It has recently come to our attention that the yellow amber through to dark orange red series can become much darker or even black if cooled very slowly or held for some time in the temperature range 575- 670°C (1070-1240°F). The problem is particularly manifest after a piece is cast, brought down to room temperature and then subsequently reheated again to slump or fuse within that temperature range. We recommend cooling down from casting temperatures no slower than 60°C (140°F) per hour through that range.
It is possible to reverse the phenomenon by reheating the glass back up to + 700°C (+1380°F) so that the original colour reappears.
We strongly recommend fusing or slumping these colours after the original high temperature casting at temperatures below 550°C (1020°F).