Detective Needed

The properties of glass remain consistent, however carefully observing individual pieces prior to firing is essential for accurate operator predictions.

Scrap pendant - black, green and transparent - pre-firing

There are four pieces of glass in the piece above:  a think, transparent rectangle is on the bottom, a cut black rectangle is sandwiched in the middle along with a scrap edge of green, and a cap of thin, transparent caps everything.

The prediction was that the corners on the clear would be slightly rounded, the corners on the black would be very slightly rounded, the uppermost corners on the green would be slightly rounded, and the irregular edge of the scrap green would look much as it does pre-firing.

Scrap pendant - black, green and transparent - post firing.  Horizontal

Post-firing I had a need to do some analysis because the piece did not look quite like I expected.  The part that surprised me was the shape of the black rectangle on the top corners.  I am now wondering if the firing temperature of the black is a little lower than the transparent and the green.  Did the outermost corners of the black reach their melting temperature before the bottom part???  This would account for the rounding of the top corners and not the bottom corners.

Could the glass have appeared to be  a consistent thickness, but really be inconsistent????  Since most of the pieces am using to run these tests are in my scrap container, it is possible that this piece of black was close to an edge when manufactured, and parts are slightly thicker than others.  This idea seems more unlikely than the temperature idea, however, because all four edges of the black rectangle were cut; there was not one raw edge in the black like there was in the green. 

Scrap pendant - black, green and transparent - post firing.  Vertical

The original plan was to suspend the pendant vertically, but I am not sure it works that way once fired.  Horizontally, with the black at the top, also allows for discrete placement of the bail.

Four pendants attached to fire paper and ready to go in the kiln; green and black in transparent in upper right

The four pieces above were all fired together on this piece of fire paper.  Notice the location .  of the black and green pendant - it adds further evidence to support the hypotheses that the black reached a higher temperature first and reacted an an independent piece.

If the temperature theory is plausible, is uneven heating in the kiln the cause, do dark colors heat faster than non-dark colors of glass, or do the chemicals in each piece of glass significantly effect firing temperatures?????