No-Clean Flux ThinningSoldering fluxes loose solvent and need additions of thinner. Flux is composed of solids dissolved in a solvent base. Over time the solvent evaporates. As the solvent evaporates the solids concentrate and eventually crystallize out. You will need to add thinner on a regular basis to prevent the flux from crystallizing. A hydrometer is used to measure the specific gravity of the flux. Normally the specific gravity (density) is checked 2 or 3 times a day. Thinner is then added to return the flux to its correct density. Hydrometers can be purchased from any scientific supply house. Kester has flux density control Data Sheets (also called nomographs) for the rosin and higher solid content fluxes. Using the nomograph you can determine how much thinner needs to be added. We sell test kits for the low residue fluxes. The test kits indicate how much thinner to add to the flux pot. Source: Kester Solder Co.