Flux is the product you use to keep the metal clean and free of oxides during soldering so that the solder can flow.
The flux used for non-ferrous metals is mainly borax. It exists in liquid form, as a paste, powder or solid. The liquid is the most used because it doesn’t require any mixing prior to soldering.
Powder flux needs to be mixed with water to form a paste. This paste has some advantages to the liquid form, like the fact that it stays in place. For curved surfaces or when the soldering area is vertical it can be helpful. The downside is that it dries quickly so you need to make it fresh every time.
Solid borax is used in situations where the solder refuses to flow. Direct application of solid borax onto the warm metal typically solves the problem. It’s also used to prime the crucible used to melt metal, by creating a glassy layer over the surface of the ceramic crucible to prevent the metal from sticking.
When using liquid flux, it’s advisable to transfer some of it to a small container, not only to make it more practical to use but also to prevent contaminating all the flux with unwanted dirt.
Before soldering, liquid flux is applied to the metal by using a paintbrush. The brush can also help to pick up solder pallions.
But if you need to reapply halfway through soldering, an eye dropper is a better way to proceed since the brush has a tendency to burn when it comes in contact with hot metal.
There are plastic and glass eye droppers but what I use more often is an improvised version: one of those single use plastic saline containers that anyone with children or contact lenses probably has around the house. I have lots of them and when the liquid flux dries and crystallises, clogging it up, I have no problems throwing it out and getting a new one. Plus it’s a form of recycling, which is always good.
Aside from soldering flux there is also barrier flux. It’s blue and usually made from methyl alcohol and boric acid. It’s used to protect the metal from oxides while soldering. If used properly, it can prevent fire scale and fire stain. It also allows you to solder twice in a row pickling the metal in between.
Barrier flux is highly flammable and evaporates quickly, so the container should always be kept tightly closed. To work properly it need to be applied to clean metal before soldering flux, every single time the metal is heated.
Just like with soldering flux, I recommend you pour a small amount into another container to prevent evaporation and contamination. Two options are:
Dropper bottles. I reused a medicine bottle with a built-in dropper.
Spray bottles. These seem to work better with the barrier flux than the soldering flux. Soldering flux leaves white crystals when it dries and tends to clog up the spray.
For the blue flux the spray is a good option because you don’t need to remove the lid, so it doesn’t evaporate easily and it helps coat the whole piece easily.
You should take great care to wear a mask so as not to breath in the drops suspended in the air (but a mask should be used at all times while soldering, anyway).