[Clayart] bisque crumbs

via Clayart clayart at lists.clayartworld.com
Tue Aug 5 17:01:27 EDT 2014

Hi Robert,

I used to work in porcelain. I found that because of the high percent  
of silica in porcelain bodies - larger pieces would crack in the  
bisque. Those cracks were the kind you could not easily see. They  
showed up when glazing. They happen during the quartz inversion at  
573C - the part of the pot in contact with the shelf would stay hotter  
longer and go through the inversion later than the rim. The rim  
(especially on large bowls and platters) would cool first and go  
through the inversion first and crack. The crack would be open at that  
point but when the rest of the pot went through the inversion the  
crack would close up.

Bisqueing higher helps - keeping those types of pots in the upper half  
of the kiln (cools slower) and/or firing down through the quartz  
inversions when all else fails are solutions.

Nothing at all to do with crumbs but - they can come from signing pots  
and not house keeping afterwards.


Quoting Robert Harris via Clayart <clayart at lists.clayartworld.com>:

> Are these crumbs little bits of trimmings that never got brushed off, or is
> your bisque just very friable, or are they large bits of grog? What temp do
> you bisque to? I've never come across bisque fragments. I bisque to 09. I
> know some people only go to 012 (thich I can seen having this problem),
> others go to 04.
> The only real difference it makes is
> 1) Fragility of the bisque.
> 2) Carbon burn out (if you have a very dirty clay full of organics, bisque
> higher or put in a soak.) I mostly do porcelain so I ignore this.
> 3) How much water the bisque will absorb (and how fast). This is very
> important for determining glaze thickness, but if you need to bisque
> higher, just use a thicker glaze slop, or dip for longer.
> Basically unless you're already firing to 04 I would try bisquing one or
> two cones higher and see how that works.
> Worst comes to the worst, buy one of those fancy sieves with rotating
> brushes and resieve every so often. With a decent sieve it'll only take
> about 5 minutes - worth it for a big platter etc.
> Robert
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Ron Roy
ronroy at ca.inter.net

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