Sharp points on rectangles

At the conclusion of firing pieces with right angles or sharper shapes you often find very sharp needle points at the corners.

This is a result of the expansion of the glass as it heats up.  At top temperature, the glass piece is larger on the shelf than when you put it in cold.  The amount of this expansion is related to the thickness of the piece and the temperature it has been fired at.

As the glass cools, it contracts.  The contraction at corners and points has greater effects on the glass than at the sides.  The corners are contracting from two sides rather than only one.  This makes them a little more resistant to contract and often leaves a little of the glass stuck at the furthermost point of expansion as it contracts.

I have found the best prevention of sharp points on the corners of rectangular pieces, and those with even sharper angles, is to nip off the tiniest bit of the corners. This very slight blunting of the corners allows the glass to expand and then retract without the corner or point catching on the separator and so creating the sharp needles.

Further information is available in the e-book: Low Temperature Kilnforming.