Different colour inner/outer bisque fired pot

[Annamarsh]

My students pots which have a 'enclosed' shape after bisque firing are white inside rather than its usual and outer pink shade. The pots are made from school buff clay and were fired to 1000 degrees.

The firing was reasonably compact, one source has said that the differing colours could have been caused by poor circulation in the kiln caused by overcrowding.

Could there be any other reason for the different bisque colours? and will it affect the pots ability to hold glaze?

[Hulk]

Hi Anna!

My guess, the differently coloured pots are thick/thicker - takes longer to reach peak temp, longer to burn out organics, etc.

As for picking up a glaze layer, easy enough to test for difference in absorption. Given that a sufficiently thick layer of glaze is applied, reaching full maturity - the clay, that is - in the glaze fire may be at issue.

The more open pots' inside can "see" the elements better - they get the radiant heat more directly than the enclosed shapes.

Back to thickness, perhaps more time at the critical temps (750, 1050, 1500F) and slow ramp to peak, giving time for thicker parts.

Back to thickness II, if all the ware in a load are not about the same thickness, loading so the thicker ones are in the hotter areas of the kiln should help.

Some air movement - supplying oxygen - is important for thorough bisque.

Edited March 15, 2021 by Hulk
bean -> be an-> be at !

[Chilly]

What @Hulk said!

Front/thicker pot is probably underfired.  

[meital]

I had the same thing happen as well. Would you suggest to bisque fire it again? Or would you glaze the pots ?

Thanks 

Meital

[Callie Beller Diesel]

Hi @meitaland welcome to the forum!

It depends a little on your circumstances. If you’re dealing with student work or sculptural things, you can just go ahead and glaze it, and make sure your next bisque kiln isn’t so tightly packed/pieces can vent organic gasses properly.

If you’re making functional work for sale to the public, I’d want to confirm the severity of the problem. If the bisque was just a little more crowded than usual, you’re again probably fine, but if it’s black cored, I’d test a piece or 2 to make sure your glazes will still fit. Black coring can affect thermal expansion rates according to digitalfire. If the ware is black cored, re-bisquing very likely won’t fix it.