The Salt That Glows
It was a grueling process. Radium and polonium are decay (or “daughter”) products in the chain that begins, in nature, with uranium; radium, in turn, decays into radon gas. The most common isotope of radium, 226Ra, has a half-life of 1,601 years—half of the radium atoms in any particular sample will decay during that period. Though relatively long in terms of human lifespans, radium’s life amounts to little more than a blip in geological time, so natural radium never has time to accumulate in large quantities. Instead, it is found in trace amounts—a fraction of a gram per ton—in uranium-bearing ores like the Curies’ pitchblende. To separate radium from the pitchblende required a long chain of chemical treatments, starting with repeatedly boiling and washing the ore, using concentrated soda and acid preparations, and ending with a painstaking process of fractional crystallization that allowed the Curies to gradually produce gently-glowing, radioactive salts containing ever-higher concentrations of radium chloride. It took the Curies until March of 1902 to isolate and purify enough radium chloride, one-tenth of a gram, to have its existence “confirmed” by the calculation of its atomic weight with a spectroscope.