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Bisque v glost


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Hi … apologies for the newbie question but I’m a long time lurker and these forums are amazing so thank you! 
anyway … taking advice from a local pottery where I’ve taken classes I’ve been bisque firing  my terracotta to 1140 (I’m in the uk ) and then my glaze firing to 1040 (which is within the range for the glaze) but I’ve been reading that the bisque should be lower than the glaze temp. Confused .., can anyone give me some advice. I’ve had no issues doing it like this … 
TIA 

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@NovicePotter, I think I can shed a little light on your question, and others I am sure will add to it. Let me welcome you to the forum first off, hope you find the answers you need here.

I have mostly fired my pieces with a lower bisque than the glaze temperature. My first reason for doing so is the obvious one, and the one you spoke of . . . it was what I was advised/taught to do.  Not that your advice is necessarily wrong, but let me explain. Bisque firing to a lower temperature than the glaze will remove organic elements from the clay, dry the clay and harden it, and have some effect on the total shrinkage of the clay body. This clay body will not be as dense as the higher temp used for the glaze firing, thus the clay will absorb some of the glaze into the surface of the clay. This absorption promotes a better bond between the clay body and the glaze in the firing. You could say that the particles get locked into the clay body with the further shrinkage that goes on in the glaze firing.  

So is what you were advised wrong? No not really, but with provisions.  Commercial ware is often fired to a higher temp in the bisque for a few reasons:

  • Higher bisque firing means the ware is fully vitrified and less fragile to handle in the glazing than lower fired bisque temps
  • Glazes are often machine sprayed on to the ware, and absorption is not needed for the glaze to stick to the body as much and often the glazes used have a binder to help glue them to the ware
  • Firing the ware at higher temps for bisque means tighter stacking, more ware in the firing, and thus more cost efficient.

Finally, if you are not having problems with your ware, and are using the clay body and the glazes your local pottery is using you are probably doing fine. If however, you want to go your own road in the future, you may need to investigate clay bodies, home mixed glazes, and other firing alternatives.  Many of us do not like to glaze ware that is fired to the glaze temp because it is hard to get the glaze to stick to the surface without additives, and it limits much of the decoration techniques many of us use.

All IMHO,

best,

Pres

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I'm of the same opinion as the potter in the UK you had advice from if 2 conditions apply.

1) If the clay is still porous enough to glaze easily/well after the 1140C bisque (approx cone 1 -2). If it is a typical earthenware with fairly high porosity then glazing to a lower temp after a hotter bisque is often done to get the clay as strong and mature as possible without overfiring the glaze. 

2) - your glazes are best when fired to the lower temperature / cone of 1040C (approx 05 - 04) versus firing them to 1140C

 

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Posted (edited)

Thanks all … really interesting. It seems to work so far but it’s very early days … I’m just loving the learning journey.

As a I have no frame of reference when you all say ‘glazes well’ … how would I know if it didn’t?  Sorry if that sounds a dumb question!

Edited by NovicePotter
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2 minutes ago, NovicePotter said:

As a I have no frame of reference when you all say ‘glazes well’ … how would I know if it didn’t?  Sorry if that sounds a dumb question!

If the glaze dries relatively quickly and covers well. If the bisque is fired too hot and the clay loses its porosity, then it can't absorb the water in the glaze and you can't get a good application.

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4 hours ago, NovicePotter said:

As a I have no frame of reference when you all say ‘glazes well’ … how would I know if it didn’t?

Just to add, much of this will end up to be surface observation. Does the glaze melt fully and for instance does it appear uniform and thick enough for 100% coverage. Sort of subjective but also can be obvious for glazes that are applied too thin and end up rough to the touch. Glazes not fully melted that should be gloss but are more matte.

Observing the finished product often reveals whether the application thickness was appropriate for the glaze as well as whether the firing schedule was appropriate.

Test tiles created while varying coverage or schedule usually reveal a best or minimum application, best schedule etc….. absent any glaze defects like crazing etc… generally visual observation and correlation to how the ware was prepared.

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