Bertram Borden Boltwood (July 27, 1870 Amherst, Massachusetts – August 15, 1927, Hancock Point, Maine) was an American pioneer of radiochemistry.

She prepared him to attend Yale, his father's alma mater, by placing him in private school at a young age, followed by the Albany Academy.

Lacking resources in English for teaching the subject, he translated two books from German, Alexander Classen, "Quantitative Analysis by Electrolysis," and the other by Charles Van Deventer, "Physical Chemistry for Beginners."

This experience brought him into contact with the rare earth metals he had studied as well as uranium and thorium, elements that would become the crux of his greatest scientific contributions.

They communicated primarily through overseas correspondence, with the exception of a short period from 1909-1910 when Rutherford invited Boltwood to join him at the University of Manchester in England.

In 1904, Rutherford gave lectures around the topic of radioactivity as a tool for geologic dating,[11] and presented calculations based on the presence of helium as a product.

[2] The following year, Boltwood made the assertion that lead was the final decay product in the disintegration of uranium, and that Pb:U ratios increase in older geological samples.

In 1907, he published results of analyzing ten mineral samples from different world locations, including a thorianite[2] that measured 2.2 billion years old.

This value was ten times greater than any previous estimated age of the Earth,[10] and geologists did not immediately accept the validity of radioactivity as a dating method.

[9] In 1907, he discovered a new element with a half life of almost 100,000 years before it decays to radium,[3] and he named it "ionium" after the ionizing action of its alpha particles.

[5] Boltwood was able to prove that ionium disintegrates to radium, and the full connection to uranium was shown in work by Frederick Soddy in 1919.

[3] Once Boltwood was offered a full professorship and chair of radiochemistry position at Yale in 1910, his career became more academic and he no longer actively pursued research.