The international prototype of the kilogram, an artifact made of platinum-iridium, is kept at the BIPM under the conditions specified by the 1st CGPM in 1889 when it sanctioned the prototype and declared: this prototype shall henceforth be considered to be the unit of mass.
The 3rd CGPM (1901), in a declaration intended to end the ambiguity in popular usage concerning the use of the word “weight,” confirmed that:
The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.
The word “weight” denotes a quantity of the same nature as a “force”: the weight of a body is the product of its mass and the acceleration due to gravity; in particular, the standard weight of a body is the product of its mass and the standard acceleration due to gravity.
The value adopted in the International Service of Weights and Measures for the standard acceleration due to gravity is 980.665 cm/s2, value already stated in the laws of some countries.
It follows that the mass of the international prototype of the kilogram is always 1 kilogram exactly. However, due to the inevitable accumulation of contaminants on surfaces, the international prototype is subject to reversible surface contamination that approaches 1 μg per year in mass. For this reason, the CIPM declared that, pending further research, the reference mass of the international prototype is that immediately after cleaning and washing by a specified method. The reference mass thus defined is used to calibrate national standards of a platinum-iridium alloy (Metrologia, 1994).