William Q Hynes on tue 2 jun 98
Greetings Clayarters! I have a question/problem for you rakuers out
there. A while back while giving a demo/workshop here at school, a large
quantity of our raku pieces blew up while firing. We take the kiln to
1850 F or until the glaze reaches maturity. The pieces were average size
bowls, some boxes and some smallish plaques. Around 1790-1800 was when we
heard the "boom" explosions, one after another, but for every four that
blew about one or two didn't. We have two theories, but please, if you
have one let me know.
Theory #1: Kneaded and wedged improperly. This is
my own theory (against all the others'....worth a shot to see what ya'll
think)... is it possible when improperly kneading to get a nice sized air
pocket which will blow during the firing? I am negligent in forgetting to
tell you the demo was with little kids (5th grade)...To me, this makes
sense.
Theory #2: Some of the pieces were just glazed and then only allowed a
small amount of time to "preheat" in a bisque kiln on low (that's how we
get ready to fire here at Ship. U.) My only question is is that does the
glaze HAVE to be perfectly dried before firing. Would a mostly dried but
slightly damp glaze cause so many pieces to blow up?
So what do you guys think? Any help you could send would be tremendously
appreciated. Thanks all in advanced. Feel free to e-mail me at the below
address or to the list itself.
Thanks everyone!!-----cookie
(e-mail at wqhyne@ark.ship.edu)
Susan Goldstein on wed 3 jun 98
Its a given that if the glaze is damp it blows. As a rule of thumb always
glaze the day before firing.
Susan
Bob Wicks on wed 3 jun 98
In a message dated 6/2/98 9:22:57 AM Eastern Daylight Time,
wqhyne@ark.ship.edu writes:
<< Theory #1: Kneaded and wedged improperly. This is
my own theory (against all the others'....worth a shot to see what ya'll
think)... is it possible when improperly kneading to get a nice sized air
pocket which will blow during the firing? I am negligent in forgetting to
tell you the demo was with little kids (5th grade)...To me, this makes
sense. >>
Bill:
First of all let me tell you that air pockets do not cause the posts to blow
up.This is a misconception that many potters have. All that air pockets do is
to cause the clay to go off center when throwing. Steam is what causes pots
to blow up, not air. Have you ever heard of an air engine? Or better yet a
stream engine. Steam generates thousands of pounds of pressure and if it
cannot be released quickly enough it blows like a hand grenade.
Theory #2: Some of the pieces were just glazed and then only allowed a
small amount of time to "preheat" in a bisque kiln on low (that's how we
get ready to fire here at Ship. U.) My only question is that does the
glaze HAVE to be perfectly dried before firing. Would a mostly dried but
slightly damp glaze cause so many pieces to blow up?
Some clays that do not have sufficient grogg or coarse materials such as sand
will not have the porosity to allow the steam to escape properly. If you fire
too rapidly the moisture in the deepest part of the clay does not escape and
the glaze on the outside melts and prevents the steam from getting out. You
will invaribly have an explosion every time when this condition exists. It's
just like putting a can of beans in the oven without opening the can before
you turn the heat on.
Suggestion: Always do a slow bisque fire first using clay with at least 10 to
20 % grogg. Always dry freshly glazed projects slowly and for consistent
results pre heat the projects before placing them in the final glaze fire. If
you follow these suggestions I'm confident you will have more success.
Good luck.
Bob
Ron Roy on wed 3 jun 98
Hey Cookie,
You are going to get a lot of answers on this one - and they are all gonna
say - you can't raku fire wet pots - period. They have to be perfectly dry
- I can remember my raku days - about 25 years ago - having to leave ware
on a hot kiln for hours to make sure they were sans H2O.
I am assuming they were bisqued - thats elementary.
>----------------------------Original message----------------------------
>Greetings Clayarters! I have a question/problem for you rakuers out
>there. A while back while giving a demo/workshop here at school, a large
>quantity of our raku pieces blew up while firing. We take the kiln to
>1850 F or until the glaze reaches maturity. The pieces were average size
>bowls, some boxes and some smallish plaques. Around 1790-1800 was when we
>heard the "boom" explosions, one after another, but for every four that
>blew about one or two didn't. We have two theories, but please, if you
>have one let me know.
>Theory #2: Some of the pieces were just glazed and then only allowed a
>small amount of time to "preheat" in a bisque kiln on low (that's how we
>get ready to fire here at Ship. U.) My only question is is that does the
>glaze HAVE to be perfectly dried before firing. Would a mostly dried but
>slightly damp glaze cause so many pieces to blow up?
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Susan Goldstein on thu 4 jun 98
I don't think you need to slow up the bisque time. If the pots are making it
through this they are coming out water free and steam will not be there to
cause them to blow. It the water from the glaze that they are absorbing and
this is causing the steam problem. This it is the timing of the glaze and
firing that must be changed.
| |
|
