CINDI ANDERSON on tue 23 nov 99
Excellent explanation. Thank you. I am curious about you and some of
your fellow ClayArters who are on the technical side. I am an engineer
by training and trade, and when I got into pottery to express my
"artistic side", I never imagined that it would be so technical. Of
course it makes total sense now, and now I can't believe that many
people who have potted for years don't have a clue about the chemistry
behind it. I personally find it totally cool that something can be
creative and technical at the same time.
Forgive me for not knowing, but I assume you are still a potter in
addition to your work with commercial clays, etc. I wonder if you find
yourself feeling more like an artist that learned a lot of chemistry to
achieve the results you wanted, or more like a chemist that likes to
pot? Or do you find it a perfect blend of the two?
Ron Roy wrote:
> ----------------------------Original message----------------------------
> 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.
> >----------------------------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
> >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.
> Ron Roy
> 93 Pegasus Trail
> Ontario, Canada
> M1G 3N8
> Evenings 416-439-2621
> Fax 416-438-7849
Hank Murrow on thu 25 nov 99
=3EExcellent explanation. Thank you. I am curious about you and some of
=3Eyour fellow ClayArters who are on the technical side. I am an engineer
=3Eby training and trade, and when I got into pottery to express my
=3E=22artistic side=22, I never imagined that it would be so technical. Of
=3Ecourse it makes total sense now, and now I can't believe that many
=3Epeople who have potted for years don't have a clue about the chemistry
=3Ebehind it. I personally find it totally cool that something can be
=3Ecreative and technical at the same time.
=3EForgive me for not knowing, but I assume you are still a potter in
=3Eaddition to your work with commercial clays, etc. I wonder if you find
=3Eyourself feeling more like an artist that learned a lot of chemistry to
=3Eachieve the results you wanted, or more like a chemist that likes to
=3Epot? Or do you find it a perfect blend of the two?
Good Evening Cindi=3B I was moved by your post concerning the
relation between art and science=3B so here's my 'deux sous' on this =
I had a very strenuous parochial education while staring out the windows in
deep dreaming. I despaired of knowing what to do with four years of Latin
and three of Greek by 1956, let alone the other classical subjects we
endured. Advance tape to 1958 and my first ceramics class.....two weeks
into the term and Bob James asked,=22Say why don't you fire a kiln?=22 So I
loaded the 5cuft Alpine and did a C/8 reduction fire (another story
there=21). Within ten weeks I discovered a subject which embraced enough
vectors of learning to engage me for the last 42 years.....and I got to
pose the questions during all that time=21 History, chemistry, physics,
architecture, design, economics(weak there), geology, ethics, and all
enfolded in wares that have happily been on folks' tables, some for
decades. The French have an expression, =22Jeter le Coeur=22, which suggests
that one throw one's passion out in front of them and follow it with =E9lan.
The happy position of the potter at the end of these millenia, is that one
may embrace as much art and as much science as one has passion for, and
throw it all out in front of themselves to move towards. And noone may say
it doesn't hold coffee.
PS: Re-wind tape to 1984 and my first trip to Europe, I was
astounded to find at 47 that I could read text in French, Spanish, =
Italian, and some German=3B despite hardly being able to recognize a word
spoken to me. I thank my teachers and I continue to be grateful to have
found my quest, and I wish as much to everyone else. Hank in Eugene