AbstractIt is generally believed that in von Kossa's technique silver cations react with phosphates and carbonates in calcium deposits and are then reduced to black metallic silver by strong light. Perusal of von Kossa's paper showed that he was aware of significant differences between reactions of silver phosphatein vitroand in calcium deposits. He regarded only the yellow coloration of calcium deposits during early stages of the reaction as diagnostic for calcium phosphate and ascribed the blackening to organic matter. Efforts to prevent this blackening were unsuccessful. Von Kossa's experiments were reproducible in our hands. Further studies showed that bright light, generally regarded as essential for von Kossa's reaction, only causes the irreversible blackening of organic matter that masks the yellow silver phosphate. When the reaction is performed in subdued light, yellow to yellowish brown silver phosphate is visualized selectively. Silver carbonate dissolves in thiosulfate and cannot be demonstrated with von Kossa's technique.