In December 1913 and April 1914, Henry Moseley, a British physicist, published data that is now famed for being the first experimental evidence for the atomic number as a physical property of the nucleus. Shortly after, in June 1914, Moseley used x-ray spectroscopy to analyze several rare earth elements provided by Georges Urbain. Moseley failed to publish his conclusions before his death in the First World War. Despite the efforts of his mother and colleagues, a posthumous publication never materialized. This essay explores the question of why. An in-depth evaluation of extant artifacts and archival materials at the Museum of the History of Science in Oxford related to Moseley’s rare earth research reveals nuances in the process by which he collected and corrected data to form his conclusions. Whereas Moseley was confident his data did not support the claim that Urbain isolated the element with atomic number 72, it failed to inspire Ernest Rutherford to see the work through publication after Moseley’s death. Archival materials reveal some of the pressures that could have prevented publication, including Rutherford’s unfamiliarity with Moseley’s process—but more importantly, the fact that this data would influence the debate over the discovery of element 72. Interestingly, it is likely this controversy led to the retention of relevant archival material. By tracing the actors that created and curated a particular collection of documents and spectra, one can explore how rare earth knowledge was produced and verified in the first few decades of the twentieth century.