The best mass standards must resist corrosion in order to have long-term stability. They should also: have high density (to limit effects of air buoyancy), conduct electricity (to eliminate parasitic forces due to static electricity), have low magnetic susceptibility (to limit parasitic forces due to magnetic fields), good thermal properties and sufficient hardness to resist wear. During the last quarter of the 19th century, when the first mass prototypes were constructed, the choice of materials was limited. Pure platinum has all the desirably properties except hardness. In addition, the technology for melting platinum and purifying it on an industrial scale had just been perfected. Alloying iridium with platinum was found to result in a material with improved hardness.

Platinum is a so-called "precious metal". The cost of a prototype is roughly the cost of the metal from which it is fabricated, which varies according to market fluctuations. Fabrication is an additional expense, though considerably less than the cost of material.

For several reasons, it is desirable to minimize the surface of a mass standard. For instance, surfaces may become contaminated by airborne dust and chemicals. One way to reduce the surface area of a metal object is by polishing it to a mirror finish. The surface of a prototype is also minimized by the choice of its geometric form. A spherical prototype would have the minimum surface area (about 62 cm²). But spheres are difficult to fabricate and inconvenient to use. The compromise between small surface area and convenience was to make the prototype in the form of a cylinder whose height is the same as its diameter. The difference in surface area between this form and a sphere is less than 15 %.

Logically, the kilogram should be called by a different name. The "grave" was an early suggestion, dating from 1793. However, the name "kilogram" (adopted in 1795) is now so embedded in our culture that changing it at this late date probably would be impractical. Proposed changes to the SI are considered by the Consultative Committee for Units (CCU). [more]

The revolutions in science that took place during the 20th century had a profound influence on technology. One need only think of how the atomic clock and the laser have influenced our daily lives. Although these new technologies have had relatively little impact on mass measurement, they nevertheless have created new ways of linking the mass of the international prototype to the fundamental constants of physics. There are several research efforts around the world pursuing these measurements and so we can be optimistic about replacing the present artefact definition of the kilogram with a definition based on physical constants. [more]