Flux (from Latin "to
flow") is a substance that facilitates the flow of
solder by preventing the formation of
oxides. Flux prevents oxides from forming by being more
reactive with
oxygen than the
metals and solder that the flux is applied to. Generally flux is combined with
water to make a fluid that can be
painted or
sprayed onto the workpiece. When heat is applied the water evaporates to leave a
glassy coating. As oxygen and other elements combine with the flux it becomes less effective.
--Partially excerpted from
The Complete Metalsmith by Tim McCreight.