Seeking More Technical Understanding Of Firing To A Cone

[Tim T]

Of course the counter example to long, low firing that shows that there is a limit to this is to get some industrial archaeologist to come up with some unfired glaze or dry powdered clay from a century or so ago - my money's on it still being powdered today!

It needs to get to a high enough temperature to get things going, but then above this point to some extent time can be traded against temperature, as both are putting more energy into the system. I say to some extent because quite a few glazes need specific temperatures and/or times, e.g. thermal decomposition of iron oxide only occurs above about 1250C, and formation of lead or zinc silicate crystals need a hold at the crystal formation temperature.

 

Nerd, if you are interested in Chinese ceramics and glazes I'd strongly recommend the Ceramic Technology book in the series Science and Civilisation in China (Vol V Book 12), by Rose Kerr and Nigel Wood. Although short on pot porn pics it is full of information, much more than Nigel's "populist" ceramic books. The downside is that as an academic text it goes for about $200 (though good value at over 900 pages) - either find a friendly library or wait for a copy to come up on Amazon or Abeboks or similar sold by someone who doesn't realise its price.

[neilestrick]

We all fire to the same cone, 6 for example, because it gives a level of simplicity to the system. We can fire multiple glazes and multiple clay bodies in the same firing. We can share recipes without having to fire that one specific glaze to its own specific amount of heat work. The reality of the situation is that most potters do not mix their own clay bodies, so tweaking a clay to be perfect at a certain cone is not an option, and a large majority of kilns are filled with more than one clay body, so altering the firing to be perfect for any one clay body is not an option. A lot people also use commercial glazes, so there's no tweaking those, either. We do what we can to find a happy middle ground where everything comes out pretty darn good, within a range of acceptable results. We are at the mercy of clay and glaze manufacturers, materials suppliers, and mother nature herself. Potters cannot afford to use lab quality raw materials, so allowances must be made along the way. But that doesn't mean the results are not acceptable or dangerous, it just means that they aren't ideal. I agree that it would be nice if every clay and glaze combination was perfection, and we should definitely work towards that, but there is a level of 'good enough' that makes for some very attractive, safe, acceptably durable work.

[glazenerd]

Neil:

I think you sorta missed my point. The examples I am using in the study threads are not lab grade materials: most are just run of the mill kaolin, some have a small percentage of ball clays like OM4, FHC, or C&C to add plasticity or alumina, and or particle size. I use lab grade to set base lines: then test other materials against that result. The big difference is the molar % of alkali, it is that simple.

All I am pointing out in the above post is the firing schedule used. Potters (not all) use firing schedules to produce the best glaze result: not the best clay result. Glazes are much more tolerant of variations than clay, and yet more attention is paid to the finished appearance rather than the supporting structure. Even if you have c6 porcelain, stoneware, and earthenware in the same kiln: they all still have a relative wall thickness. So its just a simple matter of using a firing schedule that brings them to optimum maturity. I will be doing that study in the near future as well. I know all about profit margins: been there and done that.

 

Nerd

 

Edit: by the way, the commercial clay biz needs to be faulted: making a c6 to c10 firing range claim is just flat bogus IMO and in my testing.

[Guest JBaymore]

Edit: by the way, the commercial clay biz needs to be faulted: making a c6 to c10 firing range claim is just flat bogus IMO and in my testing.

 

I've been pounding on this issue for years.  Absolutely true.  This is so that they don't have to stock more different types of clay bodies.  Cost and complexity saving for them.  Not great for the potters.  They get away with it because people do not complain.  Many don't complain because they don't know.

 

best,

 

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

[glazenerd]

 

They get away with it because people do not complain.  Many don't complain because they don't know.

 

Well, I am a "fixin" (my best Jed Clampett) to correct that problem. I know two clay bosses have been reading my threads: perhaps I should email links to the rest. Hate to break the news: flux formula limits for clay is not on their radar either. (lengthy explanation on how I plan to fix that)  and maybe we can have more consistent results. Then all we have to do is revamp the 20,000 glaze recipes floating around on the net; written pre 90's.

 

Nerd

[Tim T]

Because of frustration with clay descriptions in supplier's literature, I've slowly been building up a database of my own results, e.g. firing at 50C increments and measuring LOI, shrinkage and water absorption, estimating strength, and also wet properties and putting through sieves to analyse the grog content, plus keeping samples as colour reference. Whenever I put in a clay order, I aim to get a bag or two of new stuff to try out. It's interesting how much things differ between two clays that have almost the same description!

[Guest JBaymore]

In the supplier's defense...... one of the issues here is that .... (OK, politically incorrect stuff coming) ........ a lot of potters are cheap, cheap, cheap.  Meaning if I as a clay supplier cost you $0.01 a pound more than my competitor...... those folks will go to my competitor.  Even if my product is actually superior.... because in many cases they do not realize that fact (it is "hidden" in the technical side).

 

So I have to produce clay cheaply...... because I need to sell that clay.  If I were to really quality control and technically test the bodies I used... and be constantly revising them BEFORE the potters in the filed start having issues....... I'd be going out of business.  Because the few cents a pound that all that costs me to do.... people won't pay it.

 

In some cases ... they don't SEE the differences... because they do not know where to LOOK for the differences.  A huge portion of studio potter's out there have testing regimes that are  .... if it looks like a pot, and quacks like a pot, .... it is a pot.  I call this the "Mark 1 Eyeball Test".  I simply look at the pot and the glaze....... if it looks good....... I assume it is good.  Base on a cursory visual inspection.  Away it goes for sale.

 

best,

 

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

[glazenerd]

Well John: I see a simple fix to this problem: which means it would only take 5 years to incorporate.  Most products out there have testing standards, or product standards, or product / manufacturers group standards. The CFBS (Clay Formulation Board Standards) would set formulation limits, and criteria for individual body types, and cone standards. Makers do not want to divulge too much of their recipes: I get that. However, if a clay body was tested by this board and labeled as C6: that means it matures at C6> period. They would have to make quarterly sample submissions to maintain this label. Consumers could then check the board site for compliance/and or register complaints.

 

Nerd

[neilestrick]

I think you missed my point, too.  If we're firing multiple commercial clay bodies in the same firing, and body A needs to be slightly hotter than cone 6 to fully mature, and body B needs to be slightly cooler than cone 6, and body C needs to be right at cone 6, there's nothing we can do to make them all perfect. So we fire to 6 and formulate our glazes to work best that way. There's just no other option without mixing your own clay body, or using just one clay body in the kiln at a time, neither of which is realistic in most studios. For example, we use 5 different clay bodies in my studio. Personally, I think it's better to have the clay ever so slightly immature than the glaze. A 1/4 cone with a clay body is only going to affect the absorption rate of the clay very slightly, whereas that amount in a glaze can have a much more significant effect on how it behaves. I also don't think that a 1/4 cone, or even 1/2 cone in many cases, is going to be at all noticeable in the functionality of the pot. Perfect isn't necessarily necessary.

 

That said I'm all for the clay manufacturers stepping up and doing a better job with formulation. But like John said, it's very much money driven. When I was the tech for A.R.T. Clay, I had to reformulate a body to get it to run through the machines better, and to lower the absorption rate. The one limitation I was given was that it couldn't cost any more to produce. In the end it did cost a penny per pound more, but we made up that difference in increased production so they allowed it. There will always be the 'good enough' standard until people are willing to pay for 'perfect'. we are also dealing with artists, who are often willing to put aesthetics ahead of science. Take a look at many of the cone 6 'specialty' glazes that have come out in the last few years and you'll see what I mean.

 

I would love for clay manufacturers to do away with the cone 6-10 label. I think this came out of the time when only hobbyists were firing to cone 6, and they didn't really use enough clay to dedicate a portion of the mixing schedule or catalog to them. Times have changed, though, and cone 6 is quickly becoming the norm, and there are a lot more cone 6 bodies available. That 6-10 label has stuck, though.

[Guest JBaymore]

The CFBS (Clay Formulation Board Standards) would set formulation limits, and criteria for individual body types, and cone standards. Makers do not want to divulge too much of their recipes: I get that. However, if a clay body was tested by this board and labeled as C6: that means it matures at C6> period. They would have to make quarterly sample submissions to maintain this label. Consumers could then check the board site for compliance/and or register complaints.

 

I miss the days when Studio Potter Magazine might have taken on doing the research and then publishing a very elaborate set of results of their studies (like maybe a 7-8 page article) .... naming names........ without being constrained by possibly offending advertisers.  They used to do great equipment reviews. 

 

best,

 

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

[Mark C.]

I miss the days when Studio Potter Magazine might have taken on doing the research and then publishing a very elaborate set of results of their studies (like maybe a 7-8 page article) .... naming names........ without being constrained by possibly offending advertisers.  They used to do great equipment reviews. 

 

 

 

amen brother I miss those days myself. Its especially true now as they are the only mag that could still do this as they do not have advertising issues as conflicts.

[glazenerd]

 

I miss the days when Studio Potter Magazine might have taken on doing the research and then publishing a very elaborate set of results of their studies

Tell em to email me: will do it for them-FREE!

Maybe I should just do it, and post it here!!! They come through here anyway from time to time.

Simple test: does your clay mature at the cone you state: and does is shrink at the rate you say, and check it for absorption.

 

Nerd

 

Neil, sorry.  To make it up to you, next time I go through northern Ill, I will stop. I usually just drive through to Wisconsin.

[neilestrick]

 

 

I miss the days when Studio Potter Magazine might have taken on doing the research and then publishing a very elaborate set of results of their studies

Tell em to email me: will do it for them-FREE!

Maybe I should just do it, and post it here!!! They come through here anyway from time to time.

Simple test: does your clay mature at the cone you state: and does is shrink at the rate you say, and check it for absorption.

 

Nerd

 

Neil, sorry.  To make it up to you, next time I go through northern Ill, I will stop. I usually just drive through to Wisconsin.

 

 

Definitely stop by! It would be great to meet you in person. Bring your porcelain tests. I would love to see those.

[Guest JBaymore]

I miss the days when Studio Potter Magazine might have taken on doing the research and then publishing a very elaborate set of results of their studies (like maybe a 7-8 page article) .... naming names........ without being constrained by possibly offending advertisers.  They used to do great equipment reviews. 

 

 

 

amen brother I miss those days myself. Its especially true now as they are the only mag that could still do this as they do not have advertising issues as conflicts.

 

They now ARE doing what amounts to ads.... look in the back.

 

best,

 

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

[Tim T]

Well here's a suggestion: with the web we don't need any magazine to publish the results, and magazines in the US, UK or Oz don't travel as widely as the web anyway.

It would be simple to set up a web site to hold clay test results (and other stuff as well, possibly, in time), and there are enough potters interested in things technical to post material. The big challenge is probably in coming to agree on a common set of test procedures. The clay tests I do are good enough for relative comparisons, but I'm not sure if any of you did what was purportedly the same test, you'd get the same result, and we would need to try and achieve this.

If enough people are interested, I'm willing to put together a web site for this. Who's up for it?

[glazenerd]

Tim:

 

I am not opposed to the idea: but I am not sure how far that information would travel. One of the reasons I decided to post my porcelain and stoneware studies here is: I do not like these people. So I torture them with useless drivel, endless splattering of techno talk they will never use: and to help me resolve high school anger about being called a Nerd.  They made a movie about us not too many years ago: "Revenge of the Nerds." It inspired me. Although I never actually saw the movie: just the trailer. Armed with my new camera toy: my plan is to keep posting obscene close ups until I turn their synapses to mush. I am already looking at new cameras with even more magnification, and special optics to make different elements show up in color. I was going to take a picture of a chocolate chip cookie with special optics: and tell everyone the chips were large particles of clay..... what us Nerds do to humor ourselves.

 

Nerd

[curt]

Well here's a suggestion: with the web we don't need any magazine to publish the results, and magazines in the US, UK or Oz don't travel as widely as the web anyway.

It would be simple to set up a web site to hold clay test results (and other stuff as well, possibly, in time), and there are enough potters interested in things technical to post material. The big challenge is probably in coming to agree on a common set of test procedures. The clay tests I do are good enough for relative comparisons, but I'm not sure if any of you did what was purportedly the same test, you'd get the same result, and we would need to try and achieve this.

If enough people are interested, I'm willing to put together a web site for this. Who's up for it?

Hi Tim,

 

Like your thinking.  Though I think you are right about it being necessary to get a common set of test procedures and standards.  Wondering also if it is better to piggyback off an existing site or start something new from scratch.

 

Might suggest we see how Nerd goes and then revisit your suggestion ex-post, after we sent his tests through the ringer, crucified his procedures and methodologies, and heaped scorn on his results.  Then I am sure everyone will want to start testing.  

[High Bridge Pottery]

Could set up a Wikia http://pottery.wikia.com/wiki/Pottery_Wikia