The most widely used static method of phase equilibrium determination is the quenching method. After a specimen has been heated to a high temperature and reached equilibrium, the specimen is quenched to ambient conditions. The quenching process generally involves rapid cooling in a bath of water, mercury, or heavy oil. If successful, quenching cools the specimen to room temperature without changing the microstructure during the cooling process. Thus, the phases that were present in the material at high temperature equilibrium conditions are preserved. Note that a material can only be quenched from high temperatures below the melting point, i.e., when the specimen is in solid form. However, quenching can be useful in the study of liquidus relations of silicates or other glass-forming systems, since many of the liquid phases in these systems are viscous and crystallize slowly. These quenched specimens show, upon microscopic examination, characteristic microstructures which are identified with the liquid phase or with coexistence of liquid and crystalline phases.
Static Method Involving the Mechanical Separation of Phases
In equilibrium studies where the liquid phase of non-glass forming systems is present, it is often useful to mechanically separate the phases present in the specimen at the equilibrium temperature. A high-temperature centrifuge is used for these kind of experiments. Liquid is separated at high temperature from the solid by forced filtration through a porous filter and then analyzed chemically by micromethods. This technique can also be used to separate immiscible liquids.
Another means of separation of samples for analytical purposes is through zone melting techniques, in which different sections of a rod of known total composition are exposed to different conditions. For example, by exposing only one end of the rod to a high temperature, different phases will exist along the rod. The primary crystalline phase is found at the high-temperature end of the rod and the eutectic phase at the opposite. By sectioning the rod at different locations and analyzing each section microscopically and chemically, the composition of each and the extent of solid solution in the primary phase can be determined.
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