Authors discuss an experimental study of elastomeric mastics capable of replacing traditional mastic (calcium carbonate and linseed oil). Accelerated aging tests on test panels are devised to evaluate long-temn behavior of synthetic mastics on stained glass windows subjected to wind pressure, thermal cycles, and rain water. Testing chamber, experimental method, and results are described. Behavior of traditional mastic and alternative materials tested is charted and evaluated. See also AATA 21-1334 for reference to entire proceedings.