Creating a fixed point, however, poses many difficulties at temperatures above 1000˚C, where matter reacts violently with other matter; the highest fixed point that can currently be used is approximately 1085˚C - the freezing point of high-purity copper. Attempts to use metals with higher melting points have failed because the graphite crucible used to contain the molten reference metal reacts with it, rendering the results unusable.
At NMIJ, we devised a new method that substitutes a metal-carbon alloy in place of a pure metal, thereby eliminating the graphite-metal reaction. By including carbon in the metal alloy at the composition known as a eutectic, the graphite-metal reaction can be prevented from proceeding further. As a result, highly reproducible melting and freezing points are obtained. NMIJ has succeeded in establishing nine distinct fixed points, from 1153˚C to 2474˚C, using carbon eutectics of a wide variety of metals. We have also demonstrated that fixed points can be obtained at temperatures above 3000˚C, using eutectics of carbon and metal carbides.