[Clayart] pin holes

ronroy at ca.inter.net ronroy at ca.inter.net
Wed Jul 24 12:17:39 EDT 2019

Hi Daphne,

If the iron in a clay gets reduced in a bisque firing it will remain  
in that state to cause problems in a glaze firing. If that firing is  
in oxidation or reduction the FeO will over flux the clay either in  
localized areas (bits of iron from fire clay) or overall from the  
finer iron in darker clays.

The reason the reduced iron from a dirty bisque firing make such a  
difference is  because it's there working during both firings. Hamer  
says that FeO starts to flux around 900C and is almost impossible to  
re-oxidize after that.

I bisque fire at cone 04 - always have - we now start reduction at  
cone 011 or 012 because that works for Liz's carbon trap glaze.

When I was firing just my work I started reduction around 1050C (cone  
04) and had even reduction throughout the kiln, If I started too late  
(1100C) the c10 clay was too vitrified to get reduced. That would be  
dependent on the clay body and whatever reading your pyrometer was  

It seems obvious to me that FE2O3 can give up it's extra O2 at any  
temperature over 900C as long as the carbon monoxide can get too it.  
That is until it's involved in the melt or surrounded by anything  

You might decide to do an experiment and put some red iron oxide in  
different firings and see what happens. Sounds like there is an  
article there somewhere.


Quoting Daphne Vega <dvega at meca.edu>:

> Thank you Snail and Ron- and it was great seeing you both at NCECA! I've
> know about escaping gasses and I think that is in line with what Snail is
> saying, that these solids do not get a chance to become gas in the bisque
> if it is not a properly oxidized environment, getting released in the final
> firing causing glaze problems (which are really clay body problems)
> Ron- you are saying that the unintended reduction actually causes uneven
> maturity all over the piece (around the iron) and I get how that makes
> pinholes and bloating- so why would it matter if you could reoxidize, isn't
> the damage already done?
> My thinking has been that I bisque to a lower temp than when I body reduce
> so I'm in a safe zone (because i'm not reaching the temp needed for
> reduction)- can the Fe2O3 give up its O2 at any temp? (Am I overthinking
> this?)
> Thanks-
> Daphne
> On Mon, Jul 22, 2019 at 9:55 AM Snail Scott <claywork at flying-snail.com>
> wrote:
>> > On Jul 21, 2019, at 12:54 PM, Daphne Vega <dvega at meca.edu> wrote:
>> > ...reducing or oxidizing in a bisque kiln- I am
>> > curious why it would make a difference?
>> One important difference is that impurities like carbon need sufficient
>> oxygen to combine with them, in order to ?burn away? in the bisque process.
>> When this happens the carbon becomes carbon monoxide and carbon dioxide,
>> a.k.a. gases, not solids, so that it can leave the clay. A kiln atmosphere
>> starved of oxygen (reducing) will not have sufficient free oxygen to permit
>> this to happen, and carbon will remain trapped within the clay as it
>> matures.
>>          -Snail
> --
> www.dvegadesigns.com
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Ron Roy
ronroy at ca.inter.net
Web page ronroy.net

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