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How to fire terracotta? Can bisque be skipped ?


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#1 Frederik-W

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Posted 02 March 2012 - 07:01 AM

I have ordinary terracotta for hand building and throwing.
The packet says "for best results fire at 1100 Celcius (2012 F). Max temp 1200 Celcius (2192 F).

What is the correct bisque firing temperature?
Any advice on temperature vs time i.e. how should I program the kiln?

What I want to fire:
I made large torso, about 60cm (24 inch) high and 30cm wide, about 1cm (1/2 inch) thick walls but it varies, also large bowls - same thickness.
I want to bisque fire them all, then glaze them and then do the glaze firing. I want to prevent cracking. The items are bone dry.

As a matter of interest for my future work with terracotta:
Is it possible to skip the bisque stage and fire only once, with glaze? i.e. apply the glaze on dry terracotta and then fire all the way.
That will save a lot of time and electricity/energy. It will certainly help a little bit to reduce global warming.
I don't care if the glaze runs etc, I don't want to make intricate patterns with glaze.
Any advice on type of glaze?

Any comments and advice is welcome. Sorry if the questions sound stupid but I want to learn. Thanks.


#2 Marcia Selsor

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Posted 02 March 2012 - 07:21 AM

Terra cotta clays can vary widely for temperature range. I have some for ^2 from Alligator clay. So, it is best to follow the temperature recommended by the manufacturer.
You can once-fire. It is better to glaze greenware in a leather hard stage and add 5% bentonite to your glaze recipe. This helps with the shrinking and fit. You might like terra sigillatta as a finish for sculpture which emphasizes the clay surface.
Marcia

#3 TJR

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Posted 02 March 2012 - 10:38 AM

Frederick-W;
There is no such thing as a stupid question. Your terra cotta can be bisqued at a regular temperature. I bisque at Cone 07 and glaze at Cone 04. I am making Majolica, so I am bisquing first and then glazing. Your pieces sound rather thick. I would fire them with the bottom switch on low and the top spy open. Fire them over night like this. Don't forget to set the timer back 12 hours. Then put the spy plug in and put all switches on low for two hours, and then medium for two, then all on high.
Marcia is right. In order to raw glaze, you need more plastic clay in your glaze, like ball clay or bentonite. Roll out a leather hard test tile. Paint your glaze on and let it dry. If it sticks you are good to go. If it falls off, the clay has shrunk away from the glaze, and you need to add ball clay.
TJR.

#4 neilestrick

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Posted 02 March 2012 - 11:48 AM


As a matter of interest for my future work with terracotta:
Is it possible to skip the bisque stage and fire only once, with glaze? i.e. apply the glaze on dry terracotta and then fire all the way.
That will save a lot of time and electricity/energy. It will certainly help a little bit to reduce global warming.
I don't care if the glaze runs etc, I don't want to make intricate patterns with glaze.
Any advice on type of glaze?


When doing once-firing, it's necessary to go much slower with your firing than you would for a typical bisque or glaze load. The issue is that there is a lot of stuff that burns out of your clay, and it needs to get burned out before your glaze begins to melt, or the gasses will be trapped in the glaze, resulting in bubbles and other nasty things. So you may not be saving much on electricity at all in the end. There can also be issues with glaze fit. Most high fire single-fired glazes are rather high in clay content, like slip glazes. A commercial glaze will not have that characteristic. Adding bentonite, like Marcia said, will help, but it will be very difficult to add the bentonite to a premixed, wet glaze. It tends to clump up into a big gooey mass. Even stick blenders will have a hard time dealing with it. Ideally, it needs to be well mixed into the dry glaze before water is added.

The easiest way to deal with your pieces, and I think the best way to prevent cracking or glaze problems, is to do a really slow bisque firing, glaze them, and do a slow glaze firing. Brush-on commercial glazes will be the simplest glaze option.
Neil Estrick
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Owner, Neil Estrick Gallery, LLC
www.neilestrickgallery.com

neil@neilestrickgallery.com

#5 Frederik-W

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Posted 04 March 2012 - 12:23 AM

Thank you very much for all your advice.
I will experiment a bit and see how things go

regards





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