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Is Cone 4-10 Clay Fired To Cone 4 Underfired?


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As I search for my perfect clay, I  see quite a few that have a very wide firing range.  Is a pot that's fired to the low or middle range of that firing range less structurally sound than one fired to its highest end?  Does it make a difference other than with the glaze fit?  Does it matter in some other way?  Or not?  Are these ranges realistic?  I've been using one with a 4-10 range and also another with a 6-9 range and firing to about cone 5 3/4.  Glazes fit fine.  But what's actually best for the structure and health and happiness of the pots?  My pots are all quite young, but will they age well? Thanks in advance!

 

Irene

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Guest JBaymore

Irene,

 

Search around on the forums here about clay bodies.  Lots of discussion about this specific kind of stuff.

 

Simply put... there is no such thing as a cone 4 to 10 clay body if you are talking about strength and vitreousness.  Period.  It is either under fired at 4 or over fired at 10.  No clay body has that kind of range and can be anywhere near "optimal" at all those ranges.  It does not work that way.  Likely this body is maybe a cone 7 clay body.  Manufacturers like to do these one size fits all bodies... to cut the inventory requirements of stocking lots of different formulations.  They pass on the problem (caveat emptor) to the poor potters who have less than optimal technical awareness.

 

best,

 

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

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Hi John.  

 

So you would suggest I use a clay with a narrower firing range, say  4-6,  or just 6, to achieve the optimal vitrification and strength?  Or just 6? 

 

I searched the clay and glaze chemistry forum but didn't see any topics that seemed to address this.  Of course I didn't read all the posts.  Is there another section I should search?

 

Thanks for your input.  I'm making the pilgrimage to the pottery supply store tomorrow.

 

Irene.    

 

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Guest JBaymore

Irene,

 

Your absolute best bet for getting the max quality out of your claybody is to have a clay that matures at the cone to which you are firing.  If you can find a supplier that has a cone 5-6 specific body..... that would be the best as far as fired qualities go.

 

Even cone 6-10 clays are problematic.  They typically are cone 8-9 clay bodies, that are still OK at 10 and are a bit underfired at 6.

 

When you search the forums, do so from the MAIN forum page (the one that shows all the various sections).  And unfortunately sometimes topics pop up in other topic threads.  SO you sometimes have to just "read around" to learn such stuff.  Lots of threads on clay bodies.

 

best,

 

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

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This isn't the most sophisticated site re: computer technology or search, uploads etc., but searching Forums (which contains all forums) from the main pg. as suggested above, rather than individual ones by topic, and using a variety of pertinent keywords will help. There are also other ceramic resource boards/groups with info--of varying quality--on the Internet, if you "Google" your query (can't stand Bing, pesonally). 

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There is just so much to learn about clays ... it does not help when a supplier claims that kind of firing range on a clay body.

 

Quite honestly, as long as it does not melt into a puddle at the high end, you could claim it fires at all those temperatures ... but, it only fires to an acceptable level for functional uses or lasting durability within a narrow range. Often the supplier won't even tell you the magic number!

 

So yes, narrowing down the promised Cone range will up your chances of happiness with the results.

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I think it might also depend on what kind of work you make.  My understanding is that if you are making casseroles or other functional ware which goes in the oven it is better to have a slightly underfired clay.  If you are making vases you want the clay to be as mature as possible to prevent leaking.  Outdoor sculpture won't survive freezing if it is absorbing water.  L

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Thanks everyone.  I think I'm going to get to work on that box of cone 6 porcelain.   Do I hear groaning?  Remember, I'm still a newbie.  It's formulated for cone 6 so it's ok even tho it's porcelain right?

 

Nerd, how about a brief synopsis of your rant in layperson's terms?  I mean clayperson's terms.  Haha.  ( do I hear more groaning?)   Or just nod if you agree with what's been said.  

 

 

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Irene:  I will certainly give a nod to what has been said.

 

In a single technical aspect (molarity).. in simple terms: amount of flux in the body. The lower the temperature, the higher the percentage of flux required to achieve vitrification ( fancy word for: the cake is done!!)

 

Thats it !!!!  - brownies.

 

If you want soft and moist... more eggs/oil

If you want cake like -  less eggs/oil.

 

A cone 4-10 clay body has X amount of flux (oil), and X amount of flour (silica and clay); so it will not produce soft/moist and cake-like out of the same recipe....

 

Nerd

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Thanks Nerd, now I know why I don't like Brownies.  

 

I'd never thought of the clay's ingredients like that, makes so much more sense.

 

I always use baking to explain why peak temperature is not the same as cone x down.  I like analogies that reflect normal aspects of life, and nothing is more normal than food.

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Chilly: I have used food / ovens before in describing the firing process. Sorry, I do not remember what threads they are in. Brownies are much more fun than describing molarity, Si/AL ratios, and heat work. I have already decided to use cake frosting to describe the different glazes.... :)

 

However, I will continue my push to change the industry standard firing cycles. The trend is to fire to perfect glaze; which leads to an immature clay body: mostly applicable to a cone 6 firing. So I will use a 1" prime rib steak to make this illustration. When you put a spice rub (glaze) on a steak and toss it into a broiler (kiln); the oven will seer the outside of the steak and activate the seasonings; but the inside of the steak is still mostly raw. (medium rare). So when you serve it; the outside looks all juicy and seared until you cut into it; and the blood comes up (off gasses). In order to cook a thick prime rib to well done, you have to cook it (ramp cycle) at a temperature and cycle so the meat is cooked all the way to the center.

 

The more I study this issue; the more I am convinced that nearly 90% of pinholes and craters in a glaze is due to an immature clay body. The current remedy is to do an extended hold: which really does not mature the glaze ( if you notice the samples with pinholes, the glaze is already mature.) The extended hold simply allows the clay to complete its off gassing. Pin holes are a sure sign that the clay body is immature: which increases absorption, and decreases vitrification. Many potters that struggle with absorption, do so partly because of their firing cycles. In addition, and immature clay body can also lead to some COE issues: how a vitrified body contracts upon cooling, and a immature body contracts upon cooling: is not the same. Some fluxes actually cause expansion at high temperatures: and increase articular crazing because they have not fully matured, which would compress (vitrify) if allowed to mature.

 

There are several potters that have struggled with pinholes and craters, and well as some crazing issues that I have helped in PM. They followed my recommended cone 6 firing schedule; which remedied their problems. I will leave it up to them to post results; that is their call. The before and after results are very clear.

 

Sorry John- Neil: could no longer refrain....

Nerd

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 Doesn't most of the out gassing occur during bisqueing?   NO... but most carbons are burnt off at this point.  At bisq, the clay body is actually in an expanded state (very porous).

 

adding a hold at the proper temp   the critical temp range for clay maturity in a cone 6 firing is 2050-2190F

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Guest JBaymore

So Nerd, I was hanging in and ignoring the brownies talk and all ... then you mentioned cooking a PRIME RIB roast to well done!!!

That is just an expensive, over fired error.

 

I think that qualifies as "carbon coreing".   ;)

 

best,

 

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

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