Firing a Sealed Shape

[Ben xyz]

Apologies before for this newbie question. Have read that it is the trapped water that actually causes an explosion of a clay piece when firing vs trapped air.

Now considering firing an enclosed (hollow) form (5”x7”x4”). Will this be alright or is it important to have a hole somewhere in the form to avoid an explosion. It is a public kiln. Thanks!

[Kelly in AK]

I know a lady who puts hollow pillow like feet on her pots. She said if she doesn’t put a pinhole in them they crack off at the attachment point.

It’s certainly water to steam that causes explosions, but there are other consequences to sealing off a volume of air in clay.

A pinhole in a place that doesn’t get sealed by glaze eliminates those.

 

[Callie Beller Diesel]

I used to make all kinds of hollow knobs on lids. I would put a pinhole in the bottom of them, and it was never a problem to seal it over with glaze, but maybe it’s an issue with larger forms? I think it’s one of those things that if you want to be able to do it, test to find where the limits are with your materials and your own practice. 

[Kelly in AK]

Thank you @Callie Beller Diesel,  perfectly said! That “sealed over by glaze” bit was totally an assumption on my part. I tend to figure if it can go wrong it will and wind up extrapolating in a careful direction. 

Personal, first hand experience in ceramics is what you can count on most. If you did it and it works, you know it works.

 I’m reminded of Josh DeWeese, who told me “You don’t have to score, just use slip and really stick that handle on.” I have some of his pots, use them regularly, the handles don’t pop off. Do I still score attachments? Yeah. Can’t help it. 

[Callie Beller Diesel]

My working theory on why you get so many different instructions about slip/score/water/no water is that it varies by clay body.

When I switched from a white high fire stoneware to a mid fire red, the handle attaching method I’d been using resulted in more cracked handles than I’d ever had in my life! The white one wanted slip and score, but if I do that with my red stoneware, they take forever to grab and the slip shrinks too much and I get those nuisance edge cracks. The red one wants score plus a dab of water only. 

[Min]

2 hours ago, Callie Beller Diesel said:

My working theory on why you get so many different instructions about slip/score/water/no water is that it varies by clay body.

+1 for this!

I think it goes far beyond attaching handles and that is perhaps why it takes so long to get processes nailed down when starting out.  Ask 5 different potters how they throw / slab build / dry / make attachments / which batts work best / bisque fire/ glaze fit / glaze firing and so forth and you could very likely get at least 5 different answers for each  process. Type of clay, working environment (humidity etc), work style, clay thickness, material chemistry etc all come into play. Definitely not a one size fits all scenario for much of what we do.

[Ben xyz]

Thanks for all the responses - appreciate it. Spoke with the studio manager who said it would not be an issue with the clay they’re selling and the temps they’re firing at. Since it’s a community studio, I didn’t want to wipe out other peoples’ work if mine decided to self-destruct. Will report back in if any of my sealed forms decide to misbehave.

[neilestrick]

20 hours ago, Rockhopper said:

A sealed (or nearly sealed) form will take longer to dry than an open one

This. It takes forever for sealed forms to dry completely. The outside can be bone dry but the inside will still be leather hard. It  also takes longer for the heat of the kiln to penetrate and drive off the last bit of moisture, so do a good long preheat and ramp slowly at first.

[Pres]

Over the years I have made several hollow pieces in my pots for one reason or another, The honey jars I make have hollow round or beehive shaped handles hollow inside, with a hole in the underside of the lid where the honey scoop stem is attached with an open end at the attachment point to the lid, Years ago I was making whistles hollow with open areas. At the same time I made rattles, clay balls dried or bisqued, placed inside of a thrown hollow form with a handle area trimmed  for gluing a wooden dowel into. Many times I would put pin holes in pieces, but often not. I think in the long run it is about the joins and the dryness. If you have wet areas inside the void, the steam and the air expansion will stress join areas. I you make certain to have no wet areas and fire slowly your joins should not stress.

All of course IMHO.

best,

Pres

[Ben xyz]

The key sounds like it’s making sure the piece completely dry before firing (which takes longer than usual, due to the enclosed area). Thanks for the responses.

[Babs]

2 hours ago, Ben xyz said:

The key sounds like it’s making sure the piece completely dry before firing (which takes longer than usual, due to the enclosed area). Thanks for the responses.

And a slow candling and slower ramps at start of bisque firing

[Callie Beller Diesel]

On the plus side, when you make hollow knobs, you don’t have to worry about small extending bits drying faster than the rest of the pot.