Cone 6 clay test bars needed

[Mark C.]

I would like to do MOR test on some clay bodies as John has suggested

I will need some cone 6 test bars-I will make my own cone 10 test bars

I will need some ones help who is firing this cone range and has time to make these barsI have no access to cone 6 firing so a car kiln load fire to cone 6 is not in the cards

I'm looking for someone to supply me with at least 10 cone 6 test bars fired with no glaze to cone 6 with your strongest body you are using

The tiles need to be made carefully as per John Baymores instructions below

Make some test bars of the clay body in question about 15 cm long and 1 cm by 1 cm in cross section. These nedd to be carefully and precisely formed. Fire them to maturity lying flat on a support shelf with cone packs right next to them to assure that they are fired as you THINK they are fired. You should do at least 10 pieces of each clay body you are testing.

You will need to send me the info on what the clay body is (the name like b-mix cone 5 or whatever it is)

I checked into a lab to run this test and got fast know where fast.

I will pay for postage and whatever small costs to make this work for you.

I'll mail you a check-can you mail me the fired test bars?

send them to

Mark Cortright

212 Liscom Hill Road

Arcata Ca. 95519

I plan on running this test on them if I can get some bars sent.

If you actually want to know the general strength of a clay body, a basic Modulus or Rupture (MOR) test will give you some hard data to look at. This is not hard to do at the "potter level" of science. If, with the error that the casual approach we use induces into the equation, the numbers are so close as to be very, very similar... probably the two bodies are about the same for all intents and purposes for your usage.

 

Make some test bars of the clay body in question about 15 cm long and 1 cm by 1 cm in cross section. These nedd to be carefully and precisely formed. Fire them to maturity lying flat on a support shelf with cone packs right next to them to assure that they are fired as you THINK they are fired. You should do at least 10 pieces of each clay body you are testing.

 

Find / make some small trangular cross section supports (like small prisims) that you can then balance the clay test bars on so that they can be supported on only two knife edge points.

 

You are going to try to break the bars by pressing down on the center point while the bars are supported on the knife edges.

 

Using a pencil lightly mark the EXACT center on each of the fired test bars.

 

Spread the triangular support points apart exactly 10 cm and spanning a space between two tables with overhanging top surfaces......... knife edge to knife edge. (For the reason for the two tables see below.)

 

The space between the two tables must allow you to hang a small bucket with a plain metal bail handle (narrow point of support contact) from the exact center mark on the clay test bars. So you need table tops with projecting "overhangs".

 

You will then slowly add weight (wet clay works) to this bucket until the clay bar JUST breaks. Then weigh the contents of the bucket and the bucket itself. Record the weight results. Repeat for all 10 bars of one clay.

 

Add the numbers together and then divide by 10. This is the average result of your test

 

Then repeat this process for all the other test clay bars.

 

Compare the results between the two bodies. The one with the higher average numbers is the generally stronger*.

 

best,

 

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

[GEP]

Mark,

 

Should they be 15x1x1 cm before firing or after firing? Or does it matter? I think I might want to send you my cone 6 clay for your tests. Though right now I am swamped with orders so it might take me a while. I could probably make these quickly.

 

Mea

[Guest JBaymore]

Mark (+ GEP),

 

Congratulations on going after this one formally. Publishable paper possible here if you "do it right".

 

If you are going to do this at the "lab level" of accuracy, the bars should actually be made larger than the 1 cm x 1 cm cross section and then machined down (diamond grinding) to exactly 1 CM (+/- .01 accuracy) cross section. Check with the lab you are using to see if they can mill them for you and how much larger thay can tolerate. Also check on the preferred length for their testing equipment.

 

Real MOR numbers are stipulated on that post firing 1 x 1 cm cross section. The final fired bar samples need to be 1 cm x 1 cm cross section.

 

Forming method DOES matter. My classes tests have proved this time and again. If some are rolled from a slab and some are extruded, and some are pounded into a mold, and son are cut out the the walls of a thrown cylinder, and so on.... you'll see different numbers. I've had my classes test this.

 

So to be an accurate testing scenario, ALL of the test pieces should be made and handled exactly the same way. If potter A is making one test set... and potter B is making the next one.... there are variables creeping in that put the results in to question. For "potter level" science.,..... probably not TOO huge a potential problem... but it does induce potential errors.

 

If you are going to eventually also go with glazed bars, then it gets way more complicated to do these tests accurately. Precise (and I mean PRECISE) glaze thickness on all test pieces will matter. As still does that post firing 1 x 1 cm cross section. So you'd have to figure out HOW to actually DO that.

 

I will be VERY interested to see the results of this study. ( You using the Alfred lab?)

 

 

best,

 

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

[TJR]

Mark;

Why don't you just get a bunch of Cone six bowls and smash them with a hammer?

Oh, wait! That's what got us into this situation in the first place.

TJR.

Wear protective gear, especially around your ego.

[OffCenter]

Mark (+ GEP),

 

Congratulations on going after this one formally. Publishable paper possible here if you "do it right".

 

If you are going to do this at the "lab level" of accuracy, the bars should actually be made larger than the 1 cm x 1 cm cross section and then machined down (diamond grinding) to exactly 1 CM (+/- .01 accuracy) cross section. Check with the lab you are using to see if they can mill them for you and how much larger thay can tolerate. Also check on the preferred length for their testing equipment.

 

Real MOR numbers are stipulated on that post firing 1 x 1 cm cross section. The final fired bar samples need to be 1 cm x 1 cm cross section.

 

Forming method DOES matter. My classes tests have proved this time and again. If some are rolled from a slab and some are extruded, and some are pounded into a mold, and son are cut out the the walls of a thrown cylinder, and so on.... you'll see different numbers. I've had my classes test this.

 

So to be an accurate testing scenario, ALL of the test pieces should be made and handled exactly the same way. If potter A is making one test set... and potter B is making the next one.... there are variables creeping in that put the results in to question. For "potter level" science.,..... probably not TOO huge a potential problem... but it does induce potential errors.

 

If you are going to eventually also go with glazed bars, then it gets way more complicated to do these tests accurately. Precise (and I mean PRECISE) glaze thickness on all test pieces will matter. As still does that post firing 1 x 1 cm cross section. So you'd have to figure out HOW to actually DO that.

 

I will be VERY interested to see the results of this study. ( You using the Alfred lab?)

 

 

best,

 

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

 

 

I think it would help accuracy to make it a blind test somehow (maybe Mark sends sample of his clays to participants?). Question: Couldn't the small shocks or vibrations of grinding clay bars down stress the bars causing microscopic cracks or other weakening?

 

Jim

[Guest JBaymore]

Question: Couldn't the small shocks or vibrations of grinding clay bars down stress the bars causing microscopic cracks or other weakening?

 

Yes, that is why lab grade equipment is needed to do it "right". That is also why multiple tests are done to get rid of the "outliers".

 

 

 

I wouldn't try to do this with a dremel or an angle grinder . Too hard to hit that +/- cm mark.

 

 

Simply rough handling of a test bar in ANY stage after forming invalidates that single bar.

 

 

best,

 

.......................joh

[Mark C.]

John

do you know a lab that will do this test?I can pay for it no problem

I was going to it myself as per your instructions above in my post

Can I cut my bars down with a wet diamond saw if thats a yes then whoever sends me the bars they can be oversize and I can cut them down?

Then bisque them dry again.

Mark

[OffCenter]

Question: Couldn't the small shocks or vibrations of grinding clay bars down stress the bars causing microscopic cracks or other weakening?

 

Yes, that is why lab grade equipment is needed to do it "right". That is also why multiple tests are done to get rid of the "outliers".

 

 

 

I wouldn't try to do this with a dremel or an angle grinder . Too hard to hit that +/- cm mark.

 

 

Simply rough handling of a test bar in ANY stage after forming invalidates that single bar.

 

 

best,

 

.......................joh

 

 

Actually, I was referring to the lab diamond saws you mentioned. It seems to me even that could affect strength.

 

Jim

[Mark C.]

Another Idea John would be to send the clay for the next class you teach with students on this in the future. I'm really not volunteering you just trying for the best results.

Mark

[LilyT]

so if the bars have to be handled that carefully, how will that relate to real life pots? wouldn't

 

real life pots then be too variable?

 

i'm not trying to be difficult, just trying to see the value of such a test. perhaps knowing the

standard deviation (or standard error) of the samples would be more helpful than the

average value. (MOR +/- SD)

 

-Lily

[GEP]

No this type of test is not going to account for variations in individual pots, but having a baseline measurement is still useful. If someone is willing to do this work, I am happy to contribute. Even if the test is not perfectly apples-to-apples, it is still better information than I currently have.

 

One of my largest wholesale accounts refused to buy my work for three years, because she believed cone 6 pots were not durable enough. That was based on a bad experience she had from 20 years ago. There are a lot of people out there with outdated information, or a lack of information. The more information we can uncover, the better.

 

Also, while most of my work is cone 6, once a year I attend a cone 10ish wood-firing. I have always told my customers the wood-fired pots were more durable than my usual pots, based on a. the different claybody, b. the higher temperature, and c. I make my wood-fire pots thicker than my usual pots. I have never tried to test if I was being factual, I was repeating what I have been taught. I would be much better off if I had some test results to base that on.

 

The clay I use most often for wood-firing is Highwater Phoenix, which is fairly groggy. Already my assumptions have been challenged on this forum because I thought grog made the clay stronger. Others are saying grog makes a claybody easier to break. I can see the logic in that. Maybe I should only say those pots are better for oven use due to their thermal shock resistance, without saying the pots are stronger? I would be excited if someone using Phoenix at cone 10 would also send bars to Mark.

 

Mea

[OffCenter]

No this type of test is not going to account for variations in individual pots, but having a baseline measurement is still useful. If someone is willing to do this work, I am happy to contribute. Even if the test is not perfectly apples-to-apples, it is still better information than I currently have.

 

One of my largest wholesale accounts refused to buy my work for three years, because she believed cone 6 pots were not durable enough. That was based on a bad experience she had from 20 years ago. There are a lot of people out there with outdated information, or a lack of information. The more information we can uncover, the better.

 

Also, while most of my work is cone 6, once a year I attend a cone 10ish wood-firing. I have always told my customers the wood-fired pots were more durable than my usual pots, based on a. the different claybody, b. the higher temperature, and c. I make my wood-fire pots thicker than my usual pots. I have never tried to test if I was being factual, I was repeating what I have been taught. I would be much better off if I had some test results to base that on.

 

The clay I use most often for wood-firing is Highwater Phoenix, which is fairly groggy. Already my assumptions have been challenged on this forum because I thought grog made the clay stronger. Others are saying grog makes a claybody easier to break. I can see the logic in that. Maybe I should only say those pots are better for oven use due to their thermal shock resistance, without saying the pots are stronger? I would be excited if someone using Phoenix at cone 10 would also send bars to Mark.

 

Mea

 

 

Mea, I assume you've seen Pinnell's test results. Even if the one objection Ron Roy had about the test (that the earthenware sample was slightly oversize) is true, the tests are still probably the best we're going to get unless we all chip in (I'd do that within reason) and get the Alfred lab or some other place that can do the test even more accurately that Pinnell to do them. The problem is that we were all taught that cone 10 pots are stronger (and therefore better) than cone 6 pots and that is NOT TRUE and spreading falsehoods like that hurts all potters and the environment (because it makes some potters reluctant to switch from cone 10 to cone 6). It doesn't affect me that much because I'm not a production potter and I do cone 13, 10, 6, 3, and 04 pots, so I can always claim to some know-it-all at a show who claims that his cone 10 pots are better than pots fired a 100 degrees cooler that my cone 13 pots are, of course, much stronger and far superior to his cone 10 pots--which, of course, is not true.

 

My guess is that you are right that your grogged clay pots are better for oven use, but why tell people that if don't know that? You may be wrong and they may be less suitable for ovenware.

 

One last thing: I'm not saying that all cone 6 pots are just as strong as all cone 10 pots. I'm saying, backed up by experience and Pinnell's tests, that the very small difference in temperature between cone 6 and cone 10 firings do not determine a pot's strength; the composition of the clay does. A pot fired to cone 6 may be stronger than a pot fired to cone 10 or vice versa.

 

Jim

[GEP]

Unless I missed it, Pinnell's study did not draw any conclusions about cone 6 vs. cone 10 temps, which was the original topic of Mark's last test which drew so much criticism. I personally would want to see someone run a test with that specific comparison in mind. Yes I understand what Pinnell says, that many other factors including specific clay ingredients and glaze composition are ultimately more important than temperature, but I still think a baseline comparison would be useful for functional potters. His study also doesn't tell me anything about my cone 6 clay. I fully expect it to be less strong than Mark's cone 10 porcelain, but I think it's worth knowing for sure. I would think that anyone who is selling functional pottery for a living would want to understand their materials as much as possible, so they can stand behind it when they sell it to strangers.

 

Mea

[OffCenter]

Unless I missed it, Pinnell's study did not draw any conclusions about cone 6 vs. cone 10 temps, which was the original topic of Mark's last test which drew so much criticism. I personally would want to see someone run a test with that specific comparison in mind. Yes I understand what Pinnell says, that many other factors including specific clay ingredients and glaze composition are ultimately more important than temperature, but I still think a baseline comparison would be useful for functional potters. His study also doesn't tell me anything about my cone 6 clay. I fully expect it to be less strong than Mark's cone 10 porcelain, but I think it's worth knowing for sure. I would think that anyone who is selling functional pottery for a living would want to understand their materials as much as possible, so they can stand behind it when they sell it to strangers.

 

Mea

 

 

Actually Pinnell's tests did draw a conclusion about cone 6 vs. cone 10 in that the general conclusion of the test were that firing to a higher cone doesn't necessarily mean a stronger pot. Your last sentence is the reason I keep kicking what should be a dead horse. Customers see the potter they buy from as the expert and it is too bad that we take advantage of that trust by spewing off whatever we've been taught or believe or want to believe as fact when it is not.

 

Jim

[Mark C.]

Mea

I sent an e-mail to Pete Pinnell asking if he would consider doing the test with our raw clay bodies when his class gets around to test again. This is one option

that may work. John has not answered as well and this may be an option

I as you Mea are swamped with my busy season on the near horizon

I offered to help with some funds for Pete-Lets see what he says

 

Off center

Back when I was taught clay body and glaze calc stuff from recent Alfred Grads (with teachers like Rhodes and Cushing and Randell) there was no cone 5/6 bodies that we knew of so I have no belief system in place about them. Never been around them. I know of no potters in my county using them-I never speak of or have heard much from 40 years of customers even talking about cone temps.

I can believe a clay fired 100 degrees less can be stronger as its other factors that make up that matrix not pure temperature alone.

I do not think potters are spreading bad stories about cone 6. At least i never have heard any. I only talk about that tec stuff with potters not customers. I'm a professional when it comes to business issues with ceramics and unless asked about my clay body I keep it to myself. I do say Porcelain chips less and is extremely durable which I think we can all agree on. I hope?

What I do know is there seems to be a need for testing as we potters who are on the front lines are interested in these results and knowledge is always best and what we know about our clays and glazes is key.

I have no pony in this race just a quest for learning.

Mark

[OffCenter]

Mea

I sent an e-mail to Pete Pinnell asking if he would consider doing the test with our raw clay bodies when his class gets around to test again. This is one option

that may work. John has not answered as well and this may be an option

I as you Mea are swamped with my busy season on the near horizon

I offered to help with some funds for Pete-Lets see what he says

 

Off center

Back when I was taught clay body and glaze calc stuff from recent Alfred Grads (with teachers like Rhodes and Cushing and Randell) there was no cone 5/6 bodies that we knew of so I have no belief system in place about them. Never been around them. I know of no potters in my county using them-I never speak of or have heard much from 40 years of customers even talking about cone temps.

I can believe a clay fired 100 degrees less can be stronger as its other factors that make up that matrix not pure temperature alone.

I do not think potters are spreading bad stories about cone 6. At least i never have heard any. I only talk about that tec stuff with potters not customers. I'm a professional when it comes to business issues with ceramics and unless asked about my clay body I keep it to myself. I do say Porcelain chips less and is extremely durable which I think we can all agree on. I hope?

What I do know is there seems to be a need for testing as we potters who are on the front lines are interested in these results and knowledge is always best and what we know about our clays and glazes is key.

I have no pony in this race just a quest for learning.

Mark

 

 

Mark, No argument there! (And you know if I could find one I would--pretend I inserted one of those silly smiley faces here.) I, too, would love to see more tests done. None of us have the time. It seems to be a great project for pottery 101 or someone like John Brit (maybe he could get book out of it). If ever needed, I'd sure be willing to make a small donation to the cause.

 

Jim

[Mark C.]

Pete Pinnells e-mail bounced back

Does anyone have current one for Pete?? Chris-John maybe you have it??

you can send it to me at LHP3@liscomhillpottery.com

 

I'm friends with John Britt-I'll ask him about this test.

I just got an e-mail from him so I'll reply with this question

Its really a small world.

Mark

 

I sent John Britt the question

[OffCenter]

Pete Pinnells e-mail bounced back

Does anyone have current one for Pete?? Chris-John maybe you have it??

you can send it to me at LHP3@liscomhillpottery.com

 

I'm friends with John Britt-I'll ask him about this test.

I just got an e-mail from him so I'll reply with this question

Its really a small world.

Mark

 

I sent John Britt the question

 

 

I have one but it is old so may not be current: ppinnell@unlinfo.unl.edu

[Mark C.]

Thats the one i had that bounced back.

Mark

[yedrow]

Granting John covered this, it remains that the method used to produce the sample will effect the strength of the sample. For instance, if it is extruded it seems to me that the outer layer of clay would be of a different strength than the core of the clay. And, if the outer layer is cut away, it would have to be cut away evenly. I'm guessing that cooling the cutting blade would minimize heat fractures in the ceramic. But if you remove a layer of clay, how is it that you have the same thing that is being put on the shelf? However, intuitively it seems to me that cutting away the outer layer of an extrusion would kinda make stoneware like earthenware (less that vitrification stuff).

 

This discussion really is great. I haven't touched earthenware since I was starting out. I'm now excited to think that it could work just as well as any other body!

 

Joel.

[OffCenter]

Granting John covered this, it remains that the method used to produce the sample will effect the strength of the sample. For instance, if it is extruded it seems to me that the outer layer of clay would be of a different strength than the core of the clay. And, if the outer layer is cut away, it would have to be cut away evenly. I'm guessing that cooling the cutting blade would minimize heat fractures in the ceramic. But if you remove a layer of clay, how is it that you have the same thing that is being put on the shelf? However, intuitively it seems to me that cutting away the outer layer of an extrusion would kinda make stoneware like earthenware (less that vitrification stuff).

 

This discussion really is great. I haven't touched earthenware since I was starting out. I'm now excited to think that it could work just as well as any other body!

 

Joel.

 

 

That's what makes me think that the only way to get results that are trustworthy is to have a lab (Alfred?) or someone like Pinnell or Brit or John B take it on with students. John B misunderstood my question about cutting a sample in that I meant that even the most careful cutting with the best equipment in a lab setting would still change the sample at least a little. To me, flawed test results are worse than no test results at all. I think to avoid any cutting at all, all the clays to be tested have to first be tested for exact drying and firing shrinkage so that the all the clay bars are cut out so that all will be the same size without any cutting or other machining.

 

It is a great discussion. Any thread that challenges the many false "conventional wisdoms" of clay is a good one.

 

Jim

[Guest JBaymore]

Sorry for late replies here........ I was away. Will try to multi-quote a few things here to cover some ground up. So this posting will build for a few minutes as I copy/paste in some stuff.

 

Another Idea John would be to send the clay for the next class you teach with students on this in the future. I'm really not volunteering you just trying for the best results.

 

We've already frequently tackled this kind of thing internally with the clay bodies that our students typically will get to use...... and have some reasonable (within student level of testing accuracy) knowledge of how stuff "stacks up". I try to keep this to that realm... because we have a lot to cover in a relatively short timeframe. So this particular situation is not something that I'd be in a position to "take on" with them.

 

 

so if the bars have to be handled that carefully, how will that relate to real life pots? wouldn't real life pots then be too variable?

 

i'm not trying to be difficult, just trying to see the value of such a test.

perhaps knowing the

standard deviation (or standard error) of the samples

would be more helpful than the

average value. (MOR +/- SD)

 

Of course the forming of the pots (or sculptures or ?????) will greatly affect the "strength" overall. The walls section distribution alone will have a huge impact on the strength of a piece. Slab building using pounded out slabs versus thrown. And so on.

 

And a LOT depends here on what is actually meant by "strength" as I mentionedd already. MOR is just ONE single property of a clay body that might be attributed to the concept of "strength". How does a clay body respond to shocks creating vibreations withing the structure? How does a clay body respond to rapid heating and cooling? How does a clay body respond to compression forces? How does a clay body respond to tension forces? The list of possible testing is long.

 

Yes, the Standard Deviation model would be useful also.

 

 

 

Even if the one objection Ron Roy had about the test (that the earthenware sample was slightly oversize) is true, the tests are still probably the best we're going to get unless we all chip in (I'd do that within reason) and get the Alfred lab or some other place that can do the test even more accurately that Pinnell to do them.

 

A slight size increase in the cross section of one test piece over the cross section of another test piece can have DRAMATIC differenece in the MOR numbers. In fact unless the test pieces are exactly 1 square cm in cross section... it is not really a MOR number. It is a cvalid test for that one particular case..... but not to compare to something that has a different corss section.

 

Folks if you have never done this test on YOUR clay, go do it at least once with as much accuracy as you can muster. You will likely be astounded at the amount of pressure (weight) it will take to break that 1 sq. cm. test bar. I know my students underestimate the weight until they have done it by huge margins.

 

Also note that you can test green strength of a clay as well as bisque strength as well as finish fired strength. They are NOT necessarily directly related.

 

 

 

I'm not saying that all cone 6 pots are just as strong as all cone 10 pots. I'm saying, backed up by experience and Pinnell's tests, that the very small difference in temperature between cone 6 and cone 10 firings do not determine a pot's strength; the composition of the clay does. A pot fired to cone 6 may be stronger than a pot fired to cone 10 or vice versa.

 

Bingo!

 

We have about 4 different stonewares that we have tested all fired to cone 10. None of them match MOR wise either in the green state or in the finsih fired state. Our particular earthenware body (we use only 1) does not have as high a finish fired MOR as any of the stonewares.... but it does have a green MOR that is better than one of the stonewares.

 

 

His study also doesn't tell me anything about my cone 6 clay. I fully expect it to be less strong than Mark's cone 10 porcelain, but I think it's worth knowing for sure. I would think that anyone who is selling functional pottery for a living would want to understand their materials as much as possible, so they can stand behind it when they sell it to strangers.

 

 

Mea,

 

Yes!

 

I think that the materials science part of the "package" is not something that is really taught much anymore in the ceramic education process at an awful lot of places. So a lot of people don't even know that this kind of aspect exists.

 

It goes way beyond "functional pottery" in my opinion too. If you do an outdoor tile installation commission for a public art setting...... you'd likley want to know that 50 years from now the colors of the glazes you chose are still the vibrant hues you decided upon, instead of deteriorating from a combination of acid rain and pidgeon poop . Or that the free standing sculpture has withstood the impacts of the winter freeze thaw cycles in Denver, Colorado. And so on.

 

Personally I think that word "professional" in "professional ceramic artist" should mean something.

 

 

However, intuitively it seems to me that cutting away the outer layer of an extrusion would kinda make stoneware like earthenware (less that vitrification stuff).

 

If the work is being fired correctly, the entire cross section of the wall SHOULD be being fired to the same level of heatwork. So the amount of fluxing and the development of crtstaline growth throughout should be pretty much the same. The outside is not fired "better" than the inside. If this is true, then there IS a firing issue .

 

Also "vitrification" is only one part of the picture in derveloping strength. I think most use "vitrification" and "melting" as sort of synonomous. But in addition to the constituents possibly melting and "gluing things together" there is also the growth of crystalline structures happening in that clay body. THIS is where clay body composition and firing profiles come into BIG play in the potential strength of a body.

 

That above being said, the exterior surface is (in almost all cases of what WE do) oxidised, or re-oxidized on the cooling phase, assuming reduction firing. So there could in fact be some reactions that do affect the strength (along with the color) that are impacted by the presence of the great excess of oxygen there. And technically the exterior probably cools slightly faster than the interior, so reactions there could halt slightly faster than in the interior of the piece. But my GUESS is that for "potter level of science" this will not have enough impact to be of significance.

 

 

 

 

John B misunderstood my question about cutting a sample in that I meant that even the most careful cutting with the best equipment in a lab setting would still change the sample at least a little. To me, flawed test results are worse than no test results at all. I think to avoid any cutting at all, all the clays to be tested have to first be tested for exact drying and firing shrinkage so that the all the clay bars are cut out so that all will be the same size without any cutting or other machining.

 

One thing with ceramic engineering kinds of tests is the level of accuracy that is considered necessary. You'd be very hard pressed to create originals without any machining that hit the tolerances that are usually looked at as necessary. So the slight destruction of the surfaces is considered "acceptable". Any grinding is done, of course, as carefully as is possible.

 

If you open up the mathematical accuracy ... then it should be possible, but you then introduce other inaccuracies.

 

Remember Heisenberg's Uncertainty Principle..................... you are always going to have some inaccuracy any time you try to measure something. Just the nature of the beast. All you can do is your best approximation of the situation.

 

 

Whew,....... think that caught up there.

 

best,

 

 

 

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

[OffCenter]

 

Even if the one objection Ron Roy had about the test (that the earthenware sample was slightly oversize) is true, the tests are still probably the best we're going to get unless we all chip in (I'd do that within reason) and get the Alfred lab or some other place that can do the test even more accurately that Pinnell to do them.

 

A slight size increase in the cross section of one test piece over the cross section of another test piece can have DRAMATIC differenece in the MOR numbers. In fact unless the test pieces are exactly 1 square cm in cross section... it is not really a MOR number. It is a cvalid test for that one particular case..... but not to compare to something that has a different corss section.

 

 

 

I'm sure any increase no matter how slight will make a dramatic difference, so (If Ron Roy's objection is valid.) I'd ignore all results for that particular clay (Unfortunately, it was the most dramatic!). I meant that the test is still valuable for the other results.

 

Jim

[bciskepottery]

No this type of test is not going to account for variations in individual pots, but having a baseline measurement is still useful. If someone is willing to do this work, I am happy to contribute. Even if the test is not perfectly apples-to-apples, it is still better information than I currently have.

 

One of my largest wholesale accounts refused to buy my work for three years, because she believed cone 6 pots were not durable enough. That was based on a bad experience she had from 20 years ago. There are a lot of people out there with outdated information, or a lack of information. The more information we can uncover, the better.

 

Also, while most of my work is cone 6, once a year I attend a cone 10ish wood-firing. I have always told my customers the wood-fired pots were more durable than my usual pots, based on a. the different claybody, b. the higher temperature, and c. I make my wood-fire pots thicker than my usual pots. I have never tried to test if I was being factual, I was repeating what I have been taught. I would be much better off if I had some test results to base that on.

 

The clay I use most often for wood-firing is Highwater Phoenix, which is fairly groggy. Already my assumptions have been challenged on this forum because I thought grog made the clay stronger. Others are saying grog makes a claybody easier to break. I can see the logic in that. Maybe I should only say those pots are better for oven use due to their thermal shock resistance, without saying the pots are stronger? I would be excited if someone using Phoenix at cone 10 would also send bars to Mark.

 

Mea

 

 

 

Mea,

 

I believe Highwater uses Kyanite in Phoenix to give it that terrific thermal-shock property.

[Mark C.]

Update-I heard back from John Britt-MOR testing is not his deal

He suggested John Paccini at Laguna who I know well so I sent him the info and maybe they have done this testing already on some bodies. It will be next week at soonest for a response.

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

They as a clay manufacture may already have this data.

 

 

Kyanite is good stuff .I use it in my coating mix and I think its also in ITC or at least when you are sand blasting that ITC on the blow back feels like it .

Mark