The white powder used in forging is borax, a naturally occurring mineral also known as sodium tetraborate. Blacksmiths sprinkle it onto hot steel during forge welding to prevent rust-like oxide scale from forming on the metal surface. Without it, the layer of oxidized metal that builds up at high temperatures would block the two pieces of steel from bonding together.
Why Blacksmiths Use Borax
When steel is heated in a forge, its surface reacts with oxygen and forms a dark, crusty layer of iron oxide, commonly called scale. If you try to hammer two pieces of steel together with scale trapped between them, the weld fails. The oxide acts as a barrier, preventing the clean metal underneath from making contact and fusing.
Borax solves this problem in three ways. First, it melts and spreads across the hot steel, forming a glassy protective coating that shields the surface from oxygen. Second, iron oxides are highly soluble in molten borax, so it dissolves scale that has already formed. Third, when the blacksmith hammers the pieces together, the liquefied borax (now carrying dissolved oxides and impurities) gets squeezed out of the joint. What remains is clean metal pressed against clean metal, which bonds through diffusion at high temperature.
Borax also reacts with sulfur, phosphorus, and other contaminants that would weaken the finished weld, trapping them in the glassy slag that gets expelled during hammering.
Anhydrous vs. Standard Borax
The borax you find in the laundry aisle (borax decahydrate, sold under brands like 20 Mule Team) contains water molecules locked into its crystal structure. When heated, it bubbles and foams violently as that water boils off, which can blow flux off the workpiece and make a mess in the forge.
Most blacksmiths prefer anhydrous borax, which has been pre-heated to around 350 to 400°C to drive off all the water. It melts smoothly at roughly 740°C (1,350°F) without the foaming. You can buy it labeled as “forge borax” or “farrier flux” from blacksmithing suppliers, or you can make your own by slowly baking standard borax in an oven or skillet until it stops bubbling and turns into a fine white powder.
One quirk of anhydrous borax: left exposed to air, it absorbs moisture and gradually reverts to its hydrated form. Store it in a sealed container.
How to Apply Flux During a Forge Weld
Timing matters more than quantity. The steel is heated in the forge until it reaches a yellow glow. At that point, you pull the workpiece out and sprinkle a thin layer of borax onto the surfaces that will be joined. The powder melts on contact with the hot metal and flows into a glassy coating. Then the steel goes back into the forge to reach a bright yellow welding heat before being pulled out and hammered.
A common beginner mistake is using too much flux. Excess borax becomes a sticky, corrosive mess inside the forge. It eats through firebricks over time, and a thick layer doesn’t improve the weld. The goal is just enough to coat the joint surfaces. Experienced smiths also emphasize brushing loose scale off the steel with a wire brush before applying flux, since there’s no point in coating a thick layer of crud.
Other Fluxes and Alternatives
Borax is the most common flux, but it isn’t the only option. Historically, blacksmiths used fine silica sand, which melts at forge welding temperatures and serves a similar oxide-dissolving function. Some traditional smiths even used crushed mud dauber wasp nests, which are naturally high in silica.
Commercial forge welding fluxes often blend borax with iron filings or other additives. The iron particles in these “metal-bearing” fluxes melt and fill microscopic gaps in the weld joint, which can improve bond strength on difficult welds like pattern-welded (Damascus) steel. Boric acid is another option that some smiths use as a substitute or additive. Specialty products like EZ Weld and Crescent Weld are formulated for fine or precision forge work where standard borax might be too aggressive.
What Borax Does to Your Forge
Molten borax is acidic and corrosive to refractory materials. It will gradually dissolve the firebricks lining your forge floor, eating visible pits and channels into them over months of use. Many blacksmiths place a sacrificial firebrick or a steel plate on the forge floor to catch dripping flux. Others simply accept that floor bricks are a consumable item and replace them periodically.
The flux also splatters when the workpiece is hammered. Tiny droplets of molten glass land on the anvil, tools, and anything nearby. These cool into hard, glassy spots that are difficult to remove. Keeping your anvil face clean and working on a dedicated welding station helps.
Health Risks of Borax Dust
Borax is not highly toxic, but the dust is a respiratory irritant. A study of workers with chronic borax exposure found that dust concentrations of 4.0 mg/m³ or higher caused dryness of the mouth, nose, and throat, along with dry cough, nosebleeds, sore throat, and chest tightness. At lower concentrations (around 1.1 mg/m³), these symptoms were uncommon. Among nonsmokers, heavy long-term exposure was linked to symptoms resembling chronic bronchitis.
In a typical blacksmithing shop, exposure levels are far below industrial settings, but good ventilation still matters. Borax itself doesn’t produce dangerous fumes when it melts, but the forge environment generates plenty of other airborne particles from coal, coke, or gas combustion. Working with adequate airflow, whether from a side draft hood, an overhead vent, or simply open doors, keeps dust and combustion byproducts from accumulating at face level.