 Cobalt has been used to color glass since the Bronze Age . The excavation of the Uluburun shipwreck yielded an ingot of blue glass , cast during the 14th century BC . Blue glass items from Egypt are colored with copper , iron , or cobalt . The oldest cobalt @-@ colored glass is from the Eighteenth dynasty in Egypt ( 1550 – 1292 BC ) . The source of that cobalt is unknown . 
