Artificial Fluorite

This paper presents a condensed history of the research on artificial optical fluorite growth which was begun in the middle 1920’s and reduced to practice under N.D.R.C. Contract OEMsr-45 with the Massachusetts Institute of Technology during 1940-45.Fluorite castings were produced eventually by freezing purified molten CaF2, contained in chemically clean graphite crucibles, in a vacuum-type contrivance patterned after the atmospheric furnace which had been developed earlier for growing lithium fluoride crystals.The melt was prepared by heating selected fragments of colorless fluorspar mixed with about 2 percent PbF2 which acted as a scavenger to eliminate certain more or less unavoidable impurities, notably products of hydrolysis, and which escaped before the crystallization. In a few instances it was made from a specially synthesized material although this procedure was not wholly desirable.The crucibles were made by boring, reaming and turning graphite rods which possessed exceptional strength, homogeneity, and chemical purity. Their wall thickness was approximately 0.03 inch when feasible.Crystallization occurred preferably in a thermal field characterized by a high, nonlinear temperature gradient which remained nearly constant with time.Specimens chosen from about 1500 crystals, ranging up to 6 inches in diameter, were comparable with good natural fluorite in all essential properties and equal or superior to the best in soundness and dimensions.