Flux, like naphtha, is one of three properties a herb could potentially have. You need to achieve Herbalism level 60 to be able to find the Flux property. To create Flux, one has to combine 2 herbs with the 'flux'-property in a mortar and pestle or an Herbalist's shop.
|5x Mortar||Materials preparation: 30||none||Kiln|
|Steel ingot||Smelting: 60||Crucible and tongs||Smelting devices|
|Lump of steel||Smelting: 60||Crucible and tongs||Smelting devices|
|Steel bar||Smelting: 60||Crucible and tongs||Smelting devices|
Out of the 66 available herbs in-game which can be gathered, 11 of them produce Flux. The main method of determining what your own Flux herbs are is to gather a large sum of each herb, stack them in your inventory neatly (with quantities of each herb in 5+ so you always have some, (this process might take a few hours of gathering and sorting), and start mixing herbs until you find two which produce Flux together. Once you find those, set them aside in your inventory. Then, proceed to mix those flux-producing herbs with other herbs in your inventory, one by one, until you find more herbs which make Flux. You can then set those ones aside too until you hopefully find 11 herbs.