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Test Firing Cone Question


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#1 Brian Reed

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Posted 11 July 2014 - 03:24 PM

I have some local dug clay that I mixed up with some Lincoln fire clay and some prepared high fire stoneware clay then and ran it through my bisque firing at cone 06 electric, and it turned out pretty nice.  I very nice orange color. It throws well and looks great like I had said.  However when I ran a small piece through my cone 10 reduction gas kiln it slumped and fell into a blob.  I testing to see if at cone 06 the piece would hold water, but it leaked pretty heavy so I know it is not vitrified.  

 

Now my plan is to get an entire load of this stuff and fire it to cone 01-1 and see what happens.  I suspect it may be closer to the right temp.  however you cannot just buy a few cones you need to buy an entire box.  What do you guys usually do?  Bite the bullet and buy a bunch of cones, or just rely on a pyrometer for the test firing?

 

My other plan was to put a small piece right near the peep hole in the next firing then record when it slumps and then I will at least know what the limits of the heat would be.

 

What do you all do in cases like this?


Brian Reed

Throwing down in Washington State

http://www.reedpottery.com

Northwest Clay Club

#2 Tyler Miller

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Posted 11 July 2014 - 03:53 PM

I would just bite the bullet and buy the boxes of cones you need.  They're not hugely expensive for what they are, and you never know when you'll need them.  You'll probably go through lots tweaking your local clay.  I think I own boxes of all cones from 07-9.  Sometimes I put a whole rack of cones in to see just how my kiln's firing.  Sometimes a big spread of cones necessary for things like reduction and carbon trapping--but that's gas.

 

A useful test to run with your local clay is to figure out its exact "pyrometric cone equivalent" (PCE). Shape it like an orton cone, and fire it alongside some commercial cones.  Observe when it falls in relation to the commercial cones.  Try firing a few little cups 3-5 cones below the PCE and test for absorbency and durability.  Sometimes your firing range will be so narrow that earthenware is the way to go.



#3 Chris Campbell

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Posted 11 July 2014 - 07:27 PM

Brian ... If the adventure of finding and using locally sourced clays appeals to you then it would be useful to buy some cones. You can of course stare into peep holes to get a rough idea of what is going on, but doing that is going to get old pretty quick.

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#4 TJR

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Posted 11 July 2014 - 07:39 PM

Sounds like you might have some soluble salts in your clay.Just an idea. Fire a piece next to your unglazed regular clay. If your unglazed regular clay is flashed, you have salt, and your clay is unusable.

TJR.



#5 Denice

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Posted 12 July 2014 - 12:35 PM

Do you buy your supplies on-line or at a local supplier?  My local supplier has small bags of cones you can buy, I noticed that the small packages are not on there website. If you ordered over the phone I'm sure they would ship you what you need. Just ask them, if you call about 10. AM Central daylight speak to Virgina  316 262 2551 Evans Ceramics   Wichita, KS.   Denice



#6 Brian Reed

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Posted 12 July 2014 - 11:11 PM

Thanks for the advise everyone.  Yes I will go and buy cones and get to testing.  I was trying to find some shirt cuts. But really this is the way and I know it.  My gas kiln is just so large running a lower fire test is just going to be wasteful.  I like to idea of making some cones and putting them in my next firing made from the clay.  Simple and perfectly safe.  Thanks again.


Brian Reed

Throwing down in Washington State

http://www.reedpottery.com

Northwest Clay Club




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