bubble in my slab pottery ... feeling so frustrated

[Aminah Bradford]

Hello! i must have had beginners luck for my first few firings but lately everything I take out of the kiln has bubbles bettwen the size of 3/4 cm to 1.5 inches wide. I'm getting explosions during the bisque firing and then during the glaze fire all these bubbles show up

I am using the clay straight out of the bag. I cut a slab about an inch thick and slide it through my slab roller.  No sign of bubbles and yet after the glaze fire..... bubbles!

I am also taking scraps and wedding and then slab rolling and bubbles show up. So i cut and press and cut and press over and over til no sign of bubbles, then in the kiln, explosions and bubbles. 

I am self taught and feeling discouraged. Any tips?

Many thanks! 

Aminah

[neilestrick]

Air bubbles do not blow up. Moisture turning to steam is what blows up pots. So if you pot isn't totally dry before going into the kiln or if you're firing too quickly and the last bit of remaining moisture is turning to steam before it evaporates, then you get explosions. So either longer drying times, a preheat in the kiln, or a firing schedule that goes slower in the first few hundred degrees will solve that problem.

If you're getting bubbles in the clay, not in the glaze, after the glaze firing, we call that bloating, and it's usually from firing the clay too hot. What cone are you firing to, and what type of clay are you using?

[Aminah Bradford]

Thanks very much, Neil, for this helpful response. I have a small L&L Kiln. I am firing to cone 6. The clay is standard 370 stoneware. The bloats are only happening on slabware, not thrown pieces. 

Aminah

 

[Callie Beller Diesel]

Standard doesn’t seem to have a 370. I know that’s a Plainsman number, but I don’t think much of that makes its way down to Durham. 

[Min]

@Aminah Bradford, could you post a picture of the bubbles?  If it's a pyroplastic clay and there is even a tiny air bubble it can bloat, especially if the clay is over fired or on refires.

[Aminah Bradford]

Thanks for your thoughts on this. I must have thrown away the box! I know it's a cone six and here are some photos. I will try and bird-dog what clay it is.bubble 11.pdf

bubble 10.pdf

[Min]

Bloating for sure. Probably caused by overfiring the clay but it could also be from an improper bisque for the claybody. Using witness cones to verify the glaze firing? Also, what is your bisque schedule?

[Denice]

Slab work is usually thicker than thrown pieces,  when ever you have explosions it comes from moisture.   If you live in a humid area your slabs may not be totally dry,  sometimes Kansas can be humid for days.  Often  setting a piece in the sun would finish drying it,   if my kitchen oven was big enough I would heat it at the lowest temperature of the oven.  If it is large work you can candle it over night in your kiln.   Candling on a manual kiln is putting one control on low overnight and then continue with the firing in the morning.   Denice

[Aminah Bradford]

Thanks all! This is really helpful. Yes, I am in high humidity area. My firing schedule is I set it to 06 for bisque and to 6 for the glaze fire. I am learning so much from you all.

[Aminah Bradford]

So I fired at the slowest firing schedule that is preprogramed on my cress kiln both for the bisque and the glaze firing and didn't get the large bubbles but still tons of the 1-cm wide ones.  It's weird that it's only on the slab pieces and not on the thrown pieces that aren't big wide flat things.  so bizare.

Aminah

 

[Hulk]

Looks like bloating - here's a clip from the image you posted earlier, so others may easily see:

 

Looks like a black clay, which can be especially prone to bloating. I found that extra long/thorough bisque fire - I just parked it at 1500F for an hour on the way up to cone 04, and more time on the way down as well - helped, also careful to glaze fire to cone 5 (only, no more), and keep the ware on the thin side, for the thick-ish parts were where I found the dreaded bloats.

Air/oxygen flow would be important for the bisque; my kiln has a powered vent that pulls a small stream of air out of the kiln, hence ambient air is pulled in through the various cracks.

Bloat, what is it?

Bloating (digitalfire.com)

Could be the slab ware is more prone to the dreaded bloat on account of the clay's exposure to air is limited on the shelf side.
Could also be that the slab ware is a bit thicker? My limited black clay experience indicates that thicker is way more bloat prone.

Bisque it thoroughly!

[Callie Beller Diesel]

Another thing that I think is at work is that kiln shelves act as a heat sink. The slab pieces that are flat against the kiln shelf with no foot ring are getting more heat work than pieces that are more vertical, or have feet. With the size of those bloats though, I’d be concerned that even the upright pieces might be on the verge of doing it as well.

You mentioned that you’re using preprogrammed cycles. Have you checked the recorded end temperature against cone packs or self supporting cones? Could be your thermocouple is reading a bit cool.

[Bill Kielb]

20 hours ago, Aminah Bradford said:

So I fired at the slowest firing schedule that is preprogramed on my cress kiln both for the bisque and the glaze firing and didn't get the large bubbles but still tons of the 1-cm wide ones.  It's weird that it's only on the slab pieces and not on the thrown pieces that aren't big wide flat things.  so bizare.

Definitely check with cones to find the peak temperature. The shelf will be a consistent radiator so if your kiln is going slower than 100 degrees per hour in the last 200 f of the firing it’s going to creep up in time and overall heatwork. As to overall slow, some of these  bodies come with a suggested longer than normal custom bisque cycle just to ensure they will perform reasonably and burn out all organics as practical.

So in general for bisque, very dependent on more time to fire and for glaze often very sensitive to higher heatwork. If it cone six, aI might be inclined to test cone five, then cone 5 with a small hold (10 minute)  to get the cone to drop but keep peak temperatures down.