In the Curies’ radium mixture, barium compounds gave a green flame color.
While processing pitchblende, Marie and Pierre Curie found that the material left after uranium removal remained intensely radioactive. Chemical tests showed that much of the mixture behaved like barium compounds, including their characteristic flame emission.
The green flame was important because radium’s chemistry closely resembled barium’s, making separation difficult. The Curies relied on repeated fractional crystallization and spectroscopy to distinguish the radioactive material from ordinary barium salts.
A common mix-up is radium’s own flame color: radium compounds are often described as producing a carmine or crimson-red flame, whereas barium compounds produce green. The contrast between green barium lines and carmine radium lines helped establish that the mixture contained a new element.