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Because if the contraction and expansion of glass, just using a typical water clear resin does not usually work. Often the cure alone has enough heat to crack the glass, otherwise they tend to crack cooling down or on their first major temperature change.
For this reason you generally need a specialist flexible water clear resin to get around this issue. We do not have anything ourselves as yet, however we do know that the "Aquallusion" seems to be well regarded in the craft industry.