Welding requires a good metal to metal contact. Anything between them that gets in the way makes the weld weak, or incomplete. Fluxing helps in keeping the oxygen away from the hot metal and also eats away existing oxides.
Borax – it creates a thick molten boron glass coating that protects the settle from the oxygen in the forge atmosphere. BTW, the forge burner should be balanced to have little excess. Twenty mule team and other regular borax are sodium tetraborate. It is boric acid that has reacted with sodium to make a salt. This is done in a watery mix, and the powder is dried from it. Some of the water is still tied to the boron salt crystal. This is called hydrated sodium tetraborate – AKA borax. You can bake it at 350F for several hours and drive out all the water and make anhydrous borax, but it will try and suck moisture from the air and re-hydrate. Since the water will be driven out in use when using as a welding flux, there is no real advantage to using anhydrous borax for blacksmiths/bladesmiths.
The borax is applied to the clean and wired/welded together bilet when it is just getting to red heat, heated a bit more into the orange range and re-applied, and then applied again at the yellow welding range. It should coat and run all over the billet. One of the things that borax does to aid welding is eat away any oxides because it becomes something like a molten hot acid.
Putting a large metal baking pan under the billet when shaking the borax on saves the excess for later use. A container with holes in the top, like a parmesan cheese jar works great. You don’t want too much flux, because it will just run off on the forge floor and spray around the room like flaming meteors when you set the weld, but assure the billet is fully covered when at welding heat. Make sure the weld blows start from one end and from the center out or you won’t expel all the borax from the layers. Trapped borax will create a weld flaw that cannot be repaired. This is probably the major drawback to new smiths. The flaming meteors headed toward your crotch ( and everything within 10 feet) are the other problem.
Abrasives and shields – Many ancient techniques use things like ash, wrapping in paper, charcoal powder, fine fluorite or quartz sand, and fine clays in a slurry to coat the metal and aid welding. These still have some advocates today, but for the most, they have been replaced by better equipment and materials.
Hydro-carbon welding – This is a newer and popular method for production damascus welders. The billet is kept in a bucket of some hydro-carbon after assembly to keep all oxygen from touching the surfaces. Since it will seep into the tiniest space by capillary action, it covers and protects all the billet. Kerosene and brake cleaner type fluids are the most popular fluids. When ready to do the weld, the billet is removed from the bucket ( which is covered securely), drained a moment, and put in the forge. The fluid burns off leaving behind – CARBON – just what we really want in the space between the welds. It will slightly raise the carbon content in the weld surfaces (making the line etch slightly darker) and it also will eat up any oxygen around. From the initial setting of the weld throughout the next heats, not allowing the billet to cool below black will assure no oxides form between the layers, all yielding a very securely welded billet. When folding and drawing a billet out, a wire brush is used before and after each draw. The billet is hot cut at the end of a heat, brushed off, re-heated, brushed again, folded, and welded. If all is done at a red heat or above, the use ofa small amount of borax when folding is all that is needed.
The newer and gaining in popularity method is dry welding. It is welding up a billet in a way that allows no oxygen inside the billet at all. This is done by cleaning the bars, stacking them and clamping tight in a vise, and welding up all the seams. Modern affordable TIG and MIG welders have made this a home shop possibility today. Once fully welded shut, the billet is processes as any other damascus billet. This Is a very good system for doing a long san-mai billet for a large knife or sword.
If you are welding a billet with hydrocarbon or borax flux, you start with a clamped billet and then welding just the corners or ends together, and maybe a stripe down the middle.
Placing the stacked billet in a square tube or box that has the ends welded shut is the other common way to dry weld. The bars are cut to snugly fit the tube or box, stacked full, and the end is welded shut. Some put a tiny hole in a corner to let gases escape, but from what I have heard, most just want it full and airtight. There are all sorts of techniques like wrapping the billet in a sheet of paper, coating the inside of the tube/box with whiteout or titanium white paint, and spraying the inside with WD-40. These are for a more detailed tutorial, but are part of the process.
Once sealed, the billet can sit around until you are ready to heat it to welding temp and take it to the power hammer or press. Once fully welded solid ( you will hear and feel the difference), the canister/box is cut or ground away, and the billet is reduced, manipulated, folded, etc.as normal.
As part of the canister method, finely powdered steel is often added to the steel in the cannister. This fills the space to 100% and makes for a pattern impossible otherwise.
This is the method for mosaic damascus.”Junk Box damascus”, made from old drill bits, barbed wire, motorcycle chain, ball bearings, springs, and any other steel thing you can think of is made this way. A really cool damascus often called “bacteria” is made with various size small ball bearings and short broken pieces of old drill bits ( use both the solid part and the twist part). The pattern ends up as spirals, rods, and circles.