Firing Questions

EarthgirlOK

Not looking for trade secrets, really, but I have a couple (I think) questions. I'm doing my throwing and glazing for now in a couple of by-the-month studios. I have a huge learning curve on firing and glazing, and I'm slowly getting the throwing down. I'll post pics of my first attempts, meager as they are when I get the film to do it.

My question has to do with firing to a temperature/cone to be weather hardy. No one in the abovementioned studios seems to know this... I think I've read that outdoor bonsai pots have to be fired three times.

1. Is this the standard?
2. Does this include the bisk, glaze, and an "extra?" firing?
3. We seem to go by "cones" around here. At what "cone" are these firings generally done?

4. ("extra credit") Has anybody ever tried raku for bonsai pots? I understand that raku is low fire. But can it be done on an already fired pot? Forgive all the questions and my ignorance, but I truly am ignorant in this area.

I have some ideas that I can't wait to try, but if they WILL NOT WORK for those who have more knowledge than I then I'll save my energy for doing stuff that does work...

Thank you in advance for your help.

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Bonsainut

Dear earth girl,
So glad to hear of your clay play. Let me take a stab at these,,,

I think I've read that outdoor bonsai pots have to be fired three times.

1. Is this the standard?

They must be charging for each firing,eh! Oakay, the thing is not the total number of times but the top temperature that does the maturing of the clay. Standard procedure is twice fired. First to bisque, approx 1830f, cone 06 then second firing after glaze application to the maturation temp of the clay.


2. Does this include the bisk, glaze, and an "extra?" firing?


Sure you can. Some folks do multiple glaze firings --they add lusters, oxides, engobes, salt ,soda ,ash ,sand blast, snap crackle pop.


3. We seem to go by "cones" around here. At what "cone" are these firings generally done?


Basically up to cone 1 or 2 is considered low fired. Ware at the upper end of these temps. can be very durable though and very closly as vitrified as higher fired ceramics. Cones 3 thru 6 or 7 is mid fired , stone ware temps--tough,waterproof, these are the ones for us! 8 thru 10 and above high fired stone ware and porcelain.


4. ("extra credit") Has anybody ever tried raku for bonsai pots? I understand that raku is low fire. But can it be done on an already fired pot? Forgive all the questions and my ignorance, but I truly am ignorant in this area.


oooo check out Horst's site ..Im looking for you. he has done some high fired raku. Gimme a minute. this is good stuff earth girl, Im excited...

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Jay

Another point is that the clay body you use has a firing temp for it. There are clay bodies that are for firing to cone 10 and then there are clay bodies that are for firing to cone 7 etc. If the clay comes in a box, it will say this on the outside.


As for number of firings..... if you are going to go unglazed and you fire once to cone 10, it will be as hard as a pot that first goes through a bisque fire.

All of the above was info I learned in a studio in Sugarloaf NY


Jay

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muddslinger

Hi Earth Girl,
I agree with Bonsainut's answers to your first three questions, but I may have a little input on the raku thing.
This is something I've been wanting to do for a long time. I've tried with my regular raku clay body which can be fired to cone 7. The few experiments I did cracked. The conflict here is that to have a pot that's water/frost proof, you want it to be vitreous and hard. Conversely, to survive the thermal shock of raku firing generally requires a porous, soft clay body.
This usually means a lot of grog, although I've had some success with a smooth, talc based clay raku firing at cone 05. A clay that is porous when fired at cone 06-05 isn't so porous at cone 6-7. Keeping these things in mind, you might be able to pull it off if you brought the temp up slowly in the raku firing and allowed the pots to cool for a while in the reduction can.
My typical raku firing goes from 0 to 1800f or so in less than 45 min. I allow it to cool to 1500f before opening it and moving the pots to the reduction can where they spend about 15 to 25 minutes. Then they go into the water. If you were to take a couple hours or more to reach temperature, left them in the reduction can longer and skipped the water step, you would reduce the thermal shock quite a bit. You may also loose some of the reduction effects by allowing it to cool slowly, there by giving it a chance to re-oxidize.
I'd be interested to hear if you try any of these ideas. I probably won't be able to raku again until June.
Hope this helps.
Best,
Kevin

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stonemonkey

Hi Earthgirl

I bet you will get loads of answers on this thread so I shall put down my thoughts. Hope I don't confuse the issue to much.

As Nut said the usual is to fire twice. Once to Bisque at around 1000 c then to the final maturing temp. This may vary depending on the clay you are using and also the glaze. In general all Bonsai potters fire to Stoneware, this is between 1220 c to 1300 c. Thus using a Stoneware clay body and stoneware glazes to achieve this. Sorry us Brits use Celcius and not Farenheit. By firing to Stoneware makes the pots frost resistant. All clay bodies have a maturing tempreture.

The use of cones is common practise. They are used in conjuction with the pyrometer to measure the temprature of the kiln. The melting points of cones vary depending on the cone number. Stoneware cones in the UK are between 6 and 10 that is 1220 c to 1300 c. BUT... the tempreature that the cone bends at is also determined by the rate of tempreature increase. i.e Cone 6 at 150 c per hour will bend at 1220 c approx. If fired at 50c per hour it will bend at a bit lower temp approx 1207 c.

Lastly RAKU. This is a low temp firing process, around 1000 c. It is quite a mad process in that the pots are removed from the kiln at 1000 c and dumped in containers of combustable materials causing a localised reduction atmosphere. While as lovely as RAKU is, and the glazes are amazing, because they are fired to a low tempreture they are not quite frost resistant.

I hope this helps in your quest

Kind Regards

Andy

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EarthgirlOK

Thanks to all of you who have responded to my quest so far! I am truly blessed to have found this site and the people who contribute to it!

I'll be taking this information with me next time I go to the studio (it's about 60 miles away) and will discuss it with those who are responsible for firing, etc.

Thanks for the info! This site is wonderful!

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Sonlight NurseryGreen Country Bonsai Society

EarthgirlOK

oooo check out Horst's site ..Im looking for you. he has done some high fired raku. Gimme a minute. this is good stuff earth girl, Im excited...[/QUOTE]


WOW! Horst's website is mouthwatering!!! I want 'em all!

Thanks for the tip!

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Sonlight NurseryGreen Country Bonsai Society

Bonsainut

Yeah but, on One of the forums he describes a high fired raku, to everyone's dismay. I'm still searching...

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Keep growing,---'Nut



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EarthgirlOK

Keep lookin...

We're all waiting (I know I am) eagerly...

(of course, like the typical American, I'm barely verbal, let alone speak other languages...shame on us...) I was able to decipher some of the text with a translator.

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Sonlight NurseryGreen Country Bonsai Society