[Clayart] pin holes

ronroy at ca.inter.net ronroy at ca.inter.net
Mon Jul 22 10:52:02 EDT 2019

Hi Daphne,

Good to see you again at NCECA!

Normally we would assume any electric firing would be in an oxidizing  
atmosphere. In other words excess oxygen inside the kiln.

There are sources of combustible material however in the materials we  
use - wax for instance and unwanted carbonaceous material in many of  
the clays our bodies are made from. When these burn out they can use  
up the excess oxygen and the result is a reduction atmosphere. Paper  
clay firing would require some special firing techniques if an  
oxidizing atmosphere is wanted.

If this (reduction) happens in a bisque firing any iron in a clay body  
can be reduced from Fe2O3 to FeO. Apparently the Fe2O3 gives up it's  
extra O2 very easily. Fe2O3 is not a strong flux but FeO is. This can  
result in the clay around any bits of iron getting overfired. This can  
lead to pin holes and bloating as the over fluxed clay breaks down and  
releases gases.

You can reduce in an electric kiln by introducing anything that will  
burn like wood or gas. Not good for your elements however. There are  
special electric reduction kilns made that use glow bars. Not sure if  
there are any made in North America but I'm sure there are some made  
in China and Japan.

The Hamer dictionary has lots to say about this - quite a large  
section on the different forms of iron we use.

Let me know if you need more - RR

Quoting Daphne Vega <dvega at meca.edu>:

> Hi Ron-
> I have never thought about reducing or oxidizing in a bisque kiln- I am
> curious why it would make a difference?
> Thanks-
> Daphne
> On Sat, Jul 6, 2019 at 1:53 PM <ronroy at ca.inter.net> wrote:
>> Just a word of warning - I'm not sure downfiring would re oxidize any
>> iron that would have been reduced in a fast fired bisque. I wish
>> someone would do some experiments to see if that is possible.
>> My advise for iron bearing clay would be to fire slow enough with
>> enough excess oxygen present to burn off any organics present.
>> Keep in mind that different batches of clay in any clay body can vary
>> in carbon content. The best idea is to design your bisque firings to
>> make sure all carbonaceous content (usually coal) is eliminated.
>> Well maintained and working kiln vents and ducts are a good way to do
>> this and save your lungs and elements at the same time.
>> RR
>> Quoting mel jacobson <melpots at visi.com>:
>> > dead on david...pin holes almost always come from
>> > bad bisque firing.
>> >
>> > most know i fire for speed and perfect body development.
>> > i take everything ron says to be true.
>> > so...i fast fire bisque and i hold the kiln at about 1500 on the cooling
>> > cycle for about 30-50 minutes.
>> > all the junk comes out.
>> >
>> > ron suggested that i run experiments in down firing, or holding
>> > bisque firing.  it seems to work well.
>> >
>> > in the past i really fired slow and long for bisque.
>> > in some ways. i think i get the same results by down firing, or
>> > holding.
>> >
>> > others should try and report back.
>> > my bisque ware does not `blow'd up`.
>> > i fire dry pots.
>> > mel
>> >
>> > --
>> > Mel's Website:  www.melpots.com
>> > http://www.melpots.com/CLAYART.HTML
>> >
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>> Ron Roy
>> ronroy at ca.inter.net
>> Web page ronroy.net
> --
> www.dvegadesigns.com
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Ron Roy
ronroy at ca.inter.net
Web page ronroy.net

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