Depot Flux - Anhydrous Borax
Granular anhydrous borax from Depot Flux is possibly the simplest form of flux that works great for many types of forge welding This is not the stuff you buy in the detergent section of your local store. Due to its moisture content, regular borax will not float into your weld until the moisture has been cooked out, requiring multiple heats to get your weld. Start with anhydrous (containing NO moisture) borax for forge welding and get the weld the first time.
This is a white granular product. If you leave the can open for any extended period of time it will become hydrous.