A sulfur glass. May react with gold-bearing lead (1311, 1215, 0301, 0305) and copper (1116, 1408, 1417, 0144, 0145, 0146) glasses to create dark interface (lead sulfide, copper sulfide). Very viscous; will flow later and less than other glasses. Generally more sensitive to heat-history and more likely to show variation in color after fusing than many opals. When fired on edge, a clear distinction between outside and interior surfaces is commonly seen (a variation used by designers). This glass may become increasingly white with repeated firings. Consider using glass from the same batch for a given project.