Why was platinum-iridium used to make the kilogram? (FAQ - Mass & Density)
Platinum-iridium was chosen as the material for the kilogram for a number of reasons.
Its high density (approximately 21.5 g/cm3) means the artefact has a small surface area and therefore the potential for surface contamination is minimised. Its high density also means that it displaces a smaller amount of air than a kilogram of less dense material (stainless steel or brass for example). The weight-in-air of the kilogram (or any mass standard) depends to some degree on the density of the air in which it is weighed because the air (or any fluid in which it is weighed) exerts a buoyancy effect that is proportional to the volume of the artefact. Minimising the volume of the weight minimises the effect of a varying air density on the weight of the artefact.
The relatively inert nature of the material also minimises surface contamination and enhances the mass stability of the artefact. Platinum and its alloys are reasonably easy to machine, enabling a good surface finish to be achieved on the artifact that again reduces the effect of surface contamination. The addition of 10% iridium to the platinum greatly increases its hardness and so reduces wear.