kathy kerner on thu 17 jul 08
Clay artists,
I have been experiementing with paper clay according to Rosette Gault's "Paper Clay" a primer. The first batch I mixed was one roll of toilet paper mixed with portions a dry buffII clay. I made large(1.5') diameter sunfaces for raku. Both had small hair line cracks during the raku process(not unusual because of thermal expansion) so I slowed down the firing process. My next batch that I mixed, I increased the toilet paper to two rolls and added a small portion of grog. I am not very exact on my measurements. Three large sunfaces all blew up in the bisque kiln. I don't think that the grog made this happen. The other pieces in the kiln which were buffII from commercial mixed clay did not blow up, so I know that it was not the firing process(I fired slow to cone 03). Could I have gotten that many air bubbles trapped? I had wedged and rolled the clay with a slab roller. Maybe I was too carefree because I had thought that paper clay was nearly indestructible. Because I was hoping for large, flat, light weight, sunfaces, I was experiementing with paper clay. I have been rakuing for about 14 years and build my own raku kilns.
If anyone is looking for fire insulated brick 28k, I have 175 for sale at $3 a piece. I would deliver within 25 miles of the Milwaukee, WI area. Thanks, Kathy
kathy kerner
kkerner@earthlink.net
EarthLink Revolves Around You.
Ivor and Olive Lewis on fri 18 jul 08
Dear Kathy Kerner,
Your tell us about the diameter of your product but not the thickness.
This sounds like a Steam Explosion.
My take is that the additional amount of paper would have put a lot
more water into the clay and this has not been allowed to dry
thoroughly before you started your bisque firing. At 100 deg C, 212 F,
any water remaining would convert to steam almost instantaneously.
One single gram of water will generate more steam than would fit in a
one litre beverage carton. Since the fibres will not have burned away
there is no space for it to occupy . The pressure generated would
make the whole package behave like Dynamite and green clay will
disintegrate, fragmenting like a hand grenade.
Other people may have other views.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
David Martin Hershey on fri 18 jul 08
Hi Kathy,
Ivor is correct. The problem is steam, not air bubbles.
First, I would start with a raku type clay that is resistant to thermal
cracking for making your paper clay. Then use a very slow bisque firing
profile.
I have used this bisque profile to fire paper clay pieces up to 2 inches
thick without problems.
Segment 1: 50 f/hr to 205 f, hold 2 hours
Segment 2: 100 f/hr to 410 f, hold 0
Segment 3: 250 f/hr to 900 f, hold 30 minutes
Segment 4: 110 f/hr to 1472 f, hold 2 hours
Segment 5: 225 f/hr to 1765 f, hold 30 minutes
Segment 6: 180 f/hr to 1945 f, hold 20 minutes
Segment 7: crash cool to 1888 f, hold 40 minutes
This bisque is a hard cone 04 in my kiln. Total time about 15.5 hours.
Adjust segment 5 and 6 temps for your preferred bisque cone.
You don't want to know how I ended up with this schedule... ;^)
If you don't have a digital kiln, I think the most important thing is to
candle at around 200 f for at least 2 to 4 hours, and then fire up very
slowly until above 1100 f.
Best, DMH
David Martin Hershey
DMH Studio + Design
2629 Manhattan Ave #137
Hermosa Beach CA USA 90254-2447
310.379.6890 dmh at dmhstudio dot com
http://www.dmhstudio.com/
Joseph Herbert on sat 19 jul 08
People often blame air bubbles in clay for mishaps. A brief look at the
ideal gas laws shows a different thought.
Stated P1V1/T1 = P2V2/T2
Leaving volume constant, for the moment, the pressure in an air void at 212
degrees F is about 20 psi compared to 15 at 72 degrees. So, without air
leaking out of the void, the pressure increases about 5 psi. At 1000
degrees F the internal pressure is about 37 psi. (increase of 22) Not the
kind of thing that makes for chunks of stuff all over the kiln.
On the other hand, the phase change from water to steam is accompanied by a
volume increase of about 29 times. At 212 degrees the restrained volume
increase of 30 times means the pressure in a void with liquid water in it
goes from about 20 psi to about 600 psi, instantly. Chunks all over the
kiln.
Darn "air" bubbles.
Joe
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG]On Behalf Of David Martin
Hershey
Sent: Friday, July 18, 2008 6:36 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: paper clay blow up
Hi Kathy,
Ivor is correct. The problem is steam, not air bubbles.
First, I would start with a raku type clay that is resistant to thermal
cracking for making your paper clay. Then use a very slow bisque firing
profile.
I have used this bisque profile to fire paper clay pieces up to 2 inches
thick without problems.
Segment 1: 50 f/hr to 205 f, hold 2 hours
Segment 2: 100 f/hr to 410 f, hold 0
Segment 3: 250 f/hr to 900 f, hold 30 minutes
Segment 4: 110 f/hr to 1472 f, hold 2 hours
Segment 5: 225 f/hr to 1765 f, hold 30 minutes
Segment 6: 180 f/hr to 1945 f, hold 20 minutes
Segment 7: crash cool to 1888 f, hold 40 minutes
This bisque is a hard cone 04 in my kiln. Total time about 15.5 hours.
Adjust segment 5 and 6 temps for your preferred bisque cone.
You don't want to know how I ended up with this schedule... ;^)
If you don't have a digital kiln, I think the most important thing is to
candle at around 200 f for at least 2 to 4 hours, and then fire up very
slowly until above 1100 f.
Best, DMH
David Martin Hershey
DMH Studio + Design
2629 Manhattan Ave #137
Hermosa Beach CA USA 90254-2447
310.379.6890 dmh at dmhstudio dot com
http://www.dmhstudio.com/
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