marc mancuso on tue 15 sep 98
Hi all,
The weather is beginning to change moods and foliage here in New England.
To this southern-born and -raised boy, Autumn is never a bad thing.
Today I have a question. In reading the many helpful posts regarding the
ideal, theoretical relationship between a clay pot and what we put on the
outside of it, I realize I haven't got the slightest idea how to
practically test for specific and non-obvious glaze faults.
In looking at a pot, if the glaze hasn't crawled, foamed, or shivered (it
sounds like a rabid dog!) I guess the recipe/firing was mostly successful!
But what if I want to find out what the tensions are when the glaze
*hasn't* failed miserably? A lab will thoroughly test for leaching (she'll
just take it off into a corner and gnaw on it for a while, and then bury
it), but I've never heard of studio potter glaze-fit tests.
Anybody got some suggestions for functional ware?
I know a little about home-testing the *physical* properties of a locally
mined clay, for example. Many of these tests are simple to perform and
yield useful information. For drying shrinkage, you pound out little bars
of a known length, wait till they dry, measure again, and do some
arithmetic. For melting/maturing characteristics, you suspend thin bars
between posts, watch how/if it deforms. For absorbency, weigh dry fired
bar, soak in water, weigh again. You get the idea.
But I digress.
It's all the fault of The Glaze-Savvy People. They tell the rest of us that
the preferred state of a glaze, if it is intended to contribute to the
functionality of a piece, will not only "look healthy," but also be in
slight compression with regard to the pot. It sounds absolutely beautiful.
I want my pots to be strong and mature. I want my pots to be free of
internal conflicts. I want them to display a durable and attractive
exterior. (Great. Now *I* sound rabid!)
This ideal state is referred to all the time. In fact, this next comment is
lifted from a recent clayart posting, in this case, referring to a slip
(rather than a glaze) and the clay body it is applied to:
>The exact ratio will be determined by the other ingrediants [sic] in the
>recipe and of
>course by the clay body the slip is applied to. The key is to find a good
>match
>so that the slip fits almost but not quite like a glove.
See? Now, how am I supposed to test for that? Is it possible at home?
I'm not above getting Mr. Hammer and looking at a cross-section of a glazed
bowl every now and then. But, unless there's crackling, I don't have tests
to determine how *close* to cracking the clay/glaze bond might be. I
suppose I could wait every few weeks (or months) and soak the surface with
the India ink to expose newer cracks, adjust my recipe, glaze an identical
pot, and pray the kiln fires the same. But isn't there a more direct method?
No, I'm not about to ship my weight in functional ware to parts unknown.
Sometimes I think of questions long before I'll really need some answers.
Just in case there aren't any.
Thanks!
Marc
I went to a bookstore and asked the saleswoman,
"Where's the self-help section?"
She said if she told me, it would defeat the purpose....
Ron Roy on wed 16 sep 98
Hi Mark,
This is how I test all glazes - when they come out of the glaze fire I put
em in the freezer for 24 hours (continuing the cooling further will
aggravate any fit problem.) Then I dip em or fill em with boiling water
while still frozen. If they don't craze or dunt or shiver then you are in
the ball park. The larger the test piece the more likely the stresses will
show up. If you use the same glaze inside and out and the glaze is under
compression then the stresses will counteract each other - up to a point -
if you want to use a glaze for a liner and other glazes on the outside then
better not be too far below crazing - to test for this make some cylinders
and glaze em only on the inside - then freeze and fill with boiling water
while they are still frozen. Be prepared for cracking - do it in the sink.
This test is designed to find out if the glaze is under enough compression
to crack the clay. If your glaze is OK on one clay don't assume it will be
the same on other clays.
If you want to be realy sure that you have a good balance - keep repeating
the test on the same ware - lets say 5 times - you can then be fairly sure
you have a workable combination.
It is also a fact that clay bodies vary from batch to batch - some time
they are adjusted by the maker and sometimes they have widely variable
constituents in the formulation - testing each new batch of clay will let
you know ahead of time if you still have the same fit.
If you are making ware that has to go through repeated heating and/or
cooling - like teapots and cassarols - you should have a regular testing
program.
A glaze that winds up bigger than the clay - if applied to a bar of clay
and fired - will bow the bar (concave on the unglazed side.) A crazing
glaze will not bow it the other way because when the glaze crazes the
stress is relieved.
I am wondering if all this is clear - if not I'll be glad to try and make
it more transparent - just let me know where I have failed - RR
>----------------------------Original message----------------------------
>Today I have a question. In reading the many helpful posts regarding the
>ideal, theoretical relationship between a clay pot and what we put on the
>outside of it, I realize I haven't got the slightest idea how to
>practically test for specific and non-obvious glaze faults.
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
Cheryl L Litman on wed 16 sep 98
The simple tests I know of are:
1. Pour some vinegar in overnight and look at it the next morning, if
the color changed then there's leaching.
2. Wipe some india ink on the surface and rub off. If there are cracks
then they will show up easily.
3. Freeze a mug overnight, take it from the freezer and pour boiling
water in it. I usually try the india ink again at this point.
4. Get a 25x microscope (about $10 at Radio Shack) and examine the
surface closely - a great tip I use since Ron Roy's workshop - amazing
detail.
Cheryl Litman
Somerset, NJ
email: cheryllitman@juno.com
On Tue, 15 Sep 1998 08:39:43 EDT marc mancuso writes:
>----------------------------Original
>message----------------------------
>Hi all,
>
>The weather is beginning to change moods and foliage here in New
>England.
>To this southern-born and -raised boy, Autumn is never a bad thing.
>
>Today I have a question. In reading the many helpful posts regarding
>the
>ideal, theoretical relationship between a clay pot and what we put on
>the
>outside of it, I realize I haven't got the slightest idea how to
>practically test for specific and non-obvious glaze faults.
>
>In looking at a pot, if the glaze hasn't crawled, foamed, or shivered
>(it
>sounds like a rabid dog!) I guess the recipe/firing was mostly
>successful!
>But what if I want to find out what the tensions are when the glaze
>*hasn't* failed miserably? A lab will thoroughly test for leaching
>(she'll
>just take it off into a corner and gnaw on it for a while, and then
>bury
>it), but I've never heard of studio potter glaze-fit tests.
>
>Anybody got some suggestions for functional ware?
>
>I know a little about home-testing the *physical* properties of a
>locally
>mined clay, for example. Many of these tests are simple to perform and
>yield useful information. For drying shrinkage, you pound out little
>bars
>of a known length, wait till they dry, measure again, and do some
>arithmetic. For melting/maturing characteristics, you suspend thin
>bars
>between posts, watch how/if it deforms. For absorbency, weigh dry
>fired
>bar, soak in water, weigh again. You get the idea.
>
>But I digress.
>
>It's all the fault of The Glaze-Savvy People. They tell the rest of us
>that
>the preferred state of a glaze, if it is intended to contribute to the
>functionality of a piece, will not only "look healthy," but also be in
>slight compression with regard to the pot. It sounds absolutely
>beautiful.
>I want my pots to be strong and mature. I want my pots to be free of
>internal conflicts. I want them to display a durable and attractive
>exterior. (Great. Now *I* sound rabid!)
>
>This ideal state is referred to all the time. In fact, this next
>comment is
>lifted from a recent clayart posting, in this case, referring to a
>slip
>(rather than a glaze) and the clay body it is applied to:
>
>>The exact ratio will be determined by the other ingrediants [sic] in
>the
>>recipe and of
>>course by the clay body the slip is applied to. The key is to find a
>good
>>match
>>so that the slip fits almost but not quite like a glove.
>
>See? Now, how am I supposed to test for that? Is it possible at home?
>
>I'm not above getting Mr. Hammer and looking at a cross-section of a
>glazed
>bowl every now and then. But, unless there's crackling, I don't have
>tests
>to determine how *close* to cracking the clay/glaze bond might be. I
>suppose I could wait every few weeks (or months) and soak the surface
>with
>the India ink to expose newer cracks, adjust my recipe, glaze an
>identical
>pot, and pray the kiln fires the same. But isn't there a more direct
>method?
>
>No, I'm not about to ship my weight in functional ware to parts
>unknown.
>Sometimes I think of questions long before I'll really need some
>answers.
>Just in case there aren't any.
>
>Thanks!
>Marc
>
>
>I went to a bookstore and asked the saleswoman,
>"Where's the self-help section?"
>She said if she told me, it would defeat the purpose....
>
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