622021 TAG Penumbra COE 33 Borosilicate Trautman Art Glass Rod

Named for the shadowy edge of a lunar eclipse, Penumbra is a reactive “silvered” cobalt blue. Different colors are produced from reduction or oxidation, primarily a wide variety of blues and greens. Reduction can create a luster that you can trap under a clear encasement. However, surface haze doesn’t burn off as easily as amber/purple type glasses.