Troubleshooting Cracks in Fused Glass
part two of a two-part series. Click here to go to part one.
Here a list of the major types of cracks in fused
glass and some information about why they occur and how to prevent
Curved cracks across the middle of the
Improper annealing causes this kind of crack. It
will show up as gentle curves (sometimes as straight lines) that
break the pieces into two or three pieces. Often the crack will
curve sharply as it nears the edge of the glass. This kind of crack
is the piece's way of relieving stress. The solution is to spend
longer annealing the piece.
Cracks where two different
glasses come together.
Glass incompatibility causes these cracks, which
often show up as curved cracks around the edges of the two types of
cracks can be very small or they can cause the pieces to break apart, but they will always
be along the edges of the piece of incompatible
glass. Conduct your own compatibility tests or use "tested
compatible" glass to keep this from reoccurring.
cracks (like a spider web).
These cracks generally extend from a single spot on
the underside of the glass. They aren't usually severe enough to
cause the item to split into pieces. Sometimes shelf primer will
also be stuck to the underside of the glass. Most likely, this kind
of crack is caused by glass sticking to the kiln shelf. A close
examination of the shelf may even reveal small pieces of glass that
are stuck to the shelf. The obvious solution is to scrap the shelf
clean and apply fresh kiln wash.
Pie-shaped pieces, with smooth edges.
These cracks, which usually occur with such force
that they split the piece into five to ten pieces, are caused by
thermal shock. The edges of the pieces are often rounded because
these tend to happen early in the firing cycle (around 300 to 500
degrees Fahrenheit) and the edges round during later phases of the
firing. The cure for thermal shock is to fire more slowly in the
early part of the heating phase. You might also try cutting very
large pieces into smaller ones before firing. Finally, it's a good
idea to peek in the kiln around 400 degrees or so just to make sure
the piece is still OK.
Cracks that occur long after firing.
Sometimes a glass piece will just be sitting on a
table when you hear a sharp ping. It might be quite loud, and
perhaps there's a second (or even a third) ping. When you check it
out, you discover that the glass piece you thought was beautifully
finished has cracked. (This cracking can even be severe enough to
shatter the piece, leaving the artwork in pieces on the table.)
The reason for this disaster is undoubtedly stress
that has built up in the glass piece. Stress can come from many
factors: improper annealing, thermal shock, incompatible glass, and
even "normal" wear and tear. Improper annealing is the most likely
culprit. If you can access the firing log for the piece, check to
make sure it was properly annealed.
If you used the same schedule you've always had
success with, then perhaps this piece of glass was a bit thicker or
larger than normal. Perhaps it was a different glass than you
usually use. Perhaps your "normal" annealing schedule needs to be
adjusted to anneal just a bit longer and slower.
Copyright 2005 Brad Walker.
All rights reserved.