Note on the Dissociation of Ferric Oxide Dissolved in Glass and Its Relation to the Colorof Iron‐bearing Glasses. ¹

It is shown that above 1300"C, Fez03 is dissociated appreciably with the evolution of 0 2 and the formstion of the corresponding quantities of FeO.The same reaction is found to take place in glasses.The following percentages of Fe as FeO based on the total Fe present illustrate the extent of this dissociation in glasses.Diopside glass (CaSiOa.MgSiO8) 1400" C, 20%; 130Oo c, 30%; 1600" C, 45%.0.6 K20-0.4C ~O -~S ~O Z , 1310" C, 5.8%; 1555' C, 10.1%.0.3 K 2 0 4 .3 N a z 0 4 .4 Ca0-2SiO2, 1020" C, 4.1yc.In the two latter glasses a dissociation of loo& produced a bright green color: the glass with a dissociation of 4.17, was yellow.Iron compounds in a glass may therefore be "reduced" by heating to a high temperature under oxidizing conditions.The intensity of the color due to iron is closely related to the composition of the glass, but data on this point are not sufficiently complete to warrant generalizations. General Effects of Iron in GlassIron, in some form, is present as an impurity in every raw material used in the manufacture of optical glass.When the quality of raw materials and pots is otherwise satisfactory, the color developed by iron in glass is the most important factor in lowering transmission.The actual iron content of precision optical glass varies from 0.01 per cent to 0.05 per cent (expressed as Fez03) even when made from the purest raw materials obtainable and melted in resistant pots of low iron content.The absorption of light caused by even these small amounts of iron is significant and easily measurable, but the relation between total transmission and iron content is obscured somewhat because of the two co-existing states of oxidation in which iron is