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Mark C.

Cone 6 clay test bars needed

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Any cone 6 Laguna bodies you want me to ask about???

They as a clay manufacture may already have this data.

 

 

 

Yes! The clay I would like to test is Laguna #70, which they make in their Ohio plant.

 

Mea

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Any cone 6 Laguna bodies you want me to ask about???

They as a clay manufacture may already have this data.

 

 

 

Yes! The clay I would like to test is Laguna #70, which they make in their Ohio plant.

 

Mea

 

I'll ask him about that body Mea.

Mark

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I have a question about maturity (not me, but clay). What I call B-Mix 6 is actually called B-Mix 5 and is a cone 5 clay. I fire it to at least cone 6 but often to cone 7 or 8 to give my cone 6 glazes the same (or similar) heatwork they would get if I used a long, complicated cool down. There is no problem with the clay at those higher than recommended cones. I think B-Mix 5 is more mature and stronger when fired to cone 7 or 8 than it is at cone 5 or 6. So, am I not right to to assume that firing any clay as high as it will go without any kind of deformation is the real maturity point regardless of what the manufacturer says? For porcelain, especially, I just use the recommended cone as a guide to fire it 3 or 4 cones higher until it deforms or blisters then use one cone below the cone that deforms it to get the best translucency and ring and I believe strength.

 

Jim

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John B. -

 

Why is the 1cm x 1cm cross section so critical. If you measured with a micrometer and used the 'actual' section to do your area computation, would you not have a valid result? I know that for other 'larger' materials (i.e. concrete), you cast and test. They weren't milled down to an exact size. Now, granted, shrinkage for concrete isn't at all simliar to clay!

 

http://www.calculatoredge.com/new/rupture.htm Be aware that this site has a bunch of advertising and will hog the history in your browser and not let you 'back' out of it. But it's interesting and indicates that the cross sectional area and the support spacing could vary, as long as the computations account for it all.

 

Alice

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Guest JBaymore

Why is the 1cm x 1cm cross section so critical. If you measured with a micrometer and used the 'actual' section to do your area computation, would you not have a valid result?

 

Yes, you are right Alice, you would have valid results. I have been doing this all for so long solely with art students (many of whom are "math-ically challenged" ;) ) that I have it totally ingrained into my head to make the math work part as totally simple as is possible for them. In fact you can even have round MOR test bars also. However, note that there is some technical literature (Milligan; Swank, Caverly, Allor; etc.) that shows that varing the geometry of the test bars does introduce some influences on the numbers produced. And if the span used is not at least 10 times the cross section, that has also been shown to adds errors.

 

Why the milling? Part of that is to get to the 1 cm x 1 cm geometry. My original background on this subject is from studying ASTM standards from long ago. To quote here from Permatech (a test preparation "lab" company) on the current preparation of samples for ASTM testing, they say, "Components are often designed using material strength values derived from tests based on very specific ASTM standards. Machining of test specimens must meet ASTM standards using strict guidelines that dictate the wheel grit, speeds, and feed rates."

 

Milling the surfaces gets the surfaces to a specific state that takes out inherent surface "defects" from the original forming processes. It eliminated some "variables". For example a single speck of grog located on the very outer surface near the cental load bearing point will cause a micro-fracture in the surrounding clay structure as the clay shrinks around the already fired grog particle... and will cause the bar to fail at a different load number than if that piece of grog were not there. Surface scratches, small thickness variations, and other such forming variations will affect the results.

 

But we are talking about the "lab world" versus the "real world" in a lot of this whole discussion now. Which is not where WE live. In the "real world" those variations in forming methods and in the nature of the surfaces of a piece DO exist and WILL affect the "strength" of the given form.

 

Again we quickly come back to the question of what exactly are we trying to measure here. Is MOR the only sole factor of the physical properties that we are talking about when we get into the discussion of, "Is cone 6 clay stronger than cone 10 clay"? I personally think that is NOT the case. I think the situation will be defined by multiple properties..... and will be really hard to tie down. All we might be able to say is that this cone 6 clay has a MOR number of X.... and this cone 10 clay has a MOR number of Y. If the properties imparted by the MOR is important and IS the only ciriteria...... then we would have a basis to say yes or no if the tests were done to a scientific level of accuracy.

 

But we likely also have many other physical properties that come into play like elasticity, thermal shock resistance, and so on.

 

And most importantly we have the skill of the potter making the actual pieces. This is because the materila is not the ONLY thing that imparts what we are probably calling "strength". If there is a thin spot in the wall section of a cone 10 fired mug, and that is the place that the second cone 6 mug hits it when they are placed in the sink........ it does not matter if the cone 10 clay has a higher MOR number than the cone 6 mug... .... it will be the cone 10 mug that breaks because the clay was thin there, not because of the physical properties of the clay. Ditto for so many other factors.

The most important point here is that if the cross section of the MOR test sample is different, then the RAW force numbers are going to be different and not directly comparable. Your math has to account for the cross section. You'd be comparing apples and oranges if one sample was 1.2 cm2 and another was 1 cm2 and you simply looked at the weight (force) necessary to break the bar. You do have to account for the geometery of the test samples in the calculations, and bars other than square do look to induce some errors as does span to cross section ratios of less than 10:1.

 

Sorry if I introduced any confusion into the discussion in the attempt to simplify it.

 

best,

 

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

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Guest JBaymore
So, am I not right to to assume that firing any clay as high as it will go without any kind of deformation is the real maturity point regardless of what the manufacturer says? For porcelain, especially, I just use the recommended cone as a guide to fire it 3 or 4 cones higher until it deforms or blisters then use one cone below the cone that deforms it to get the best translucency and ring and I believe strength.

 

If you do a MOR test (back to that ;) ) for a specific clay and start firing it hotter and hotter, you typically see the MOR numbers increase to a point (true maturity), and then start to decrease in sort of a "bell curve" idea. However the bell curve graph typically has a more gentle slope on the left side of the peak (lowere cone) than on the right (higher cone). MOR drops off fast as the clay gets "overfired" and the glassy phase starts to really take over the crystalline nature of a well developed mature body. It is not all about glass in the body... it is about a good mixture of glass as well as the development of crystalline structure that "melds" the material into a whole.... like fiberglass resin permeating the fiberglass cloth.....with the addition of some fine carbon fibers in there too.

 

This decrease of MOR usually starts to happen before there are too many overt visual signs that the body is "overfired".

 

"Translucency", "ring", and actuial "strength" may or may not be linerally related (I've never really "tested" for that combination of thoughts....so I can't say anything about that.) My GUESS is that translucency and MOR are not necessarily linerally related (high "ring" factor equates with high MOR).

 

 

best,

 

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

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It is not all about glass in the body... it is about a good mixture of glass as well as the development of crystalline structure that "melds" the material into a whole.... like fiberglass resin permeating the fiberglass cloth.....with the addition of some fine carbon fibers in there too.

 

This decrease of MOR usually starts to happen before there are too many overt visual signs that the body is "overfired".

 

"Translucency", "ring", and actuial "strength" may or may not be linerally related (I've never really "tested" for that combination of thoughts....so I can't say anything about that.) My GUESS is that translucency and MOR are not necessarily linerally related (high "ring" factor equates with high MOR).

 

 

Interesting. Thank you.

 

Jim

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And most importantly we have the skill of the potter making the actual pieces. This is because the materila is not the ONLY thing that imparts what we are probably calling "strength".

 

 

In my experience, poor throwing skills, overfireing, and over-reducing, are the most common ways of damaging the serviceability of one's ware, regardless which body or cone. I just think that if you make your ware right it will be plenty strong enough.

 

Joel.

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Heres Johns response today.

I'm sticking out on quick and easy resting results.

 

HI Mark --- All is well here ---getting into our busy season . Hope all is well with you. Sorry to say we have never done any MOR testing on the clays here at Laguna. It's one of the few pieces of equipment we don't have ---. Best regards Jon Pacini Laguna Clay Co.

I also sent another e-mail asking about the Ohio Plant heres his answer

Hi Mark ---Correct, Ohio would not have that info either. At least as far as I know ---jon

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Guest JBaymore

Wow.

 

That response tells you something about the assumptions we likely most all make about how much real technical work goes into the formal development of commercial clay bodies for handcraft potters, or the ongoing monitoring of the properties OF those materials over time for potential changes that could affect the final end user's products.

 

And they are probably the largest producer (tonnage) of pre-formulated clays slated for the handcraft market. So while this is only the response from one company... my guess is the answer would be the same for just about all of the clay producers. (I'd LOVE to be wrong there :) )

 

It is kind of surprising that they have never even looked at this property of the bodies and don't even have the in-house ability to really formally do so. Or have not at the least sent out samples to labs to test.

 

It certainly begs the question, "What other properties are not monitored all that well?"

 

It probably helps to answer why potters seem to get significant variations in the commercial clay batches frequently and without warning.

 

Interesting.

 

One message here is test, test, test in your own studio if quality and consistency is important to you and don't put a new batch into production until you've fired a test run from the new batch.

 

best,

 

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

 

PS: My days long ago working as a technical consultant for the now long gone Cutter Ceramics spoiled me. :lol:

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Heres Johns response today.

I'm sticking out on quick and easy resting results.

 

HI Mark --- All is well here ---getting into our busy season . Hope all is well with you. Sorry to say we have never done any MOR testing on the clays here at Laguna. It's one of the few pieces of equipment we don't have ---. Best regards Jon Pacini Laguna Clay Co.

I also sent another e-mail asking about the Ohio Plant heres his answer

Hi Mark ---Correct, Ohio would not have that info either. At least as far as I know ---jon

 

 

 

Darn, well thanks for asking anyways Mark.

 

I'd still be interested in providing test bars for some cone 6 vs. cone 10 MOR tests, though I see now that such tests would not be that useful unless all the bars are produed with a high degree of uniformity. Can that be done across the internet? Not sure. I don't think I have the expertise on the subject to suggest how the bars should be produced. If you or anyone else wants to come up with some uniform procedures for producing the bars, count me in.

 

Mea

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