Brooks, I don’t know what kind of equipment you have , but I
have used opticon to seal back up an opal doublet. If it is
actually a crack this should work. If you have access to an oven
and vacuum machine you can do this very effectively by building a
chamber out of plumbing parts at your local hardware store. Cost
about $50 plans are at http://www.gis.net/~adamas/chamber.html
John