The Essential Guide to Using Flux in Forge Welding
Forge welding is one of the most essential and traditional techniques in blacksmithing. It allows you to join two pieces of metal by heating them to a high temperature and hammering them together until they fuse. However, one of the challenges in forge welding is preventing oxidation during the heating process — and this is where flux plays a critical role.
In this post, we will explore when to use flux, why it’s important, and how to apply it properly to ensure strong, clean welds every time.
What is Flux and Why Is It Used?
Flux is a chemical agent used in forge welding to protect the heated metal surfaces from oxidation and to help clean away scale and impurities. When metal is heated to welding temperature (typically around 2200°F or 1200°C), it reacts with oxygen in the air and forms scale — a brittle layer of iron oxide that prevents proper bonding between the pieces.
Flux works by:
Preventing scale formation on the metal surface
Dissolving existing scale and impurities
Helping molten metal flow between the weld faces to create a strong joint
Without flux, the oxide layer will interfere with the weld, resulting in weak or failed joins.
When to Use Flux in Forge Welding
You should use flux whenever you are performing forge welding on steel or iron, especially in the following cases:
Joining two pieces of clean steel together: Flux prevents oxidation and allows the metal to fuse.
Welding higher carbon steels: These form thicker oxide layers that flux helps dissolve.
When using a coal forge: The atmosphere can be more oxidizing compared to gas forges, increasing scale formation.
Welding small or thin pieces: Oxidation can burn through or weaken thin metal if not protected.
Some modern forge welders may use fluxless welding in controlled atmospheres (like inert gas or vacuum), but for traditional blacksmithing with open forges, flux is almost always necessary.
Types of Flux Commonly Used in Blacksmithing
The most popular flux for forge welding is borax (sodium borate). It’s readily available, easy to use, and very effective. Borax melts at forge welding temperatures, creating a glassy layer that protects and cleans the metal surfaces.
Other fluxes include:
Commercial flux blends: Some smiths use proprietary mixes combining borax with other substances for improved performance.
Boric acid: Sometimes mixed with borax for enhanced scale removal.
Homemade flux recipes: Some blacksmiths experiment with mixtures including sand, salt, or powdered glass, though these are less common today.
For beginners, pure borax is the best starting point.
How to Use Flux in Forge Welding: Step-by-Step
1. Prepare Your Metal
Make sure the edges to be welded are clean and free from rust, oil, or heavy scale. Light surface scale is acceptable because flux will help remove it.
2. Heat Your Pieces to Welding Temperature
Place your pieces in the forge and heat them evenly. You’re aiming for a bright yellow-orange glow (around 2200°F / 1200°C). Avoid overheating, which can burn the metal or cause excess scale.
3. Apply Flux
Once your metal reaches welding temperature, remove it from the forge and sprinkle a thin layer of borax onto the weld surfaces. The borax will immediately start to melt and bubble, helping clean the metal.
4. Bring the Pieces Together and Weld
Quickly return the pieces to the forge for a few seconds to get the flux glowing and molten. Then, remove them and hammer firmly. The molten flux will fuse the surfaces, allowing the metals to join.
5. Inspect the Weld
If done correctly, the weld should be strong, with minimal scale or slag. You can knock off any remaining flux residue with a wire brush after the piece cools.
Tips for Successful Forge Welding with Flux
Don’t use too much flux: Excess flux can cause inclusions or weaken the weld. A light dusting is enough.
Keep your weld faces aligned and clean: Good fit-up is essential for strong welds.
Work quickly but carefully: Flux and metal cool rapidly; strike while hot!
Practice makes perfect: Forge welding is a skill that improves with experience and timing.
Conclusion
Using flux correctly in forge welding is a fundamental skill that helps create clean, strong metal joints. Borax remains the tried-and-true flux of choice for traditional blacksmiths, protecting steel surfaces from oxidation and allowing the weld to form flawlessly.
With practice, the application of flux will become second nature, helping you unlock new possibilities in your forging projects — from toolmaking to decorative ironwork.
If you would like to discuss a potential project, please do not hesitate to contact us.
