For attainment of 0.01-dB/km absorption losses in the 2-4 micrometer range, iron, cobalt, nickel, and copper concentrations in fluoride fibers must lie in the 0.2-10 range. (1 PPB = 1 mol/109 mol glass.) Commercial grades of zirconium salts tend to form colloids and to contain hundreds or thousands of times more than the tolerable concentrations. We have found filtration to be partially effective for colloid removal. Working with rela-tively concentrated (2 molar, about 17% by weight) solutions of Zr salts, we have introduced about 7000 PPB each of Fe, Co, Ni, and Cu. We have determined that all of these metals can readily be removed, by laboratory-scale ion-exchange methods, to meet the absorption stan-dards for 0.01-dB/km absorption.
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