David Hendley on sun 7 mar 99
These postings by John Hesselberth and Ruth Ballou (below),
saying that limit formulas for glazes are not reliable for predicting
good glazes, have been bothering me all day.
Let me explain why and then present some analogies.
Probably the number one topic for the 2 years I have been
reading Clayart is glazes.
I would estimate that, according to numbers of messages sent
in, the level of glaze expertise of the group is approximately:
10% ------ Have devoted a lifetime to the study of glazes. Lots
of experience, completeley comfortable with glaze calculation
and empherical formulas.
10% ----- Know a little about glaze formulation. Have tried a glaze
calculation program, would like to know more.
10% ----- Know that there is a difference between a recipe and a
formula. Know that some people develop glazes, but have never
tried it.
70% ------ Can anyone give me a reliable cone 6 clear glaze?
Of course, this is just from the people that send in messages. Figure
for every poster there are 5 lurkers. Lurkers are generally less
experienced and less sure of themselves. Make those percentages
2%,, 2%, 2%, and 94%.
People like Craig Martell, Ron Roy, David Hewitt, and Tony Hansen
time and time again try to motivate folks to learn about what makes
up a glaze and how to make good glazes. Glaze calculation and
limit formulas are the best ways to "look" at a glaze before actually
mixing it up and firing it. No, it's not perfect, but it is a "a giant step
for potter-kind" for someone from that 94% group to make that
small step.
Now they are getting a mixed signal. Why bother to learn glaze
calculation if it is not reliable anyway?
Well, because, even if it is not 100% accurate, it is light-years
better that what most potters now do.
Remember in the 1950's (you youngsters, trust me) how
most of the population smoked cigarettes? Then in 1964
it was decided that there was enough evidence to link smoking
to lung cancer, and warnings were put on cigarette packages.
Well, guess what? You can get lung cancer even if you never
smoked a cigarette and were never in the same room with one.
So, would it have been useful to say, at a time when most of the
population smoked, "Wait a minute folks, some people who don't
smoke still get cancer, and some people who do smoke don't
get cancer"?
No.
How about when seat belts were first installed in automobiles?
It was decided that seal belts can help avoid injury and death
and people were encouraged to use them.
Guess what? You can wear a seat belt and still be killed in an
accident. In fact, in certain rare instances, a seat belt can actually
increase the extent of injuries.
So, should we say, "Don't use seat belts because they are not 100%
effective, and can even be detrimental"?
No.
Well, we're still trying to get the great majority of potters to
"stop smoking" and "buckle their seat belts".
I am continually amazed at the the number of silica-starved
matt glazes that I see on functional pottery.
The choice is between blindly using glazes that have been
passed around by friends and learning how glazes work.
And calculation and limit formulas, which are just guidelines
that others have found to be useful, is the place to start.
The work that John is doing is actually (hopefully, if conclusions
can be drawn from the data) serving to modify glaze limits
or create new, more accurate and detailed ones.
Best wishes,
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com
>I agree with most of what you way; however I must take issue with the
>implication that if you stay within established limits you will have a
>durable, non leaching glaze. There is absolutely no data in the
>published literature to confirm that. With the very limited testing I
>have done, I have seen glazes within limits that leach and at least one
>glaze outside of limits that doesn't leach. I do believe the odds are
>higher of having a durable glaze if you stay within limits, but that is
>only a gut feeling at this time--I do not have enough data to support
>that statement. If anyone does, I would welcome seeing it.
>
>We must remember that the limits were derived, for the most part, by
>examining glazes people were using and the criteria of getting "good
>glass", i.e. a visual examination of the glaze. So, for example, the
>limits for Bristol glazes were established by looking at the unity
>formulas of the Brisol-like glaze variants people were using at the
>time--nothing more scientific than this!
>
>Currently there is no way to assure getting a durable glaze--inside or
>outside of limits--other than to do your own testing on the criteria that
>are important to its function. Staying within limits guarantees nothing.
>
>
>John Hesselberth
>Frog Pond Pottery
>P.O. Box 88
>Pocopson, PA 19366 USA
>EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
>
At 01:11 PM 3/5/99 EST, you wrote:
>----------------------------Original message----------------------------
>Using durable glazes that do not leach toxic materials for the interiors of
>functional pots is definitely the responsible way to go. John Hesselberth
>has posted extensive research on his website,
>http://www.frogpondpottery.com. Glaze limits were not developed to predict
>glaze durability. Glaze limits resulted from research done in the 19th
>century and the first part of this century. They were used to show where
>one might get a "good glass." As I understand it, even the term "good
>glass" is not well defined, other than by general statements such as
>surface appearance and being free of surface defects. A visual inspection
>of the glaze sufficed.
>
>To depend on glaze limits to predict whether a particular glaze is safe or
>not could lead the conscientious potter to make a big mistake. If a glaze
>is outside the limits, a potter might erroneously decide that the glaze is
>not safe and go to great expense and trouble to replace a glaze that is, in
>fact, safe. The Floating Blue or Broken Blue glaze that so many ^6 potters
>use is a case in point. It has now been tested by the Alfred lab and been
>found to have very low leaching levels. However, it's outside the limits on
>several oxides. Conversely, depending that a will not leach if it is
>formulated within the limits, could also easily prove to be wrong. The only
>way to be sure is to have the glaze tested at a lab.
>
>I don't mean to imply that glaze limits are not useful. They are one tool
>to use in understanding why a particular glaze looks and behaves the way it
>does. They are useful when you're trying to correct a problem. But there
>are many exceptions. For example, no one knows what to do with boron. Up to
>a point, it improves glaze durability, then durability suffers. No one
>knows what that point is, and it may vary from glaze to glaze. Glaze limits
>are guidelines, useful for understanding, but not hard and fast rules. It
>helps to also have some understanding of the roles of the individual
>oxides, their characteristics, what each brings to the glaze melt. This
>information is not reflected in glaze limits.
>
>It seems to me that some terms are being used without adequate definition.
>For example, what do we mean by a balanced glaze? We may agree that
>durability is important for glazes on the insides of vessels used for food.
>How do we define durability? What hardness should a glaze be? It seems if
>we're going to use these terms, then we need to have some data to back up
>our definitions. Industry has standards and specifications for
>freezing/thawing of tiles, which kinds of tiles can be used on floors or
>walls, glazes on sanitary ware, etc. Are these standards appropriate for
>potters? I'd think we'd find them too limiting. But each of us must take
>responsibility for the glazes we use and we need to be sure of the
>standards we're applying.
>
>Ruth Ballou
>Silver Spring, MD
>
Cheryl L Litman on mon 8 mar 99
David,
Of this 2% who are comfortable with glazes and have spent a lifetime
learning & experimenting, I'd like to see a book published: the
definitive guide - based on current state of the art. Something written
in a style that non-technical people can follow with moderate effort.
Unless one is lucky enough to live in the same town as one of these
guru's and can take a semester long course, we're stuck with reading
tantalizing tidbits on clayart and one week workshops which are all too
brief.
Since I started teaching kids I've given a lot of thought to my personal
frustrations about learning - glazes being one of those areas. My
analogy is that I feel I'm being asked to assemble a very complex jigsaw
puzzle - without being able to see the picture AND I'm being handed one
random piece at a time. I'm drowning in other words. At Ron's glaze
class, he pulled out a dozen books at one point and showed us how many
have false and misleading info. It's no wonder that many just give up.
So, who's up to writing this book?
Cheryl Litman
Somerset, NJ
email: cheryllitman@juno.com
On Sun, 7 Mar 1999 23:47:37 EST David Hendley writes:
>----------------------------Original
>message----------------------------
>These postings by John Hesselberth and Ruth Ballou (below),
>saying that limit formulas for glazes are not reliable for predicting
>good glazes, have been bothering me all day.
>Let me explain why and then present some analogies.
>
>Probably the number one topic for the 2 years I have been
>reading Clayart is glazes.
>I would estimate that, according to numbers of messages sent
>in, the level of glaze expertise of the group is approximately:
>
>10% ------ Have devoted a lifetime to the study of glazes. Lots
>of experience, completeley comfortable with glaze calculation
>and empherical formulas.
>
>10% ----- Know a little about glaze formulation. Have tried a glaze
>calculation program, would like to know more.
>
>10% ----- Know that there is a difference between a recipe and a
>formula. Know that some people develop glazes, but have never
>tried it.
>
>70% ------ Can anyone give me a reliable cone 6 clear glaze?
>
>Of course, this is just from the people that send in messages. Figure
>for every poster there are 5 lurkers. Lurkers are generally less
>experienced and less sure of themselves. Make those percentages
>2%,, 2%, 2%, and 94%.
>
>People like Craig Martell, Ron Roy, David Hewitt, and Tony Hansen
>time and time again try to motivate folks to learn about what makes
>up a glaze and how to make good glazes. Glaze calculation and
>limit formulas are the best ways to "look" at a glaze before actually
>mixing it up and firing it. No, it's not perfect, but it is a "a giant
>step
>for potter-kind" for someone from that 94% group to make that
>small step.
>
>Now they are getting a mixed signal. Why bother to learn glaze
>calculation if it is not reliable anyway?
>Well, because, even if it is not 100% accurate, it is light-years
>better that what most potters now do.
>
>Remember in the 1950's (you youngsters, trust me) how
>most of the population smoked cigarettes? Then in 1964
>it was decided that there was enough evidence to link smoking
>to lung cancer, and warnings were put on cigarette packages.
>Well, guess what? You can get lung cancer even if you never
>smoked a cigarette and were never in the same room with one.
>So, would it have been useful to say, at a time when most of the
>population smoked, "Wait a minute folks, some people who don't
>smoke still get cancer, and some people who do smoke don't
>get cancer"?
>No.
>
>How about when seat belts were first installed in automobiles?
>It was decided that seal belts can help avoid injury and death
>and people were encouraged to use them.
>Guess what? You can wear a seat belt and still be killed in an
>accident. In fact, in certain rare instances, a seat belt can actually
>increase the extent of injuries.
>So, should we say, "Don't use seat belts because they are not 100%
>effective, and can even be detrimental"?
>No.
>
>Well, we're still trying to get the great majority of potters to
>"stop smoking" and "buckle their seat belts".
>I am continually amazed at the the number of silica-starved
>matt glazes that I see on functional pottery.
>The choice is between blindly using glazes that have been
>passed around by friends and learning how glazes work.
>And calculation and limit formulas, which are just guidelines
>that others have found to be useful, is the place to start.
>The work that John is doing is actually (hopefully, if conclusions
>can be drawn from the data) serving to modify glaze limits
>or create new, more accurate and detailed ones.
>
>Best wishes,
>David Hendley
>Maydelle, Texas
>hendley@tyler.net
>http://www.farmpots.com
>
>
>>I agree with most of what you way; however I must take issue with the
>>implication that if you stay within established limits you will have
>a
>>durable, non leaching glaze. There is absolutely no data in the
>>published literature to confirm that. With the very limited testing
>I
>>have done, I have seen glazes within limits that leach and at least
>one
>>glaze outside of limits that doesn't leach. I do believe the odds
>are
>>higher of having a durable glaze if you stay within limits, but that
>is
>>only a gut feeling at this time--I do not have enough data to support
>>that statement. If anyone does, I would welcome seeing it.
>>
>>We must remember that the limits were derived, for the most part, by
>>examining glazes people were using and the criteria of getting "good
>>glass", i.e. a visual examination of the glaze. So, for example, the
>>limits for Bristol glazes were established by looking at the unity
>>formulas of the Brisol-like glaze variants people were using at the
>>time--nothing more scientific than this!
>>
>>Currently there is no way to assure getting a durable glaze--inside
>or
>>outside of limits--other than to do your own testing on the criteria
>that
>>are important to its function. Staying within limits guarantees
>nothing.
>>
>>
>>John Hesselberth
>>Frog Pond Pottery
>>P.O. Box 88
>>Pocopson, PA 19366 USA
>>EMail: john@frogpondpottery.com web site:
>http://www.frogpondpottery.com
>
>
>>
>At 01:11 PM 3/5/99 EST, you wrote:
>>----------------------------Original
>message----------------------------
>>Using durable glazes that do not leach toxic materials for the
>interiors of
>>functional pots is definitely the responsible way to go. John
>Hesselberth
>>has posted extensive research on his website,
>>http://www.frogpondpottery.com. Glaze limits were not developed to
>predict
>>glaze durability. Glaze limits resulted from research done in the
>19th
>>century and the first part of this century. They were used to show
>where
>>one might get a "good glass." As I understand it, even the term "good
>>glass" is not well defined, other than by general statements such as
>>surface appearance and being free of surface defects. A visual
>inspection
>>of the glaze sufficed.
>>
>>To depend on glaze limits to predict whether a particular glaze is
>safe or
>>not could lead the conscientious potter to make a big mistake. If a
>glaze
>>is outside the limits, a potter might erroneously decide that the
>glaze is
>>not safe and go to great expense and trouble to replace a glaze that
>is, in
>>fact, safe. The Floating Blue or Broken Blue glaze that so many ^6
>potters
>>use is a case in point. It has now been tested by the Alfred lab and
>been
>>found to have very low leaching levels. However, it's outside the
>limits on
>>several oxides. Conversely, depending that a will not leach if it is
>>formulated within the limits, could also easily prove to be wrong.
>The only
>>way to be sure is to have the glaze tested at a lab.
>>
>>I don't mean to imply that glaze limits are not useful. They are one
>tool
>>to use in understanding why a particular glaze looks and behaves the
>way it
>>does. They are useful when you're trying to correct a problem. But
>there
>>are many exceptions. For example, no one knows what to do with boron.
>Up to
>>a point, it improves glaze durability, then durability suffers. No
>one
>>knows what that point is, and it may vary from glaze to glaze. Glaze
>limits
>>are guidelines, useful for understanding, but not hard and fast
>rules. It
>>helps to also have some understanding of the roles of the individual
>>oxides, their characteristics, what each brings to the glaze melt.
>This
>>information is not reflected in glaze limits.
>>
>>It seems to me that some terms are being used without adequate
>definition.
>>For example, what do we mean by a balanced glaze? We may agree that
>>durability is important for glazes on the insides of vessels used for
>food.
>>How do we define durability? What hardness should a glaze be? It
>seems if
>>we're going to use these terms, then we need to have some data to
>back up
>>our definitions. Industry has standards and specifications for
>>freezing/thawing of tiles, which kinds of tiles can be used on floors
>or
>>walls, glazes on sanitary ware, etc. Are these standards appropriate
>for
>>potters? I'd think we'd find them too limiting. But each of us must
>take
>>responsibility for the glazes we use and we need to be sure of the
>>standards we're applying.
>>
>>Ruth Ballou
>>Silver Spring, MD
>>
>
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: Terraopera on tue 9 mar 99
David, I'm one lurker who totally agrees with you. I finally learned how to
formulate a glaze (after 10 years of stumbling in the dark) and now can't
immagine why I waited so long. Having thorough knowlege makes working with
pottery much more rewarding and the material is not as difficult as it at
first seems. I would urge anyone that is seriously involved with clay to
buckle down and find out the hows and whys of glaze formulation.
John Hesselberth on wed 10 mar 99
Dave and others,
I hope nothing I have written is being interpreted to mean glaze
calculations and limit formulas are meaningless. Quite the contrary, I
believe the work of Herman Seger and all those who have followed in this
area is outstanding and, as you say, a huge step forward to begin to gain
some understanding of this area. Seger, in particular, was an absolute
genius to have broken through the lore of the times to develop a new way
of thinking about glazes. I believe any potter mixing their own glazes
should be REQUIRED to gain a basic understanding of what is going on
before they set out on their glaze formulation journey. They are, after
all, dealing with some materials that can be exceptionally dangerous to
themselves and others if mishandled. No, I'll say it more strongly. Any
potter mixing their own glazes without an understanding of glaze
calculation is completely irresponsible and is a danger to themselves and
the community.
On the other hand, I have been bothered by the tenor of a lot of messages
on Clayart over the last several months that have implied or stated
directly that everything is OK and you will have a durable, safe glaze if
you stay within limits. This is simply not the case. The limits are not
perfect and, while a good place to start, you must do your own testing to
prove that any particular glaze is suitable for the use for which it is
intended. There are good, durable glazes outside of limits and poor
glazes within limits. I have seen enough data to be certain of that.
Glaze calculation is extremely useful to help you understand what is
likely to happen and to help you decide what direction to move to
eliminate a particular problem or get a specific quality to your glaze.
It is not the be all and end all when it comes to predicting durability
given our current state of knowledge. The more we can get potters to
calculate AND test instead of just mixing and firing, the sooner we will
gain a level of understanding that may allow prediction of durability.
But it is not just calculate; it is calculate and test.
What I am happy to see over the past few weeks is a good lively
discussion of this topic. That's an indication to me that the issue is
being thought about. I have posted a number of messages on this general
topic over the past year that were greeted with deafening silence and
major yawns.
Peace, John
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"
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