Abstract: A flux control device for maintaining the specific gravity and the surface level of a liquid flux contained in a fluxing vessel within predetermined ranges, includes a first reservoir for storing a concentrated flux, a second reservoir for storing a diluent, detectors for checking the liquid level and the specific gravity of the liquid flux in the fluxing vessel for any deviation from predetermined ranges, and first and second conduits extending between the vessel and the first reservoir and between the vessel and the second reservoir, respectively. Means are provided to apply a gas pressure selectively to the first and second reservoirs so that the concentrated flux and the diluent may be fed to the fluxing vessel through the first and second conduits, respectively, upon pressurization of the first and second reservoirs.