There have been a number of excellent posts on the subject of art glass
in recent days.  As has been noted, many cities had art glass foundries
from the 1890s-1929s, and as a consequence there was an enormous variety
in glass colors, types and qualities.  Unless you are fortunate enough
to have some of the new-old-stock Seeburg windows cited by Art Reblitz
in the last MMD issue, you will have a very hard time replacing broken
pieces with a glass that will match close enough to pass inspection.

If an individual piece in a window is merely cracked across, it is best
to just leave it as is.  If a piece is broken out but you have all the
bits, a professional glassworker can glue it together and replace it so
it is hardly noticeable.

For missing glass, though, you will either have to find a worker who has
access to lots of antique glass, or replace the entire window.  I could
give detailed instructions for replacing individual pieces, but I am
afraid that might just encourage somebody to start off half-cocked and
end up ruining the looks of their window.

Walt Wentz