The method of preparing finely divided metallic “blacks” is described and measurements on the transparency of these blacks in the infrared are presented. The use of sodium and thallium chlorides, distilled as finely divided powders on radiometer vanes, yields a selective receiver the far infrared. A new type of transmission band, shown by coarse powders of quartz and calcite is described.
A. H. PFUND, "The Optical Properties of Metallic and Crystalline Powders," J. Opt. Soc. Am. 23, 375-377 (1933)