[Clayart] advice on bisque firing

[ronroy at ca.inter.net]

Hi Carol,

Porcelain is problematic because it has so much free silica - which  
goes through a volume change at 573C during cooling. If cooling is too  
fast or bisque not strong enough, rims will go though the contraction  
before the feet
and the rims will crack.

Keeping bowls and plates in the upper part of the bisque firing will  
help because cooling is slower there.

Keeping rims a little thicker will help.

The cracks that happen for that reason are not noticeable and are  
sometimes missed until after a glaze firing. Thats because the cracks  
are open when they happen but close up when the feet go through the  
contraction a little later.

If the problem persists simply fire bisque down through the inversion  
to stop the rims from cooling too fast - say from 700C down to 500C.

So don't fire bisque any lower than C04.

RR


Quoting carol at knighten.org:

> Hi -
>
> I'd like suggestions re a bisque firing.  I like to bisque to C/04, that is
> the only part of the bisque firing where I know what I want to do, or why.
>
> I'm firing an english grolegg porcelain from Tacoma Clayart center.  My ware
> is relatively small utilitarian kitchen ware, dishes, plates, bowls.  Bowls
> are never over 16 inch diameter. Nothing is over 3/8 inch thickness.
>
> Other than getting the ware fully dry before going above water boiling
> temperature, what are the relavant parameters?
>
> Thanks,
> Carol
>
>
>
> --
> ============================
>     Carol Marians
>     (541) 296-4528
>   carol at knighten.org
> http://carol.knighten.org
> ============================
>
>
>



Ron Roy
ronroy at ca.inter.net
Web page ronroy.net