Glaze Fire Closed Forms

[Shelly M]

Hello everyone!

I have two large closed form pieces. I made a pinhole in each prior to bisque firing. I have now glazed them but despite waxing the pinhole, I can't locate it. I'd rather not wash all the glaze off the piece to find it.  I can't remember if it is necessary to have a small pinhole when glaze firing to prevent explosions. It would seem the glaze would eventually cover it up during the firing process anyway?  Thanks for your replies.

[JohnnyK]

The way I see it, Shelly, one of three things may happen. 1: nothing, the piece will be fine. 2: the glaze will melt and the expanding air in the piece will blow it out of the hole, venting the piece. 3: the piece will explode. If the piece is that important to you , I would guess as to the place you think the hole might be and clean the glaze off to reveal the hole, put a toothpick or whatever fits in  the hole, reglaze, unplug the hole and fire. How are you supporting the piece to keep the glaze off the kiln shelf when you fire it?

[Shelly M]

46 minutes ago, JohnnyK said:

The way I see it, Shelly, one of three things may happen. 1: nothing, the piece will be fine. 2: the glaze will melt and the expanding air in the piece will blow it out of the hole, venting the piece. 3: the piece will explode. If the piece is that important to you , I would guess as to the place you think the hole might be and clean the glaze off to reveal the hole, put a toothpick or whatever fits in  the hole, reglaze, unplug the hole and fire. How are you supporting the piece to keep the glaze off the kiln shelf when you fire it?

the bottom was left unglazed with enough room at the foot for glaze to run down. I could dremel a hole in the bottom to be on the safe side but thinking the unglazed bottom might be enough to make a difference?

 

[Mark C.]

This is usually not an issue-just fire slowly

 

[neilestrick]

The pot will not explode without a hole in it. Only moisture causes explosions, and that's hard to do in a glaze firing unless you just glazed it and put it into a kiln that's firing really fast, like Raku.

The pinhole is not even necessary in a bisque firing. Closed forms can be fired just fine without any holes. You just have to let them dry for a really long time, or give them a long preheat in the kiln, because they take forever to dry. The outside can be bone dry, with the inside still leather hard. The trapped air will not cause an explosion, though. In a firing, the trapped air only expands about 1.6X- not enough to blow apart a pot. When water turns to steam, it expands 1,700X, and that will blow up a pot.