  We describe the wartime challenges associated with the rapid developments in
plutonium chemistry and metallurgy that were necessary to produce the core of
the Trinity Device. Beginning with microgram quantities of plutonium metal late
in 1943, initial measurements showed a wide and confusing variance in density
and other properties. These confusing results were the first clues to the
astounding complexity of plutonium. As this complexity was revealed, it
introduced new challenges for the fabrication of kilogram-scale parts. In a
remarkable period from January 1944 to June 1945, Manhattan Project scientists
made rapid progress in understanding plutonium chemistry and metallurgy. By
early 1945, they had discovered five of the six ambient-pressure phases of
unalloyed plutonium and reported the density of these phases to within a value
of 0.1 g/cm$^3$ of those accepted today. They solved the stability problem
introduced by these phases with a rapid alloy development program that
ultimately identified gallium as the preferred element to stabilize the
delta-phase, producing a plutonium alloy still of scientific and technical
interest today. We conclude with a description of post-war developments in
these areas, including applications of wartime plutonium metallurgy to civilian
applications in nuclear reactors. We dedicate this paper to the memory of Ed
Hammel, the Manhattan Project plutonium metallurgist whose previous description
and documentation of plutonium history during the war has been essential in our
research.
