Coe

[yedrow]

Since this came up in another thread...

How does COE work in a relative sense? I understand the dilatometer sense. But the COE of one glaze relative to one body doesn't seem to have the same effect as the COE of another glaze on the same body, at least as far as failure goes. Am I correct?

I've been looking at glazes that are within Insight or in the Roy/Hessleberth book and I find what seems to me to be a pretty wide range of COEs. I've been assuming that a COE of ~7.8 (Insight) to be the upper limit, and ~6.5 to be the lower limit. Is this even close to right?

 

Joel.

[Ben]

What do you mean by failure?

I have had glazes which craze and some that shiver on the same clay and clay bodies that are broken into pieces by the action of the glaze misfit with a glaze that fit other clay bodies well.

[Guest JBaymore]

Yedrow,

 

Your question is not clear.  Can you re-phrase it?

 

Different clay bodies have different COEs.  Differernt glazes have different COEs.  The goal is to match them up so that the glaze neither crazes nor shivers on a given body.  While there is a "typical" range for body COEs for a given cone range....... it can vary all over the place with individual bodies.

 

See the tutorial on Digitalfire in using Insight that covers adjusting the fit of a glaze.  It gives a pretty good description of all this.  I think that is in the "free" section.

 

best,

 

..................john

[yedrow]

Ben, by failure I mean shivering, dunting, crazing, etc.

 

John, I understand. I need to know the COE of the body I'm using. My supplier doesn't have this information. I'm guessing I'll have to extrude some bars to get a ball park figure and go by that. This is really probably not very doable without a dilatometer reading of both the clay and the glazes. I was drawing inferences from what I took to be an average set of glaze COEs, from original sources. Is there any value to drawing such inferences? I don't think I can afford dilatometer testes. But, that being said, if I only had the clay tested perhaps I could.

I guess my main question would be that range of acceptable COEs, `6.5-7.8, good for a rough draft on a glaze designed in Insight?

 

Joel.

[Ben]

You can "measure" ceo relatively by using a series of glazes with coe's that range from low to high. Glaze test tiles made from a clay body in each glaze and fire. The results will be a relative measurement of that bodies coe based on the calculated coe for the glaze series.

 

For example if clay "A" is glazed with 5 glazes (glaze 1~5) and it crazes on 1~4 you'll know that the clay has about the same coe as glaze #5.

 

This testing method is further described in "Mastering Cone Six Glazes." An excellent book that I recommend for anyone interested in glazes.

Ben

[Guest JBaymore]

This testing method is further described in "Mastering Cone Six Glazes." An excellent book that I recommend for anyone interested in glazes.

 

That testing method was presented by Tony Hansen (Insight / Digitalfire) a long, long time ago.  It is also described on the Digitalfire website in great detail.  The test glaze series for this is a pretty easy test series to do...... not too "cosmic" a concept.

 

Lacking a dilatometer, this is the only way to find (in a relative sense) the COE of a clay body.  No one has (as of yet) found a way to model the interractions of body chemistry and physical makeup to some form of calculations. 

 

"Mastering Cone 6 Glazes" is a fabulous book...... that probably should have a different name...... because it tends to limit the audience for the info in it that applies to a broad understanding of glaze chemistry and function.....not just a cone six.  I had the pleasure of presenting on a glaze chemistry panel at NCECA with Ron Roy (and Paul Lewing) and not only does Ron know his stuff...... he is an absolutely GREAT guy. 

 

Ron is almost always at NCECA hanging out in the CLAYART room....so you can likely get to talk to him if you go.

 

best,

 

..........................john

[Guest JBaymore]

I'm guessing I'll have to extrude some bars to get a ball park figure and go by that. This is really probably not very doable without a dilatometer reading of both the clay and the glazes.

 

Most suppliers do not have the COE's of their bodies available. So you are not alone. 

 

Many people have the ASSUMPTION that the clay suppliers are all staffed by real "tech weenies" and they have done all sorts of sophisticatred testing on their products.  That is not always the case.  IOn many cases the supply companies were started by studio artists as an additional way to make some income in the field.... and the business just grows to a larger one.  Many of the bodies are developed in the same way thay you would do so in your own studio by trial and error testing... and the basic tests that you can also do.  No "fancy equipment".  Not monitored or developed by some ceramic engineer.

 

Your comment quoted above shows me that you are apparently confusing firing shrinkage with the Coefficent of (reversible) Thermal Expansion.  I mentioned this in another posting on this COE subject.  The two are NOT the same thing and are not necessarily related.

 

The COE is a property of the ALREADY FIRED CERAMIC BODY.  Firing shrinkave is the change in physical sze that occurs as the chemsitry and physical makeup of the raw clay body is exposed to a certain level of heat work.  As the chemical water is driven off the clay crystalsm, and the stuff that can melt that is contained in the biody is acted upon by the fluxes in the clay body, the physical mass contracts.  Something that was 100 mm long comes out of the kiln at 90 mm long...... for a 10% firing shrinkage.

 

But if you take that 90 mm long piece of FIRED clay and put it into a situation whre it is heated again...... as it is heated up again it is going to get LONGER once again...... not by very much.... but some tiny percentage.   And as it is coolled off again, it is going to return to the original 90 mm long size.  THAT expansioon and subsequent contraction upon heating and cooling is the COE.

 

Now that piece of ceramic is already "fired" on the original firing cylcle when you put it in the kiln abnd took it to the cone you want to fire to.  So at peak temperature, it is now "fired".  Following the example above.... it is now at the 90 mm firing shrinkage size plus the bit of tiny expansion because that fired ceramic is now also HOT fired ceramic.  As the kiln then cools down on this original firing cycle.... that FIRED ceramic is also cooling off.  It then shrinks back to the 90mm size.  That is the impact of the COE of that particular body.  If the glaze that is ON the body shrinks about the same amount as the body........ then it fits.   If not... it crazes or shivers if the mismatch is off enough to exceed the physical strrength of the glaze layer or the glaze / body interface layer.

 

Does all that help?

 

best,

 

.......................john

[yedrow]

John, that was very informative.

 

I'm referring to the size change that occurs during kiln cooling and during use, in an oven for instance. I may be all wrong about the terms I'm using. It wouldn't be the first time.

As I understand it, the clay and glaze will shrink to the maximum loss in size at the maximum heat-work they experience. Then, as the materials cool they will contract. In this case, COE would be seen as -COE, or coefficient of contraction. At room temperature then they will expand and contract relative to temperature change and if they get very (~400 f.) hot they will experience however much of a crystobalite bump the body possesses. Also, if the glaze has a greater COE than the clay, it will craze, if a lesser COE it will dunt. Given both are excessive enough to generate a sufficient differential.

Isn't that what it means when Insight lists COE? If not, please explain since I'm totally confused 

 

Joel.

[Guest JBaymore]

As I understand it, the clay and glaze will shrink to the maximum loss in size at the maximum heat-work they experience. Then, as the materials cool they will contract. In this case, COE would be seen as -COE, or coefficient of contraction. At room temperature then they will expand and contract relative to temperature change and if they get very (~400 f.) hot they will experience however much of a crystobalite bump the body possesses. Also, if the glaze has a greater COE than the clay, it will craze, if a lesser COE it will dunt. Given both are excessive enough to generate a sufficient differential.

 

Isn't that what it means when Insight lists COE? If not, please explain since I'm totally confused 

 

Joel,

 

Bingo!  You got it. 

 

I'm here a lot at the moment trying to stay on top of the little hack job that apparently has been done to the site as much as I can and do whatever "damage control" and forwarding of info to the Admins as I can.   Hence the long and fast replies.  Cutting into my studio time though!

 

best,

 

....................john

[yedrow]

Thanks for taking care of the site John. I was wondering if I had a virus on my mac.

 

Joel.

[Guest JBaymore]

Thanks for the thanks Joel. 

 

I (and the other Mods) actually can do litle at this point other than gather up and forward potentially useful information and facts to the Admins, and communicate what is happining with foplks that might not know.  This is a server level issue and it is the IT guys that are pulling their hair out (I already lost most of mine by pulling it out as a potter  ).

 

best,

 

..............................john

[jrgpots]

. 

 

 (I already lost most of mine by pulling it out as a potter  ).

 

best,

 

..............................john

 

It is probably a better idea to use horse hair on the pots instead of your own anyway....lol.

 

Jed

[Mudlark]

Taylor and Bull discuss thermal expansion in detail in their book Ceramic Glaze Technology, including a simple series of tests using an electric oven and water to determine the resistance to crazing of a sample glaze.

Book details:

Library of Congress Cataloging-in-Publication Data

Taylor.J.R.

Ceramics glaze technology

Includes bibliographical references

1.Glazing (Ceramics) 2.glazes. 1.Bull.A.C

11.Title

TP812. T39 1986 666'.444 85-29700

 

It is worth chasing up your local library to get this book on loan as it also answers many other questions that crop up when you experiment with glazes.