Grand-K, the (former) official standard kilogram has been losing mass as determined by comparison with several official copies. The divergence of the platinum-iridium standard kilograms is not a mystery at all. It is due to the history of metallurgy. Platinum-iridium was chosen because the two elements of which it is made are chemically HIGHLY INERT. They are unaffected by nearly all chemicals. However, iridium has an "evil twin", osmium, with which it is found as an alloy called variously iridiosmium or osmiridium. At the time Grand-K was crafted, it was impossible to remove ALL osmium from iridium, so osmium remained as a trace element. To make the platinum-iridium standards, the powders of the constituent metals were thoroughly mixed and then sintered into a solid block. Technology at the time could not melt this alloy without contaminating it. Sintering left countless tiny channels which penetrated the final block. Unlike platinum and iridium, osmium is NOT chemically inert. It combines with oxygen from ordinary air to form osmium tetra-oxide. Also, unlike nearly all oxides of heavy metals, osmium tetra-oxide is volatile and evaporates at ordinary temperatures. Hence, the effect of air on the standard weights is two-fold: At first, they gain weight as the oxygen is combined into osmium tetra-oxide; then the osmium tetra-oxide evaporates. This process is very slow, but over decades the results accumulate, and the cylinders lose weight. The exact losses depend upon temperature, humidity, and oxygen content of surrounding air. Mystery solved.