"busted" Busts

[Laura DeBoer]

My sculpture class made busts based on Philippe Faraut's DVD, and every single one cracked apart in the firing. They were definitely dry--they air-dried for a week, then soaked at 150 degrees in the kiln for 12 hours, vented, before firing slowly (about 21 hours) to Cone 05. What went wrong? I have never really fired any sculptures, just high school art projects and my own functional pottery, never anything as thick as these busts. They took over a week to make--did we work on them too long, perhaps letting them get too dry between work sessions? Does anyone have any advice for what I could do next time?

[Chris Campbell]

My number one suspicion is they were not dry.

 

A week is not much time to dry a thick sculpture.

Candling them won't totally dry them either.

It takes time and patience because you want them to dry evenly and

thoroughly.

 

I suspect your outsides were dry while the insides were not and that set

up a lot of stress on the work.

 

To test for dryness ...

You should be able to rest your hand anywhere on the piece and

it should not feel cool ...

 

I used to wonder why people gave directions like "dry slowly' ... heck, what does that mean?

A day, a week, a month?

As in most everything in pottery ... it depends.

Could be a day, a week or a month.

[Paul]

I have to agree with Chris, particularly since every piece cracked. Sculpture can be deceptive in terms of varying thicknesses, which can cause stress fractures...but moisture is the killer of all killers in bisque firing.

[wayne]

Hi Laura,

I have never had a problem firing my sculptures. I do however let the work dry five weeks before I even consider firing. Five weeks is not a magic number it just seems to work well for me. I then fire on low (vented), continue on medium (closed kiln) and finish on high to cone 05. The entire process takes about ten hours. I can give you my entire proceedure if you like but Chris and Paul are correct, the work is not dry enough.

[Laura DeBoer]

Thanks, everyone! I will definitely let things dry longer next time.

[anthonyfoo]

My sculpture class made busts based on Philippe Faraut's DVD, and every single one cracked apart in the firing. They were definitely dry--they air-dried for a week, then soaked at 150 degrees in the kiln for 12 hours, vented, before firing slowly (about 21 hours) to Cone 05. What went wrong? I have never really fired any sculptures, just high school art projects and my own functional pottery, never anything as thick as these busts. They took over a week to make--did we work on them too long, perhaps letting them get too dry between work sessions? Does anyone have any advice for what I could do next time?

 

 

Hi Laura,

 

This happens a lot to portraiture sculptors. I have a friend who went through the exact same thing. Her piece was the only one that survived the bisque stage from her class. Her saving grace was that her bust was drying on the rack for over 2 months.

 

I agree a week is definitely not enough time for regular clay to dry out completely, especially in the thick areas where it's hard to hollow out. Any moisture remaining is a killer.

 

As a result of her experience, she switched over to using paper clay for all her sculptures and is having a great time with it.

 

Hope this helps.

[Heather-JoplinMO]

Even though the outside looked and felt dry, there was still moisture present inside...enough to create steam and explode outwards destroying the pieces.

Unless you live in a very arid place like the Arizona or Texas, a piece like a bust would need to dry for months to get bone dry because of the humidity in your studio.

Which if you are mopping and wet cleaning like a good teacher would insist on for safety it would be.

Unlike smaller work where you can take almost dry greenware and dry it in a low heat kiln, large pieces like the portrait busts need to be absolutely bone dry before they are fired. They also need to get to bone dry slowly (partially covered with plastic). If dried too quickly the outside will shrink more rapidly than the inside and the surface will crack.

As the clay moves from leather hard to bone dry, the color of the clay will change ... it will become chalky and will not be cool to the touch.

Touch the inside of the piece too where you hollowed it out to test to see if it feels cool. If it does, wait.

Always err on the side of caution when drying large pieces. If in doubt, wait.

 

Another suggestion is to try using P-clay (Paper clay) for your students next portrait bust project.

The paper fibers in the clay body allow the pieces to dry more evenly... they wick moisture from the inside to the outside.

You can also force/speed the drying process without being afraid of the outside cracking.

 

Heather in Joplin