Radium chemically resembles barium, making radium separation difficult.
Radium and barium are both alkaline-earth metals in group 2 of the periodic table. They commonly form divalent ions and similar salts, so radium in uranium ore tended to follow barium through many chemical treatments rather than separating cleanly.
The Curies exploited small differences in solubility instead of relying on a dramatic chemical distinction. Their process repeatedly converted the mixed material into salts and used fractional crystallization to enrich radium. Radium compounds were generally slightly less soluble than the corresponding barium compounds, allowing successive crystallizations to concentrate the rare radioactive element.
This was painstaking work because radium occurred only in trace quantities among large amounts of ore residues. Spectroscopy and radioactivity measurements helped track the separation: radium produced characteristic spectral lines, while its intense emissions made even tiny amounts detectable. A common mix-up is calcium, which is radium’s important biological analogue because radium can enter bones in place of calcium; chemically, however, barium was the major obstacle in purification.