John Post on mon 23 nov 98
Hi Ron,
In your posting the other day you stated:
>I also use a friends mugs every day - many times a day and boil water in
>them. The clay is a stoneware with 1 to 2% absorbency when boiled for 2
>hours from perfectly dry.
It got me wondering how much absorption should a cone 6 vitrified clay body
have?
I looked in Hamer and it has this about vitrification...
"A vitrified body is usually accepted as having a small porosity. When a
body has reached the point of being absolutely non-porous it is also so
glassy that it fractures more easily."
So in your experience in developing cone 6 clay bodies, how much absorption
should the body have in order for it to be as strong as possible and still
be considered vitrified?
Can I determine my clay body strength based on its absorption, or are there
other factors that come I need to consider?
I'm testing several local commercial clay bodies and trying to find one
that truly matures at cone 6-7.
Thanks,
John Post
Sterling Heights, Michigan USA
rp1mrvl@moa.net
Ron Roy on fri 27 nov 98
Hi John,
Just a little background before I answer your question. I do maintain many
commercial clay bodies. It is not easy and depends on a rigorous testing
program and a particular formulation strategy. All of the clays we use have
natural variations in shrinkage and firing maturity. Because I am using
test data built up over the years I can choose and rely on certain clays to
be more uniform than others. I still however choose to have many different
clays in each body - so if one takes a jump in any direction it will not
upset the bodies too much. From that you can extrapolate - knowing how many
clays are in each body can and should be a way of discriminating between
clay bodies.
I know that having the absorption - after boiling a dry sample for 2 hours
- under 3% will ensure enough vitrification so the ware will not leak or
absorb enough water to be a problem. So I try to keep each body between 1
and 1.5 % absorption and specify that it be fired at one cone. Porcelains
are kept as close to 0% as possible.
The ability if ware to resist cracking is a complex subject and several
factors contribute to it - vitrification would be one - firing and cooling
another and glaze fit - the most important in my experience. If you are
using the same glaze inside and out and it has a lower expansion rate than
the clay - but not too low - can make ware very strong. Crazed glazes
weaken ware but at least you don't have to worry about the glaze breaking
the pot because it winds up to big for it.
Testing glazes for fit on a body it not that hard to do - but you need to
have some assurance that the clay will be at least similar from batch to
batch - I do recommend testing every batch of clay you buy - if there are
no batch numbers on the box - don't buy it. In fact knowing what I know now
- if the manufacturer is not testing their product it's a gamble and you
will have to pay sooner or later - unless you do the testing before you use
it.
As you can see I have strong feelings about this - it's hard enough to make
a living making pots without shoddy suppliers making it more difficult. If
you find companies that are paying attention to quality - support them -
they have to charge more for their clay because they do test. Just a matter
of encouraging those who are applying some craftsmanship.
Be glad to go into this in more detail if you want.
RR
>In your posting the other day you stated:
>
>>I also use a friends mugs every day - many times a day and boil water in
>>them. The clay is a stoneware with 1 to 2% absorbency when boiled for 2
>>hours from perfectly dry.
>
>It got me wondering how much absorption should a cone 6 vitrified clay body
>have?
>
>I looked in Hamer and it has this about vitrification...
>"A vitrified body is usually accepted as having a small porosity. When a
>body has reached the point of being absolutely non-porous it is also so
>glassy that it fractures more easily."
>
>So in your experience in developing cone 6 clay bodies, how much absorption
>should the body have in order for it to be as strong as possible and still
>be considered vitrified?
>
>Can I determine my clay body strength based on its absorption, or are there
>other factors that come I need to consider?
>
>I'm testing several local commercial clay bodies and trying to find one
>that truly matures at cone 6-7.
>
>Thanks,
>
>
>
>John Post
>Sterling Heights, Michigan USA
>rp1mrvl@moa.net
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
John Post on sun 29 nov 98
Hi Ron,
Thanks for your posting. It was very informative as usual.
>So I try to keep each body between 1
>and 1.5 % absorption and specify that it be fired at one cone.
Tom Buck e-mailed me privately and said this is what you would say.
(Thanks Tom).
>...but you need to
>have some assurance that the clay will be at least similar from batch to
>batch - I do recommend testing every batch of clay you buy - if there are
>no batch numbers on the box - don't buy it.
I've been developing cone 6 glazes using the approach in Ian Currie's book
"Stoneware Glazes A Systematic Approach". I decided that I need to
standardize the clay bodies that I use in my studio so that my results with
the Currie method will be reproducible over time.
The clay suppliers within driving distance of my home do not test their
clay bodies and I have seen variations in the fired results. Many of these
bodies have claimed firing ranges of cone 2-10. I'm in the process of
testing a variety of these clay bodies at cone 6-7 but...I really would
rather buy my clay from a supplier who maintains the quality from batch to
batch.
>I do maintain many
>commercial clay bodies...
I'm looking for a white stoneware body and an irony stoneware body that
mature at cone 6. I have been e-mailed by others on the list who are
interested in this as well. Can you share with us whose clay bodies you
maintain and their phone #.
>As you can see I have strong feelings about this - it's hard enough to make
>a living making pots without shoddy suppliers making it more difficult. If
>you find companies that are paying attention to quality - support them ...
Keep voicing these opinions Ron. I have learned more about glazes and clay
from Clayart than I did in some of my college courses.
John Post
Sterling Heights, Michigan USA
rp1mrvl@moa.net
Ron Roy on wed 2 dec 98
>----------------------------Original message----------------------------
John Post is asking me about clay bodies I maintain and I have replied to
him off list. I am unsure that posting the suppliers I work for is
appropriate here - anyone got some advice about this?
In the meantime - if you are close enough to Toronto or Orlando and are
thinking of buying tested clays - email me off list.
RR
>
>I'm looking for a white stoneware body and an irony stoneware body that
>mature at cone 6. I have been e-mailed by others on the list who are
>interested in this as well. Can you share with us whose clay bodies you
>maintain and their phone #.
>
>>As you can see I have strong feelings about this - it's hard enough to make
>>a living making pots without shoddy suppliers making it more difficult. If
>>you find companies that are paying attention to quality - support them ...
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
Jean Stephenson on mon 7 dec 98
I have been asked to make unglazed stoneware suitable for using in the oven.
I have made bread domes in the past for baking sour dough, etc. but since
this will be used for different food types, I would like to make sure that
the clay body [cone 10 or I guess I could do cone 6, but I usually work in
10] is safe. Any suggestions? Thanks. Jean
-----Original Message-----
From: Ron Roy
To: CLAYART@LSV.UKY.EDU
Date: Thursday, December 03, 1998 6:47 AM
Subject: Re: Microwaves & Lowfire ceramics/Ron Roy's Reply
----------------------------Original message----------------------------
>----------------------------Original message----------------------------
John Post is asking me about clay bodies I maintain and I have replied to
him off list. I am unsure that posting the suppliers I work for is
appropriate here - anyone got some advice about this?
In the meantime - if you are close enough to Toronto or Orlando and are
thinking of buying tested clays - email me off list.
RR
>
>I'm looking for a white stoneware body and an irony stoneware body that
>mature at cone 6. I have been e-mailed by others on the list who are
>interested in this as well. Can you share with us whose clay bodies you
>maintain and their phone #.
>
>>As you can see I have strong feelings about this - it's hard enough to
make
>>a living making pots without shoddy suppliers making it more difficult. If
>>you find companies that are paying attention to quality - support them ...
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 thu 10 dec 98
Hi Jean,
I can't see a problem here - you need to know if they require the clay to
be vitreous enough to exclude water or not. Some bakers even like the ware
to have some water in it. It is more important to use a low expanding (on
heating) body with little or no cristobalite in case there is uneven
heating.
If there is no glaze I don't expect it will trap any steam and blow up - if
there is no glaze to seal the pores. If water can get in it would probably
not be wise to use it in a microwave.
Actually unglazed ware solves most of the problems for baking as there is
no clay/glaze fit to have balanced.
RR
>----------------------------Original message----------------------------
>I have been asked to make unglazed stoneware suitable for using in the oven.
>I have made bread domes in the past for baking sour dough, etc. but since
>this will be used for different food types, I would like to make sure that
>the clay body [cone 10 or I guess I could do cone 6, but I usually work in
>10] is safe. Any suggestions? Thanks. Jean
>-----Original Message-----
>From: Ron Roy
>To: CLAYART@LSV.UKY.EDU
>Date: Thursday, December 03, 1998 6:47 AM
>Subject: Re: Microwaves & Lowfire ceramics/Ron Roy's Reply
>
>
>----------------------------Original message----------------------------
>>----------------------------Original message----------------------------
>John Post is asking me about clay bodies I maintain and I have replied to
>him off list. I am unsure that posting the suppliers I work for is
>appropriate here - anyone got some advice about this?
>
>In the meantime - if you are close enough to Toronto or Orlando and are
>thinking of buying tested clays - email me off list.
>
>RR
>
>
>>
>>I'm looking for a white stoneware body and an irony stoneware body that
>>mature at cone 6. I have been e-mailed by others on the list who are
>>interested in this as well. Can you share with us whose clay bodies you
>>maintain and their phone #.
>>
>>>As you can see I have strong feelings about this - it's hard enough to
>make
>>>a living making pots without shoddy suppliers making it more difficult. If
>>>you find companies that are paying attention to quality - support them ...
>
>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
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
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