Paul Bush on mon 15 nov 99
I'm hoping I can get help answering a question that doesn't seem to show
up in any of my many reference books: generally speaking what is the
latitude in cones of various commercial clay bodies. Can a ^10 clay be
used at ^6 ox, for example, and vitrify enough to be rendered fully
functional? I'm mostly interested in knowing how much below a clay's
rated cone, it can be used for functional ware.
Thanks,
Paul Bush in Portland, OR
Ron Roy on tue 16 nov 99
Hi Paul,
I look after the clay bodies for two manufactures - I recommend a range of
one cone - it's difficult enough to maintain that properly.
To answer your question properly - if the clay is fully vitrified at cone
10 (no absorbancy) - it will probably not leak at cone 9 and even cone 8
might be OK - but not cone 6.
If the clay is designed for cone 10 reduction and has more than say 1% iron
- the iron is a flux in reduction - and even at cone 9 oxidation it may
leak.
Many commercial bodies are either over or under fluxed for the recommended
cone - you should be testing them regularly to see what is going on - or
pay the price sooner or later unless your supplier is testing - then you
can relax.
RR
>----------------------------Original message----------------------------
>I'm hoping I can get help answering a question that doesn't seem to show
>up in any of my many reference books: generally speaking what is the
>latitude in cones of various commercial clay bodies. Can a ^10 clay be
>used at ^6 ox, for example, and vitrify enough to be rendered fully
>functional? I'm mostly interested in knowing how much below a clay's
>rated cone, it can be used for functional ware.
>
>Thanks,
>
>Paul Bush in Portland, OR
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Hank Murrow on tue 16 nov 99
>----------------------------Original message----------------------------
>I'm hoping I can get help answering a question that doesn't seem to show
>up in any of my many reference books: generally speaking what is the
>latitude in cones of various commercial clay bodies. Can a ^10 clay be
>used at ^6 ox, for example, and vitrify enough to be rendered fully
>functional? I'm mostly interested in knowing how much below a clay's
>rated cone, it can be used for functional ware.
Thanks, Paul Bush in Portland, OR
Dear Paul; Claymakers are generally erring on the side of wider firing
latitude, I believe. For example, I use Trail Mix and Mac 6 in the anagama
fire, reaching temps of C/12+ with wonderful results; and yet these are
both nominally C/6 bodies. I should add that I customarily tumblestack up
to 4 pots high, which is a more difficult load. That said, i have
experienced bloating on occasion, which I got rid of by just bisquing a
little higher. I think you really have to try a standard set of tests for
every new bod your supplier comes out with to see what it's useful for. A
nuisance, but the alternative is to give up studio space and production
time to mixing it yourself. This is mostly what I do. Good Luck, Hank in
Eugene
CINDI ANDERSON on wed 17 nov 99
I had the same question but would like to expand it a little for some of
you great chemists. What if you're not talking about leak-proofness,
but rather strength and fit to the glaze? For example, would a cone 10
clay fired at cone 6 be 50% more likely to chip than if it were fired at
cone 10, or 10% more likely (in very rough terms; if Paul is anything
like me we're looking for orders of magnitude rather than exact
numbers?)
Ron, when you mention testing, how would you do this? I understand how
you would test waterproofness, but how would you test strength against
chipping? Other than drop testing a bunch of pieces from different
heights :-) I've heard people mention a freeze/boil test on this list,
but if this is the type of test you would use, how would you equate that
to what would happen in "real" use.
Cindi
------------------------------------
Ron Roy wrote:
>
> ----------------------------Original message----------------------------
> Hi Paul,
>
> I look after the clay bodies for two manufactures - I recommend a range of
> one cone - it's difficult enough to maintain that properly.
>
> To answer your question properly - if the clay is fully vitrified at cone
> 10 (no absorbancy) - it will probably not leak at cone 9 and even cone 8
> might be OK - but not cone 6.
>
> If the clay is designed for cone 10 reduction and has more than say 1% iron
> - the iron is a flux in reduction - and even at cone 9 oxidation it may
> leak.
>
> Many commercial bodies are either over or under fluxed for the recommended
> cone - you should be testing them regularly to see what is going on - or
> pay the price sooner or later unless your supplier is testing - then you
> can relax.
>
> RR
>
> >----------------------------Original message----------------------------
> >I'm hoping I can get help answering a question that doesn't seem to show
> >up in any of my many reference books: generally speaking what is the
> >latitude in cones of various commercial clay bodies. Can a ^10 clay be
> >used at ^6 ox, for example, and vitrify enough to be rendered fully
> >functional? I'm mostly interested in knowing how much below a clay's
> >rated cone, it can be used for functional ware.
> >
> >Thanks,
> >
> >Paul Bush in Portland, OR
>
> Ron Roy
> 93 Pegasus Trail
> Scarborough
> Ontario, Canada
> M1G 3N8
> Evenings 416-439-2621
> Fax 416-438-7849
Ron Roy on mon 22 nov 99
Hi Cindi,
The problem with under vitrified clay is - it takes up water in use - this
eventually rehydrates the clay a little - the clay gets bigger and the
glazes craze - and the system gets weaker fast.
I don't know of any non destructive test for chipping - I am sure you could
improvise one. How about a rod - anchored with a hinge at one end - you
could drop it from different heights on different clay/glaze samples.
When you ask for numbers - I don't have a clue - you will have to do your
own research and it will pertain to the glaze and clay combinations you do
them on.
What makes stoneware stronger than earthenware is the interlocking effect
of mulite in the clay - this does not normally start to form until about
1100C. In some cases - underfired clay can be stronger than vitrified clay
. You would test just the clay without glaze to find this out. Normally
pressure is increased on a bar till it cracks - a modulus of rupture. There
are tools to do this - they have a way of showing the pressure it took.
Higher fired clays are usually stronger because the is more mulite produced
in them.
What is more important - at least in my mind - is the relationship - in
terms of expansion and contraction - between clay and glaze. Crazing
weakens ware - some compression of the glaze makes ware stronger - too much
compression on the glaze (glaze has less expansion/contraction so winds up
too big for the pot) results in shivering and cracking - especially if
there is a crazed glaze on the outside.
My freeze boil test is designed to find clay/glaze combinations that have
glaze under too much compression. If that kind of glaze is used - say on
the inside of tea pot - you would want to know if it is going to crack when
you pour boiling water in. Doing that when the tea pot is frozen just makes
it a tougher test.
I remember when I first started proposing this test - many potters thought
no pot could pass - and of course many can - and all should. There have
been many cases of mugs cracking when something hot is poured - sometimes
it takes weeks and months - but it should never happen - not if we are
trying to make a living at this.
RR
>----------------------------Original message----------------------------
>I had the same question but would like to expand it a little for some of
>you great chemists. What if you're not talking about leak-proofness,
>but rather strength and fit to the glaze? For example, would a cone 10
>clay fired at cone 6 be 50% more likely to chip than if it were fired at
>cone 10, or 10% more likely (in very rough terms; if Paul is anything
>like me we're looking for orders of magnitude rather than exact
>numbers?)
>
>Ron, when you mention testing, how would you do this? I understand how
>you would test waterproofness, but how would you test strength against
>chipping? Other than drop testing a bunch of pieces from different
>heights :-) I've heard people mention a freeze/boil test on this list,
>but if this is the type of test you would use, how would you equate that
>to what would happen in "real" use.
>
>Cindi
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
| |
|
