling
>instance would be the inside of a pitcher used to store orange juice,
>but I have no Secrest pitcher.
>
>Thanks for sharpening my observations. But now, what to make of this?
>
>Marcia
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Ron Roy on sun 14 mar 99
I will be teaching a glaze calculation course again this summer at Canador
College.
Canador college is about 200 miles North of Toronto and there is a bus
service from Toronto Airport
This is a hands on course - mixing and firing cone 6 glazes in the pottery
studio and learning how to use computers and software in a computer lab.
The date if the first course is July 5th to July 9th
There is a limit of 12 registrants -
If there are at least 6 registered for the next week they will run another
course.
Course fee is $167.00 + a lab fee of $30.00 CDN.
Residence is - single $90.00 CDN (town houses)
(that's about $200 AMR.)
They do not accept registration till April 1st
If you want details Email me at ronroy@total.net
If you want a brochure Email Kieth Campbell
campbelk@canadorec.on.ca
I'll be back from NCECA March 22.
RR
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Eric Ciup on mon 15 mar 99
Another reason for the pitting and dulling of your glaze may be abrasion. You sa
Try scratching an unpitted area of glaze with the bare clay.
Eric Ciup
Montreal
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