Weird Question On Refiring Bisque

[SkitzoidLady]

Last night my daughter and I did a bisque firing in the kiln at cone 04.  I've had some pieces shatter that last couple times we've bisque fired, and I have no clue why.  Yesterday's firing went great except for a kitchen utensil crock.  I was pretty proud of that piece because I just started playing with making vases and such from slab work.  Every other piece came out great, except for that one.  I made a matching spoon rest, so was pretty disappointed when the bottom shattered out of the utility crock.  

 

So here are my questions. 

 

1.  Can I try to attach green ware to the bottom of that piece and refire it?  Is it even worth it?  I'd hate to ditch it as I spent a lot of time on it, plus the cost of the clay factor.  If fixing it is a pipe dream, I'll move on.

 

2.  I am stumped as to why my pieces are shattering.  It's not every piece.  In fact, our kiln was filled to the gills yesterday, and that was the only piece that shattered.  I've tried moving thicker pieces to the center of the kiln.  I thought maybe it's the clay, but I'm using different clays.  It doesn't matter if the kiln is super full or half empty.  The pieces have been completely dried, and I always bag them and dry slowly.  Anyone have an idea?

 

Please recognize that I'm new to this stuff, so if my questions sound dumb, it's because I'm a greenhorn.

 

 

[Mark C.]

You blew the bottom out of that piece-it was to wet (not dry) 

You can dry the work more or slow the bisque fire down more (slower)-I suggest a slower bisque

Those hand built thick pieces need to be fired slowly.

Its not worth fussing with a new bottom just make another new one.

[Roberta12]

I have been firing my own kiln for almost 5 years now.  I have heard and heeded the warnings about "too wet to fire" however, there was this one mug a couple of weeks ago.....I really wanted to see how the underglaze worked on it, so I put it on the top shelf.....knowing I could be pushing the limits a bit..... But hey! it's stoneware, very porous....it will be fine!!!......even put a preheat on it.....That sucker exploded like none other!!!  I would post a picture but I can't figure out how to get my pics from my phone to this forum.....I had pieces and dust alllllll through the entire kiln.  I agree with Mark....too wet. 

 

Roberta

[bciskepottery]

Agree that the pattern suggests the clay was still wet when fired.  It may have been dry on the surface, but not completely.  It also looks like the bottom may have been thicker than the side walls, so you might have had a combination of uneven drying and uneven expansion/shrinking of the clay due to variation in thickness. 

 

For your next bisque, consider holding the temperature at 200F for one hour; that temperature is just below boiling point and an hour hold will allow any moisture to be released as steam.  Once the steam is released, the kiln can start its temperature climb. 

[MatthewV]

Many tips on firing work that is too wet:

 

1. candle, preheat or hold (all words for the same thing) at 100°C (~200°F) for 3-6 hours. Or if you have something truly wet, 8-12 hours.

2. put thicker pieces anywhere but the bottom shelf as much as possible.

3. put thicker/heavy bottom pieces upside-down

 

---

Now you can still finish a broken piece. If and only if gravity is working with you. Just put the two pieces together carefully with glaze between them. In the glaze firing, the glass will fuse them together forever. Here is an example:

 

[Min]

Put a piece of glass at right angles to a peephole after candling for X number of hours, dependant on thickness / dampness, if you see any steam then you still have too much water in the pots to start increasing the temp and need to candle longer.

 

If you hold a pot to your cheek and it feels cool chances are it's still not dry. If you have another pot (from the same room) that you know for sure is dry, like it's been there for ages, put that to your cheek then the pot in question and it's easy to feel if it's cooler.

[GiselleNo5]

I have seen many explosions due to moisture and that looks like what you've got. My friend had two teapots she spent hours on, looked just like that. Not long enough drying time.

 

I think if you can take MatthewV's suggestion you'll be fine as it's a utensil holder and won't be filled with liquid.

[oldlady]

agree with everyone else, too thick and too wet.  have seen some classes recently where everyone was making things so thickly that i wonder why they did it.  someone was trying to make a vase from 2 slabs laid over a pipe and joining the "wings".  each slab was 1/2 inch thick.  funny thing, it was in the kiln room broken about 4 days later.  much too soon for all that thickness.

 

1/4 inch is thick enough for nearly everything.  any thicker and the clay will not take a shape easily.  bending clay around a support is easier and the piece will dry more evenly if it is thinner.

 

the next one will be much easier.  and after you have made 6 of them, it will be a cinch.  when you are at the early learning stage, you need to realize you are learning a skill, not making a product.  it is really hard to get that idea right when it takes so much work just to make one item.

[SkitzoidLady]

This is great information.  This piece dried for 2 days under a bag, then another 5 days in our garage.  We've had some warm weather, so I was sure it was dry enough.  Maybe with thicker pieces, I need to go to 2 weeks total.Thanks to everyone who responded. 

 

One of our major issues is that we have an older kiln that we are trying to get used to.  We have no programming like the newer kilns.  What blows my mind is that we had other pieces in the kiln that were the same thickness as this piece that did fine. ~shrug~

[TJR]

Touch your piece to your cheek. If it feels cool, there is moisture inside.

For bisquing I fire my electric kiln on low for two hours, then all switches up to medium for one hour, then all switches on full.

Bisque should take approx 8 hours.

Try rolling your slabs on sticks. Your cylinder looks pretty uneven.

Great decoration. Sorry for your loss.

TJR.

[Benzine]

I will add slightly what TJR said.  "Cool" is a relative term.  I say, if a piece is still wet, it will feed cold.  Even thinner wares, that have been drying weeks feel cool to me.

[Marcia Selsor]

I agree with the others, too.

It definitely looks like it was still damp.

 

You should dry thick pieces upside down or on sticks to let the bottom dry.

ou can also fire thick pieces raised on cols to let the steam scape.

It definitely looks like it was still damp.

 

Marcia

[glazenerd]

Will add two thoughts:

1. After a couple of days, flip the piece over so air can dry the bottom of the piece.

2. If I have pieces I know might be a tad wet, I hold at 200F for a few hours: then do the mirror test. I keep holding until it passes the mirror test. When the moisture no longer fogs the mirror: then I proceed with the bisque firing.

Nerd