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cone 6 reduction clay question

updated wed 30 jun 99

 

pam pulley on mon 28 jun 99

Our pottery co-op uses a variety of Standard's clay types for firing
Oxidation and reduction. 153 is our best for both the cone 6 and 10
firings we do(and for teaching students). Our problem seems to be in the
225, light brown clay. Works fine in oxidation but in reductions it going
all to pieces...literally. I lost 75% of of my pieces in the last firing.
We thought it might be to the change in firing techniques of not candling
overnight and will be checking that out. But our distrubutor did a cone 6
reduction firing and lost most of the kiln load. He hasn't called the
Standard yet, but has anyone out there had this problem with this clay.


Pam in the sweating palm of Michigan.


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Lori Pierce on tue 29 jun 99

Hello Pam...yes indeed, I had the same problem with Standard clay many years
ago. The firing took place in a hard brick gas kiln, reductiion fired to
cone 9-10. I fired my own clay body, the other people working with me
purchased their clay from an Atlantan supplier of Standard clays. All of the
pots made with the Standard body spiral cracked as tho cut with a knife on
the wheel, top to bottom. Not one piece of mine had this problem. Standard
replaced the clay, tho not the value of the pots, when I sent them a
sampling of the shards and an explanation of what I perceived wrong with the
body. I wish I still had a copy of the letter for you. I believe the beta
quartz had converted to crystobalite, in the reduction kiln, and with the
compression caused by the inversion, the body had shattered. Free silica in
the body must be kept below 20%: if the flint or quartz is very finely
ground it will form crystobalite more quickly. My experience taught me that
this type of body could not be used in a reduction kiln with a long slow
firing and cooling cycle. I hope this helps...Hard to believe Standard is
still ignoring the needs of their customers with so many more potters today
NOT making their own clay bodies and glazes, or so it seems from the list.
Our amazement when we took down the kiln door and saw those fractured pots
was awesome. Good luck, Lori in New Port Richey Fl
-----Original Message-----
From: pam pulley
To: CLAYART@LSV.UKY.EDU
Date: Monday, June 28, 1999 9:23 AM
Subject: Re: CONE 6 REDUCTION CLAY QUESTION


----------------------------Original message----------------------------
Our pottery co-op uses a variety of Standard's clay types for firing
Oxidation and reduction. 153 is our best for both the cone 6 and 10
firings we do(and for teaching students). Our problem seems to be in the
225, light brown clay. Works fine in oxidation but in reductions it going
all to pieces...literally. I lost 75% of of my pieces in the last firing.
We thought it might be to the change in firing techniques of not candling
overnight and will be checking that out. But our distrubutor did a cone 6
reduction firing and lost most of the kiln load. He hasn't called the
Standard yet, but has anyone out there had this problem with this clay.


Pam in the sweating palm of Michigan.


_______________________________________________________________
Get Free Email and Do More On The Web. Visit http://www.msn.com

Ron Roy on tue 29 jun 99

Hi Pam,

If you fire an iron bearing clay in reduction you turn the iron into a big
time flux - so the net result is probably overfired clay. There can be
compatibility for the same body fired in both oxidation and reduction but
the key is - the less iron present the better.

The opposite is also true - if you formulate an iron bearing clay to
vitrify properly at whatever cone in reduction - if fired in oxidation the
body will probably be underfired.

RR

>----------------------------Original message----------------------------
>Our pottery co-op uses a variety of Standard's clay types for firing
>Oxidation and reduction. 153 is our best for both the cone 6 and 10
>firings we do(and for teaching students). Our problem seems to be in the
>225, light brown clay. Works fine in oxidation but in reductions it going
>all to pieces...literally. I lost 75% of of my pieces in the last firing.
>We thought it might be to the change in firing techniques of not candling
>overnight and will be checking that out. But our distrubutor did a cone 6
>reduction firing and lost most of the kiln load. He hasn't called the
>Standard yet, but has anyone out there had this problem with this clay.
>
>
>Pam in the sweating palm of Michigan.
>
>
>_______________________________________________________________
>Get Free Email and Do More On The Web. Visit http://www.msn.com

Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849

Web page: http://digitalfire.com/education/people/ronroy.htm