How was the prototype international kilogram made?

The prototype international kilogram was a solid cylinder made from an alloy consisting of 90 percent platinum and 10 percent iridium.12

Adopted in 1889, this cylinder was designed with a height equal to its diameter to serve as the international standard for mass. It replaced an earlier standard that had been represented by a solid cylinder of pure platinum. This platinum-iridium alloy was chosen for its durability and resistance to corrosion and chemical attack.12

It is important to note that the kilogram is no longer defined by this physical artifact. Due to concerns regarding the stability of the prototype's mass over time, the General Conference on Weights and Measures redefined the kilogram in 2019 based on Planck’s constant, a fundamental physical constant.13