Literally meaning "flow", a flux is a liquid in which some solid substances melt at a lower temperature then their usual melting point. To some materials a certain liquid may act as a good flux while it isn't for others. Temperature and pressure are the two factors of great influence. Water, at normal pressure and temperature, acts as a flux for sugar but doesn't for gold. In relation to jewelry it used to aid goldsmiths soldering, dissolve the chemicals needed to create some types of synthetic gemstones. It is also used in similar conditions as a medium to heat, diffusion treat and (glass) fill gemstones.