Cone 6 vs Cone 10 clay strength

[Mark C.]

I realize there has ben a lot said on this (cone 6 is the new 10 and so forth) so I wanted to find out for myself what's up. A friend gave me some fired glazed cone 6 work that they did not want.

Now Im short on testing machines around the studio so what I did was find one of my cereal bowls with a lip split on it.

Now I have two bowls one fired to cone 10 with a serious lip split and one bowl that's fired to cone 6 in perfect shape

I put on safety glasses and started hit these two bowls together at the weak lip split point. It took many taps as I increased the hit .

After about 4-6 hits the cone 6 bowl broke. I thought the lip split cone 10 would surly be the 1st to go but no. I did this test once again and the cone 6 bowl broke again-the lip split bowl is still ringing true.

Test results from non scientific testing cone 6 for me and I know many disagree is a weaker body.

This clay by the way was not frost but another body and my body is porcelain.

I have some more testing to do but this was done yesterday.

Mark

[neilestrick]

Was the cone 6 piece porcelain, too? I'm wondering if the lip split actually made it harder to break, like it gave it a point where it could flex a little, like stress relief. Just a thought. I think ultimately you need two bowls made with cone 6 and 10 versions of the same body. And even if the cone 6 is a little weaker, 4-6 hits aint too bad!

[Chris Campbell]

Mark ... A tad "non-scientific" ???

Hope you are wearing protective gloves, eyeware, faceguards, work boots, tooth guards, apron, work jeans and such as you thwack these pots together!

AND ... can you get someone to take photos and post them?

[Mark C.]

I'm a BIG FAN of real world stuff-so when I go outside and its raining I do not need a weatherman -I think Bob Dylan covered this long ago

I'm also a big safety fan so heres the said photo of what this test takes-heavy boots and leather are now put away.I wear this stuff in the studio always as you never know when the s---t hits the fan.

You can see the crack on my bowl on left-never knew a crack made it stronger-I love learning new stuff every day.The cone 6 pots are on the right 3 of them said cone 6 bowl is busted. That was the one without a crack and was weaker-all are white bodies

as to exact bodies yes in a perfect world thats true but real world stuff here so lets keep it real.

I got the cone 6 pots from my mentee who lives 900 miles from me over a international border no small feat there-again real world-I am checking on what bodies these are so the untill then the Warren commission findings are still top secret-we may never know if it was more than one gunman

Now I do own a lab coat but when its weaker to me I know its weaker. If you believe different thats great also as I like various opinions but I'm saying I tested it for me. Please run a test on your own as more info is better. I just wanted to find out for myself before drinking the cool aid and now I'm passing on drinking it as I know its weaker.

I'll post the bodies when I get the info.

For you folks who are not wearing hard hats while throwing think about what could happen?

Folks this post is about 1/2 in jest the other 1/2 is real world stuff

Mark

[TJR]

I'm a BIG FAN of real world stuff-so when I go outside and its raining I do not need a weatherman -I think Bob Dylan covered this long ago

I'm also a big safety fan so heres the said photo of what this test takes-heavy boots and leather are now put away.I wear this stuff in the studio always as you never know when the s---t hits the fan.

You can see the crack on my bowl on left-never knew a crack made it stronger-I love learning new stuff every day.The cone 6 pots are on the right 3 of them said cone 6 bowl is busted. That was the one without a crack and was weaker-all are white bodies

as to exact bodies yes in a perfect world thats true but real world stuff here so lets keep it real.

I got the cone 6 pots from my mentee who lives 900 miles from me over a international border no small feat there-again real world-I am checking on what bodies these are so the untill then the Warren commission findings are still top secret-we may never know if it was more than one gunman

Now I do own a lab coat but when its weaker to me I know its weaker. If you believe different thats great also as I like various opinions but I'm saying I tested it for me. Please run a test on your own as more info is better. I just wanted to find out for myself before drinking the cool aid and now I'm passing on drinking it as I know its weaker.

I'll post the bodies when I get the info.

For you folks who are not wearing hard hats while throwing think about what could happen?

Folks this post is about 1/2 in jest the other 1/2 is real world stuff

Mark

 

 

Holy crap, that was beautifull- some parts actually made sense. Did you come across the body of Jimmy Hoffa on your way to the results?

What Mark is saying here, gang is that a cone 10 clay body is harder than a cone 6 body.

I really like the visual!

TJR.

[bciskepottery]

from the Clayart archives . . .

 

From: Ceramic Arts Discussion List [CLAYART@LSV.CERAMICS.ORG] on behalf

of Pete Pinnell [ppinnell1@UNL.EDU]

Sent: Thursday, December 20, 2001 11:15 AM

To: CLAYART@LSV.CERAMICS.ORG

Subject: clay body strength

 

For the final project in my Clay and Glaze class this semester, we mixed

about 50 clay bodies for testing, including red and white earthenwares,

stoneware, porcelain, and sculpture bodies. Besides other tests, we

extruded numerous bars of each body and broke them to measure MOR

(Modulus Of Rupture, which is a measure of the bending strength). There

are other strength tests that can be done (chipping tests, for

instance), but MOR is a quick and easy way to predict how well a body

will hold up to the bumps of everyday use.

 

Out of all these tests, there were a number of interesting trends:

 

1. Any amount of grog weakens clay bodies, especially in sculpture

bodies that are essentially underfired. Some of the sculpture clays were

so weak at cone 04 that we couldn't measure them- the bars broke at

initial contact before any stress was applied. Any texture in the clay

tended to have the same result, though the texture from using 50 mesh

fireclay seemed to have only a minimal effect. Really fine grogs- those

less than 80 mesh- also had little effect.

 

2. Glaze made a huge difference in strength. Crazed glazes lowered

results 50% or more from the strength of the same bar unglazed. Uncrazed

glazes raised the strength of the bars from 50 to 100 %. I had read this

before, and assumed that it was mostly related to the lack of surface

flaws on a smooth glaze (cracks like to start at a flaw- take away the

flaws and it's more difficult for a crack to start). What I found

interesting is that the amount of compression also mattered. We glazed

the porcelain bars with three different versions of my Pete's Clear

glaze, which ranged from mild compression for the original version to a

very low expansion version that places the clay in a very high

compression. Consistently, the higher compression versions produced

higher MOR results.

 

3. Clays have to be fired to maturity to get good strength. Even firing

porcelain bodies to cone 9 rather than 10 lowered strengths a good deal.

As an aside, I define maturity as the point at which a body achieves its

best strength and glaze fit, and no longer suffers from marked moisture

expansion. Absorption, in my opinion, is not a good indicator except

within one clay body group (such as "high fire porcelain"). Porcelains

may need to have less than 1% absorption to avoid moisture expansion

problems, while mature white earthenwares can have upwards of 20%

absorption (which is why those cheap white tiles on our shower walls

don't develop delayed crazing).

 

4. "Smooth" counts for more than "glassy", which seems to contradict one

bit of standard wisdom I've heard in the past.

 

5. Quartz seems to be a problem- at least in a minor way. Porcelain

bodies that used a combination of pyrophyllite and quartz were stronger

than those which used only quartz as a filler. It's a bit of a mixed

bag, though, because glazes on pyrophyllite bodies tended to craze more.

 

What were the strongest clays? This will surprise you- it certainly did

me. The strongest clays, consistently, were (drum roll, please) red

earthenware clays fired to a full cone 04.

 

Yep, that's right. Plain old Redart based, smooth red earthenwares. They

were stronger than smooth, brown or gray stonewares, and even stronger

(over all) than porcelain, which I had assumed would be best.

 

Yes, it was very important to fire them to a full cone 04: cone 06

didn't hack it. Surprisingly, taking them to cone 1 did not increase

MOR, though they certainly were denser and felt more solid and chip

resistant. Within red earthenwares, we got consistently higher strength

from those using wollastonite as a secondary flux (5 to 10%), rather

than talc. It seemed best to use red clay in amounts of 50 to 70%, and

while Redart alone (for the red clay portion of the body) gave the best

strength, we got much better workability (and only a tiny bit less

strength) by using a mixture of red clays, such as Redart mixed with

Ranger Red (from Texas) and Apache Red (from Colorado).

 

As with porcelain, the clay was made much stronger with glazes that fit,

and higher compression glazes were strongest of all. Our all-time

champion (for strength, NOT workability) was the following recipe,

glazed with Linda Arbuckle's Majolica and fired to a full cone 04.

 

Redart, 60%

KT 1-4 Ball Clay, 30%

Wollastonite, 10%

 

I thought you might find this interesting. I only teach a Clay and Glaze

class one semester every three years, so while I plan to do some follow

up tests (these tests raised as many questions as they answered), don't

look for those results any time soon!

 

Pete Pinnell

University of Nebraska at Lincoln

[Mark C.]

I'm very familiar with Pets test results on this but who uses ( Our all-time

champion (for strength, NOT workability) was the following recipe,

glazed with Linda Arbuckle's Majolica and fired to a full cone 04.Yes, it was very important to fire them to a full cone 04: cone 06

didn't hack it.) And who fires to this 04?

I know Pete knows his stuff but I am not working in a lab-I make my own tests and thats what this is about.Real world everyday pottery thats used every day piled high in the sink tossed around and bumped dropped etc.

I'm talking about my own real world rock paper test and rocks won.

The Pottery sold as utilitarian wares to the public is my testing grounds and I wanted to see for myself which is stronger and guess what most cone 6 bodies I feel are not as strong -at least the two I have to test.

I suggest all to do their own testing and not drink others cool aid I'll send a flawed cone 10 pot for testing your cone 6 body (send me a PM) and others can make their own choices.

I may just be making all this up and the strongest body is a cone 08-who knew?

I do not need a weather man to tell me which broke the easiest. I hope others make the test.

Keep in mind not many use Linda Arbuckle's Majolica for production work for the public.

The bodies are

Plainsman M340 & M350 and B-Mix. Butter dish is M370 which is Plainsman's version of B-Mix 5

all from Canada.

Mark

[atanzey]

Mark -

 

Here's the problem I see. You can draw the conclusion that the ^6 you tested is weaker than the ^10 you tested, but with however many commercially available clay bodies out there, not to mention potter's own blends, I don't believe you can (at least as yet) draw the generalized conclusion that ^10 is always stronger than ^6. The variences between clay mixtures and silica and grog and 'true vitrification temp' (rather than being deemed ^6 for some commercially convenient reason) would surely produce enough test fodder for a doctorate degree in ceramics.

 

That said, if anyone is out there hitting my pots with a hammer, they deserve shards! I'm happy you're convinced, but I don't need convincing. If I go to ^10 in the future, it will be for reasons other than strength.

 

Interesting stuff - and it brought out the Pete Pinnell thread, which I would have never thought to research.

 

Alice

[Mark C.]

My sense is that in general they are stronger (cone 10) but as noted their are exceptions-The right body the right maturation point (temp).

In general I want my body to be as durable as I can get it that also goes with most glazes I use-Yes my reds are softer and so forth.As I'm making utilitarian wares.

i started with stoneware in 1969 and switched to al porcelain in mid 80s for color -I have found that stoneware chips much easier than porcelain (I'm sure someone will disagree) but I found that to be true

I have been waiting awhile to make a test of the cone 6 body storyand now that Its done I've convinced myself (I did not use a hammer)

Others may find different results. My point is to try.

You can believe others

or do your own testing

I choose the later

Neils point below is spot on

Mark

[neilestrick]

I may just be lazy (my wife says I am), but I am of the opinion that which one is stronger is not as important as which are strong enough. I have cone 6 and cone 10 pots in my cupboards that get used every day- eaten from, run through the dishwasher, banged about by toddlers, etc.- very much real world testing. Some are crazed, some are not. They are dark, light smooth and groggy- little bit of everything. It is very rare that a pot breaks in our house, even with a fair amount of abuse heaped upon them. Maybe 1 every couple of years. So I feel like pretty much all of them are strong enough, even though there may be a great difference in their strength when compared to each other.

 

That said, I do make sure that I'm firing my clay to full maturity, and try to make my glazes as durable as possible.

[yedrow]

I would think that for a completed piece there would be a host of variables, like wall thickness, clay-glaze interface quality, clay-glaze coefficients of expansion relationships, vitrification of clay and glaze, etc. But I'm no engineer and am just guessing. All in all though, I would say that a well made and well fired pot would be noticeably stronger than a poorly made pot, either ^6 or ^10.

 

One question I had when I read Pinnell's paper was the nature of the cross section. The more vitrified ^10 cross section would also have less surface area per unit volume, so would it be less strong than a less vitrified ^04 cross section (think I beam)?

 

I agree with Neil that strong enough is strong enough. I would like to know though which is stronger. Perhaps there is a college that likes to test shear strength on materials for free, heh.

 

Joel.

[OffCenter]

Your "tests" are absolutely worthless, Mark, except that it helps you cling to your belief that cone 10 pottery is stronger than cone 6 pottery so that you can tell your customers such nonsense. The truth is that it is maturity and lack of impurities (including additions like sand and grog) that determine the strength of fired clay. You seem to be so attached to what you were taught in the 70's (or whenever) that you probably think heavy reduction firing makes clay even stronger--it doesn't, it makes it weaker.

 

Hell, I could grab a hammer and go into the back yard and break one of my cone 13 pots and then a cone 6 pot and make all kinds of stupid pronouncements about cone 6 pots being stronger and not needing a weatherman to confuse me with facts! The difference between cone 6 and cone 10 is only a little over 100 degrees. Some clays fired to maturity at cone 6 are stronger than some clays fired maturity at cone 10. Some clays fired to maturity at cone 10 are stronger than some clays fired to maturity at cone 6. It depends on the clay, not a 100 degree difference in temperature.

 

Thanks, Bcisketpotery, for posting Pete's test results. I especially like his finding that of the clays they tested scientifically a cone 04 clay was the strongest! Now, excuse me while I go take a hammer to a pot made of Lizella red so I can claim that it is the strongest clay in the world.

 

Jim

[bciskepottery]

Jim -- wear appropriate safety gear and stand to the side when you swing that hammer . . . don't want you getting hurt when the hammer ricochets back off that super hard Lizzell Red earthenware.

[Chris Campbell]

Well, the trouble with scientific studies run with consistent rules is that they are not very sexy. You present the results of what you verify and only results you can verify and that's kind of boring.

[Lucille Oka]

Let's face it we make breakable, chipable product; no way around it.

[Mark C.]

Your "tests" are absolutely worthless, Mark, except that it helps you cling to your belief that cone 10 pottery is stronger than cone 6 pottery so that you can tell your customers such nonsense. The truth is that it is maturity and lack of impurities (including additions like sand and grog) that determine the strength of fired clay. You seem to be so attached to what you were taught in the 70's (or whenever) that you probably think heavy reduction firing makes clay even stronger--it doesn't, it makes it weaker.

 

Hell, I could grab a hammer and go into the back yard and break one of my cone 13 pots and then a cone 6 pot and make all kinds of stupid pronouncements about cone 6 pots being stronger and not needing a weatherman to confuse me with facts! The difference between cone 6 and cone 10 is only a little over 100 degrees. Some clays fired to maturity at cone 6 are stronger than some clays fired maturity at cone 10. Some clays fired to maturity at cone 10 are stronger than some clays fired to maturity at cone 6. It depends on the clay, not a 100 degree difference in temperature.

 

Thanks, Bcisketpotery, for posting Pete's test results. I especially like his finding that of the clays they tested scientifically a cone 04 clay was the strongest! Now, excuse me while I go take a hammer to a pot made of Lizella red so I can claim that it is the strongest clay in the world.

 

Jim

 

Jim I did use a hammer or say reduction strengthens clay (not true) or mention any of this to my customers.

I just did a non scientific test and posted what I found out-sorry it bothered you as well as when I learned about pottery seems to. I have been learning about clay for 40 years now and that includes the 80's 90s and 2000s.

Next time Ill keep my results to myself-This is starting to feel like another site. I'm tough skined but dude you need to lighten up.

I have a huge sense of humor and I realize some have non.

Not sure what the 70's have to do with this test?

I thought this site was to share ideas and help each other.

Their is no reason to be mean

Mark

 

 

[WillowTreePottery]

Interesting discussion. So many variables affect this. I work in both c 10 and c 6 , mostly c 6 and c 04 in the winter when my gas kiln is frozen shut.

 

 

 

I find that the glaze fit seems to be the critical variable that affects my work regardless of the recommended vitrification temp. True test would be to fire the clay bodies raw and then repeat your crash tests.

 

 

This is a very non scientific observation as well, the pots that have survived in my kitchen cupboard (unchipped) for more than 25 yrs are all cone 10 red stoneware Plainsman 443 glazed with good old standard white or off white glazes. Other clays have come and gone, we've managed to break, crack, or damage the ones that were put to everyday use. Cone 6 stuff is still around but much of it is pretty badly crazed.

 

 

Not everyone makes functional dinnerware, or whacks stuff around when they cook like i do. I am not saying that one firing temp is better than another just another non scientific observation about the survival rate of my pots over 25 yrs.

[OffCenter]

Your "tests" are absolutely worthless, Mark, except that it helps you cling to your belief that cone 10 pottery is stronger than cone 6 pottery so that you can tell your customers such nonsense. The truth is that it is maturity and lack of impurities (including additions like sand and grog) that determine the strength of fired clay. You seem to be so attached to what you were taught in the 70's (or whenever) that you probably think heavy reduction firing makes clay even stronger--it doesn't, it makes it weaker.

 

Hell, I could grab a hammer and go into the back yard and break one of my cone 13 pots and then a cone 6 pot and make all kinds of stupid pronouncements about cone 6 pots being stronger and not needing a weatherman to confuse me with facts! The difference between cone 6 and cone 10 is only a little over 100 degrees. Some clays fired to maturity at cone 6 are stronger than some clays fired maturity at cone 10. Some clays fired to maturity at cone 10 are stronger than some clays fired to maturity at cone 6. It depends on the clay, not a 100 degree difference in temperature.

 

Thanks, Bcisketpotery, for posting Pete's test results. I especially like his finding that of the clays they tested scientifically a cone 04 clay was the strongest! Now, excuse me while I go take a hammer to a pot made of Lizella red so I can claim that it is the strongest clay in the world.

 

Jim

 

Jim I did use a hammer or say reduction strengthens clay (not true) or mention any of this to my customers.

I just did a non scientific test and posted what I found out-sorry it bothered you as well as when I learned about pottery seems to. I have been learning about clay for 40 years now and that includes the 80's 90s and 2000s.

Next time Ill keep my results to myself-This is starting to feel like another site. I'm tough skined but dude you need to lighten up.

I have a huge sense of humor and I realize some have non.

Not sure what the 70's have to do with this test?

I thought this site was to share ideas and help each other.

Their is no reason to be mean

Mark

 

 

 

 

Mark, my post was meant to rebut your post strongly, not be mean. Admitting that your "test" wasn't scientific and adding a bit of humor shouldn't exempt it from criticism when, in the end, you did present it as proof of your belief that clay fired to cone 10 is stronger than clay fired to cone 6.

 

Jim

[Mark C.]

(you did present it as proof of your belief that clay fired to cone 10 is stronger than clay fired to cone 6)

 

 

 

I started this test not knowing which would be stronger.Really I had an open mind.

I'll gladly mail the two bowls to you and you do your own tests and let us know what the outcome is?

I learned the outcome for me on these two bodies

Double blind is really the best.

Mark

[Guest JBaymore]

I thought I mentionmed this in this thread.... but I guess not.

 

There a re specific test and methods of testing used to test clay body strength in various aspects OF that general idea of what we might mean by "strength". There are lots of types of "strength"....... tensile, compressive, brittleness, abrasion, etc. What are we talking about here? And what are we "measuring"?

 

Unless the tests are done in a tightly controlled and scientifically accurate manner, excluding and controlling variables, the results are at best motre anecdotal than factual.

 

Glaze impacts stuff like MOR (Modulus of Rupture) greatly.

 

Note also that physical structure affects potential strength also. Take a plain fully fired clay bar and test the MOR. Then take the same kind of bar and encase it on all four sides with a well fitting glaze. That added glaze layer creates sort of a torsion box structure around the core of pure clay. VERY different.

 

This is a complex subject.

 

best,

 

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

[Marcia Selsor]

Does this thread remind anyone of Clayton Bailey's Dr. Gladstone compression tests on Ming Dynasty vases back in the early 70s ?

Marcia

[OffCenter]

(you did present it as proof of your belief that clay fired to cone 10 is stronger than clay fired to cone 6)

 

 

 

I started this test not knowing which would be stronger.Really I had an open mind.

I'll gladly mail the two bowls to you and you do your own tests and let us know what the outcome is?

I learned the outcome for me on these two bodies

Double blind is really the best.

Mark

 

 

The problem, Mark, is that your "test" is so flawed that it really is worthless. Of the two pots that you banged together until one broke, the one that broke could have broken first for many reasons that have nothing to do with the strength of the clay -- shape, grog, glaze, angle of hit, etc., etc., etc. Had the cone 10 pot broken first, would you really have jumped on the computer and instead of posting "Cone 6 vs Cone 10 Clay strength" subtitled: "It's like I thought originally" posted "Cone 6 vs Cone 10 Clay strength" subtitled: "I was wrong"? Actually, I was too kind in my original post. Your "test" is worse than worthless; it is a disservice to people coming here to learn about clay. (I can just hear some newbie saying to a classmate, "Yeah, some guy on the Internet proved cone 10 clay is stronger than cone 6.") If you're going to take the time to do a test then do it right or not at all. Do a MOR test (being very careful to be accurate, follow all the procedures, and try to make it a blind test if possible). You not only don't take the time to do a MOR test but you dismiss the results of such testing by one of the most knowledgeable potters in the world by some almost-macho-sounding crap about your tests being real world tests.

 

It is very possible that a carefully done MOR test could have proven that that particular cone 10 clay body was stronger than that particular cone 6 clay body but even that would not prove that in general cone 10 clay bodies are stronger than cone 6 clay bodies. The truth is that some cone 6 clay bodies fired to maturity are stronger than some cone 10 clay bodies fired to maturity and vice versa. It is the composition of the clay (and in the case of pots all kinds of other factors like timed cooling, glaze fit, etc.) that determine strength, not firing the kiln 100 degrees hotter.

 

Jim

[Chris Campbell]

I really don't mean to pile on but I have great respect for accurate information.

 

Here is the definition of a double blind test:

 

1- an experimental procedure in which neither the subjects of the experiment nor the persons administering the experiment know the critical aspects of the experiment; "a double-blind procedure is used to guard against both experimenter bias and placebo effects"

 

So, in order for yours to even come close you would have had to have NO knowledge of which bowl was which.

 

The best you can say for your experiment is that on this particular day, with these particular bowls, held by a non-ambidextrous male, struck in a more or less forceful way ... One broke before the other did.

[Guest JBaymore]

I really don't mean to pile on but I have great respect for accurate information.

 

Here is the definition of a double blind test:

 

1- an experimental procedure in which neither the subjects of the experiment nor the persons administering the experiment know the critical aspects of the experiment; "a double-blind procedure is used to guard against both experimenter bias and placebo effects"

 

So, in order for yours to even come close you would have had to have NO knowledge of which bowl was which.

 

The best you can say for your experiment is that on this particular day, with these particular bowls, held by a non-ambidextrous male, struck in a more or less forceful way ... One broke before the other did.

 

 

Precisely.

 

best,

 

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

[OffCenter]

I really don't mean to pile on but I have great respect for accurate information.

 

Here is the definition of a double blind test:

 

1- an experimental procedure in which neither the subjects of the experiment nor the persons administering the experiment know the critical aspects of the experiment; "a double-blind procedure is used to guard against both experimenter bias and placebo effects"

 

So, in order for yours to even come close you would have had to have NO knowledge of which bowl was which.

 

The best you can say for your experiment is that on this particular day, with these particular bowls, held by a non-ambidextrous male, struck in a more or less forceful way ... One broke before the other did.

 

 

Wish I had said that last sentence.

 

Jim