How did Marie Curie isolate radium from pitchblende?

Marie Curie isolated radium from pitchblende by employing a novel method that combined chemical analysis with quantitative measurements of radioactivity.1

Because the radioactive substance was present in only minute quantities, Curie used an ionisation chamber to trace the radioactive property through various chemical fractions. She discovered that the activity concentrated in fractions containing barium. To separate the radium from the barium, she utilized a process of fractional crystallization, taking advantage of the fact that radium chloride is slightly less soluble than barium chloride.12

This work was exceptionally difficult due to the massive quantities of raw material required. By 1902, Curie successfully prepared a decigram of pure radium salt, confirming its identity through its atomic weight and spectral lines. She later isolated metallic radium in 1910 with the assistance of André-Louis Debierne. This pioneering approach of using radioactivity to guide chemical separation became a fundamental technique in the study of radioelements.123