Michael McDowell on sun 12 oct 03
Here's a question for Ron Roy. Ron, I might have sent this to you as
a private e-mail. But since you are educating so many of us here on
clayart. I thought the question might be relevant to some others as
well. Since you sent me formulas for a series of glazes of different
expansion rates a few years back, I have taken to testing every new
batch of clay to determine its expansion and the prospective fit of
my glazes. You originally sent me formulas that ranged from 400 to
600 in your numbering scheme with Insight software. I have since had
to expand that series to include glases down to 350 and up to 650 to
find fitting glazes for all my different clays.
Most recently I have had a series of tests come out which exhibited
crazing on cups glazed on the inside only with glaze expansion of
500 and above. Cups glazed with expansion rates of 450, 400, and 350
all are uncrazed after a couple of cycles of freezing and filling
with boiling water. Even the test with expansion of 350 shows no
indication of compression defects. It seems like a rather large
range of compatible expansion rates in my limited experience. Do you
think it would be wise to go ahead and use glazes with expansions as
low as 375 based on these results? Or should I try and stay closer
to the point at which crazing first ceases? Around 450 in this case.
Michael McDowell
Whatcom County, WA
Michael@McDowellPottery.com
http://www.McDowellPottery.com
David Hewitt on wed 15 oct 03
Micheal,
As I haven't seen an answer from Ron, may I give my own opinion.
I do not think that you should be concerned at using a glaze that has a
calculated expansion of 350 if tests on the clay that you are using
prove to be satisfactory in that it neither crazes or shivers. These are
calculated figures which are of the order of 3.5 x 10-6 / oC linear and
are not a precise science.
If asked how much lower should the expansion of the glaze be, relative
to the clay body to avoid crazing, the generalised answer is 10%. Again
this is not a precise figure in practice.
David
In message , Michael McDowell writes
>Here's a question for Ron Roy. Ron, I might have sent this to you as
>a private e-mail. But since you are educating so many of us here on
>clayart. I thought the question might be relevant to some others as
>well. Since you sent me formulas for a series of glazes of different
>expansion rates a few years back, I have taken to testing every new
>batch of clay to determine its expansion and the prospective fit of
>my glazes. You originally sent me formulas that ranged from 400 to
>600 in your numbering scheme with Insight software. I have since had
>to expand that series to include glases down to 350 and up to 650 to
>find fitting glazes for all my different clays.
>
>Most recently I have had a series of tests come out which exhibited
>crazing on cups glazed on the inside only with glaze expansion of
>500 and above. Cups glazed with expansion rates of 450, 400, and 350
>all are uncrazed after a couple of cycles of freezing and filling
>with boiling water. Even the test with expansion of 350 shows no
>indication of compression defects. It seems like a rather large
>range of compatible expansion rates in my limited experience. Do you
>think it would be wise to go ahead and use glazes with expansions as
>low as 375 based on these results? Or should I try and stay closer
>to the point at which crazing first ceases? Around 450 in this case.
>
>Michael McDowell
>Whatcom County, WA
>Michael@McDowellPottery.com
>http://www.McDowellPottery.com
--
David Hewitt
David Hewitt Pottery
7 Fairfield Road, Caerleon, Newport,
South Wales, NP18 3DQ.
Tel:- +44 (0) 1633 420647
Fax:- +44 (0) 870 1617274
Web:- http://www.dhpot.demon.co.uk
Ron Roy on wed 15 oct 03
Hi Michael,
Nice to see you back!
Good question - I'm not sure I can give a definative answer.
Just to make sure I understand - you did say "glazed only on the inside" -
right?
This is not the ultimate test - a crazing glaze on the out side and the low
expansion on the inside is the ultimate situation because - when a glaze
crazes is also puts a very slight (micro) crack in the clay - so crazed
pots are weaker because the cracking has already started.
I have been amazed at how low the expansion of a glaze can be before it
shows up with some ware - with others there is a much smaller danger range.
There are many factors - how much cristobalite is present, how fast the
firing is (clay glaze bond), the actual shape of the ware - square corneres
tend to concentrate stress. How heavy the glaze coat is is a big factor -
and conversly how thin the clay is.
I would be more comfortable with a higher expansion in your case just to be
sure. If the glazes are not crazing at 450C and crazing at 500C then I
would feel better between 450C and 400C - like around 425C.
Remember to keep the glazes on the thick side when testing - and the clay
on the thin side - and remember - the bigger the pot the more buildup of
stress. Flat bottomed straight sided cassarols are the ultimate test I
think.
I wish I would have had time to do more experiments in this area - as it is
- I work for many professional potters in this area and I do get a few
questions each year about cracking pots - many times more questions about
crazing - so in the end I think your experience is fairly normal - there
can be quite a low glaze expansion before there is a manifestation of the
problem. But it would be a huge problem for any production potter if
dunting did start - because of any of the above factors - you would not
find out till they started coming back. So you can see why I think it is
better to be on the conservative side in this.
I read somewhere - potters feel safest when they get some crazing
occasionally - but hasten to add I don't think that is necessary.
RR
>Here's a question for Ron Roy. Ron, I might have sent this to you as
>a private e-mail. But since you are educating so many of us here on
>clayart. I thought the question might be relevant to some others as
>well. Since you sent me formulas for a series of glazes of different
>expansion rates a few years back, I have taken to testing every new
>batch of clay to determine its expansion and the prospective fit of
>my glazes. You originally sent me formulas that ranged from 400 to
>600 in your numbering scheme with Insight software. I have since had
>to expand that series to include glases down to 350 and up to 650 to
>find fitting glazes for all my different clays.
>
>Most recently I have had a series of tests come out which exhibited
>crazing on cups glazed on the inside only with glaze expansion of
>500 and above. Cups glazed with expansion rates of 450, 400, and 350
>all are uncrazed after a couple of cycles of freezing and filling
>with boiling water. Even the test with expansion of 350 shows no
>indication of compression defects. It seems like a rather large
>range of compatible expansion rates in my limited experience. Do you
>think it would be wise to go ahead and use glazes with expansions as
>low as 375 based on these results? Or should I try and stay closer
>to the point at which crazing first ceases? Around 450 in this case.
>
>Michael McDowell
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Michael McDowell on wed 15 oct 03
Ron,
Thanks for the reply. I am very glad to be back!
Your response was very informative as always. I'm not looking for
definitive answers anyway. Just nice of you to share with me what
your comfort zone would be on this.
I did have one experience with dunting showing up on expansion
tests. A clay I had been using, and had tested to have expansion fit
around 400. When I got a new batch, I just started using it,
assuming the fit would be the same. Turns out this was one of the
last batches of this clay before they closed the mine. I had a
beautiful firing. The glazes looked great, but they were all
shivering! I had to take that entire load straight to the trash bin.
Not even seconds. Those sharp edged shards of glaze flaking off the
pots might cause serious harm.
Well, I still had 750 lbs of this clay, so I belatedly tried the
expansion series on small cups glazed on the inside only. That was
when the dunting appeared. The cups came right out of the kiln
shattered by the outward pressure of the lower expanding glaze. This
was when I had to expand your original series to 650 in order to
find a glaze with high enough expansion to fit this clay. I made it
a project to devise a few glazes to fit this body, and now use it
only make mortar & pestles where the glaze is on the outside only.
Never would have done it without glaze software!
Michael McDowell
Whatcom County, WA
Michael@McDowellPottery.com
http://www.McDowellPottery.com
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