Flux in forging serves one critical purpose: it prevents oxygen from reaching the metal surface during heating, which would otherwise form a layer of oxide scale that ruins the weld. When two pieces of steel need to be joined through forge welding, flux dissolves the oxides as they form, lowers their melting point, and allows them to flow out of the joint when the pieces are hammered together. Without flux, those trapped oxides create weak spots, inclusions, and failed welds.
How Flux Works at the Chemical Level
When steel heats up in a forge, oxygen in the atmosphere reacts with the iron to create iron oxide, commonly called scale. That dark, flaky crust you see on hot steel is the enemy of a forge weld. For two pieces of metal to bond, their surfaces need to be clean enough that the iron atoms can diffuse into each other under heat and pressure. A layer of oxide between them acts like a wall.
Flux solves this problem in two ways. First, it coats the hot metal surface and physically shields it from the oxygen in the forge atmosphere. Second, and more importantly, it chemically combines with any oxides that have already formed. This reaction lowers both the melting temperature and the viscosity of the oxide, turning it from a stubborn crust into a thin, runny liquid. When you bring the two pieces together and strike them with a hammer, that liquefied oxide squirts out from between the surfaces, leaving clean iron-to-iron contact. The result is a solid metallurgical bond rather than two pieces of steel glued together by slag.
Common Types of Flux
The most widely used flux in blacksmithing is plain borax, the same substance sold as a laundry booster under the “20 Mule Team” brand. It’s cheap, effective, and available at most grocery stores. Many smiths use it straight from the box, though some prefer anhydrous borax, which has been heated beforehand to drive off moisture. Removing the water content reduces the foaming and bubbling that happens when regular borax first hits hot steel.
Silica sand is the other traditional option, with a long history in European blacksmithing. Clean quartz sand works as a flux because silica reacts with iron oxide at high temperatures to form a glass-like slag that flows easily. Some smiths use ground glass or flint silica as alternatives, since they’re chemically similar. Boric acid is another option, sometimes mixed with borax to create custom blends.
More unusual fluxes exist in specific traditions. Japanese swordmakers historically used rice straw ash. Charcoal ash works in a pinch. Even crushed mud dauber nests (which are essentially fine clay and silica) have been used. Commercial forge welding fluxes sold at blacksmithing suppliers are typically blends of borax and iron filings, sometimes with added boric acid, formulated to stay on the metal longer and work across a wider temperature range.
When and How to Apply Flux
Timing matters. Flux should go on the metal after it has reached a bright yellow heat but before it hits full welding temperature. Most smiths pull the workpiece from the forge when the steel glows yellow, quickly sprinkle flux onto the surfaces that will be joined, then return it to the fire to bring it up to welding heat. Applying flux too early, when the steel is only at a red or orange heat, means it may not melt and flow properly. Applying it too late means oxide has already built up unchecked.
The application itself is straightforward. With the hot steel on the anvil or held in tongs, you sprinkle a thin, even layer of powdered flux across the joint surfaces. You want enough to cover the area but not so much that excess flux pools inside the forge. Once the flux melts and forms a glassy coating on the steel, you’ll see the surface take on a wet, shiny appearance. That’s your visual cue that the flux is doing its job. The piece goes back into the forge, heats to welding temperature (a bright, almost white-yellow), and then you hammer or press the joint together quickly before the heat drops.
What Flux Does to Your Forge
Borax is aggressive stuff at forge welding temperatures. It turns into a dark, gooey, sticky liquid that drips off the workpiece and onto whatever is below it. When it cools, it hardens into a dark, glassy substance that can cement objects together and eat into refractory linings. Even high-quality refractory materials like Kastolite 30 are not immune to flux damage over time.
How quickly your forge floor deteriorates depends on how much flux you use and how often you forge weld. A smith who is heavy-handed with the borax and welds large billets regularly could damage a forge lining in just a few sessions. Someone who uses flux sparingly and welds smaller pieces might get dozens of sessions before needing repairs. To protect the forge floor, many smiths place a sacrificial firebrick or a thin steel plate at the bottom of the forge to catch dripping flux. Cleaning out cooled flux promptly, before it bonds permanently to surfaces, also extends the life of the forge.
Forge Welding Without Flux
Flux is not strictly required for every forge weld. Some experienced bladesmiths successfully forge weld Damascus steel billets without any flux at all, relying instead on careful atmospheric control and technique. The key is minimizing the oxygen that reaches the metal surfaces during heating. A reducing atmosphere inside the forge (slightly fuel-rich, with less available oxygen) helps prevent oxide formation in the first place.
One common approach for fluxless welding involves sealing the billet. Some smiths TIG weld the edges of a stacked billet shut before putting it in the forge, trapping very little air between the layers. Others spray a thin coat of WD-40 or kerosene onto the billet surfaces before stacking. These hydrocarbons burn off during heating and consume oxygen in the immediate vicinity of the joint, creating a local reducing atmosphere between the layers.
Fluxless welding has a practical advantage beyond saving money on borax: it eliminates the risk of flux inclusions. These are tiny pockets of trapped flux inside a finished weld that create weak points in the steel. In high-layer-count Damascus, where dozens or hundreds of layers are folded together, even a small flux inclusion can show up as a flaw in the final blade. Experienced smiths at the American Bladesmith Society have demonstrated that clean, strong welds at high layer densities are achievable without any flux, provided the forge atmosphere and technique are dialed in.
For beginners, though, flux remains the safer and more forgiving option. It compensates for imperfect forge atmospheres, inconsistent temperatures, and the slower working speed that comes with learning. Borax is cheap insurance against a failed weld, and mastering its use is a natural first step before experimenting with fluxless methods.