Vince Pitelka on thu 21 apr 05
> In lower fire ware that is structurally weaker and not vitreous, crazing
> is a more important strength factor.
Lee -
I was thinking about this, and I don't think it's that simple. We all
expect porous low fire ware to be more fragile, while we expect high-fire
ware to be much more durable. A large percentage of low-fired wares have a
crazed glaze surface, and yet they hold up to regular use, because people
treat them carefully. I think this even applies to people who do not know
the technical stuff about clay. We just get acclimated to the idea that if
a pottery piece makes that punky "clunk" sound when it bumps against another
piece, it is more fragile and needs to be handled very carefully.
Also, no one has discussed the degree of crazing. Maybe one of our glaze
gurus will have an opinion on this, but there are degrees of crazing,
depending on the glaze fit. If the glaze shrinks more, the crazing will be
more severe, and it is logical to assume that the craze cracks will be
deeper, weakening the piece more than if the craze cracks were shallow.
This might not be readily visible on the surface ware. It would be easy to
say "crazed is crazed" in all pots at one temperature, but I don't think
that is really true.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Jonathan Kaplan on thu 21 apr 05
On Apr 21, 2005, at 7:35 AM, Vince Pitelka wrote:
>> In lower fire ware that is structurally weaker and not vitreous,
>> crazing
>> is a more important strength factor.
>
> Lee -
> I was thinking about this, and I don't think it's that simple. We all
> expect porous low fire ware to be more fragile, while we expect
> high-fire
> ware to be much more durable. A large percentage of low-fired wares
> have a
> crazed glaze surface, and yet they hold up to regular use, because
> people
> treat them carefully. I think this even applies to people who do not
> know
> the technical stuff about clay. We just get acclimated to the idea
> that if
> a pottery piece makes that punky "clunk" sound when it bumps against
> another
> piece, it is more fragile and needs to be handled very carefully.
>
>
Well, we need to re-phrase this. While yes, ware that has a high
absorption, eg porous is more fragile, I have formulated bodies at cone
06-04 with less than 1% absorption and craze proof. I have also done
the same at cone 1, cone 3 and cone 6. So the generalization that just
because the ware is low temperature ware does not mean that they are
weak. Most of the commercially available ones are junk. Clay bodies
that ARE properly formulated can be made extremely strong.
It depends on so much.
Most low temperature hobby casting slips are very high in absorption
and are very weak and glazes will craze.
Most low temperature plastic clay bodies are very high in absorption
and are very weak and glazes will craze.
Includes low temperature white clay and terra cotta.
Delayed moisture crazing will happen in all the above bodies even if
they are glazed on the bottoms and stilted. These ball clay/talc bodies
are garbage.
There are ways to build successful, strong, non-crazing clay and glaze
systems at most temperature ranges providing you know what to work with
and how to work with them and what ratios between plastics and
non-plastics need to be taken into consideration.
Jonathan
Jonathan Kaplan
Ceramic Design Group
PO Box 775112
Steamboat Springs CO 80477
(970) 879-9139
(please use this address for all USPS deliveries)
Plant Location:
1280 13th Street Suite K
Steamboat Springs CO 80487
(please use this address for all UPS, courier, and common carrier
deliveries only!!)
info@ceramicdesigngroup.net
www.ceramicdesigngroup.net
Ron Roy on sat 23 apr 05
Hi Vince,
Just a short description of how crazing progresses - it always happens
gradually - as ware cools - first the primary crazing - cracks are wide
apart.
Some times you can see this as a pot is unloaded - then - after a few hours
or weeks or months or years - the secondary crazing happens.
It is the natural way the strain is relieved.
Are finely crazed pots weaker than not so finely crazed - I would imagine
they are - but in the end the probability is they will all wind up finally
crazed.
RR
>Also, no one has discussed the degree of crazing. Maybe one of our glaze
>gurus will have an opinion on this, but there are degrees of crazing,
>depending on the glaze fit. If the glaze shrinks more, the crazing will be
>more severe, and it is logical to assume that the craze cracks will be
>deeper, weakening the piece more than if the craze cracks were shallow.
>This might not be readily visible on the surface ware. It would be easy to
>say "crazed is crazed" in all pots at one temperature, but I don't think
>that is really true.
>Best wishes -
>- Vince
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on sun 24 apr 05
Again - clays fired below the temperature when mullite begins to form will
not usually have the mechanical strength and will not be as chip resistant
as stoneware and porcelain.
Some degree of that kind of mechanical strength could be gotten by mixing
mullite into the clay before firing - I would guess that it would still not
be as strong as stoneware and porcelain.
Kyanite of sufficient mesh size introduced into raku bodies will help in
this way - if you are making large tiles for instance - the mechanical
strength will be greatly improved.
RR
>Well, we need to re-phrase this. While yes, ware that has a high
>absorption, eg porous is more fragile, I have formulated bodies at cone
>06-04 with less than 1% absorption and craze proof. I have also done
>the same at cone 1, cone 3 and cone 6. So the generalization that just
>because the ware is low temperature ware does not mean that they are
>weak. Most of the commercially available ones are junk. Clay bodies
>that ARE properly formulated can be made extremely strong.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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