Bubble formation

Question:

I had 2 kiln loads where every piece had huge bubbles. The 3rd time I did a test fire and put 2 pieces on Thinfire and one on the kiln shelf. The one without thin fire got a bubble. Theories?

Response:

Your experiment showed that the Thinfire prevented the bubbling.  The question you are asking is why.

Fibre paper is a porous material allowing air to move through it and from under the glass.  If the shelf has only slight depressions, the Thinfire or Papyros will allow air out from under the glass avoiding bubbles.

Thicker fibre paper can provide a different and more level surface if it is thick enough to span any depressions in the shelf, while allowing air out too.

First, it is apparent that your shelf is not absolutely smooth and level.  This has been shown by your experiment where Thinfire prevented bubbles where previously there had been bubbles.  It showed that without the additional cushion that the depressions, although slight are enough to cause bubbles without additional fibre paper separators.

Second, although you do not give your schedule, the firing is too hot.  There is dog boning of your thin glass.  The bubbles on the thinner glass have burst and thinned greatly.

Firing hot causes the glass to become much less viscous than needed to perform a full fuse and allows the trapped air to push bubbles into and through the glass.  Lower temperatures with longer soaks/holds enable the glass to better resist the formation of large bubbles.  Also firing more slowly enables air to escape and allows the use of lower temperatures while still being able to achieve the fuse you want.

More information is available in Low Temperature Kilnforming, an evidenced based guide to scheduling.