What Is Blacksmith Flux?

Blacksmith working on an axe with his forge in the background.

If you recently took up metallurgy, you might have started to hear discussions about fluxes.

Do you need them, what type to use, and the various advantages of different fluxes are all common discussions on blacksmith message boards.

Here, I’ll break down some of the basics so that you can get the most out of your flux.

What is blacksmith flux? Blacksmith flux is a useful and ancient tool in metallurgy. It is used to clean impurities from metals or prevent the formation of oxides, both of which make it easier to join the metals neatly.   

There are different types of fluxes. The material of the flux will determine what cleaning or reducing properties it has.

Learning a bit about the historic and modern meaning of flux, how to use it, and a few tricks about what types of fluxes work best will all help you become an even better blacksmith. 

The Definition of a Blacksmith Flux

Simply put, a flux is an aid that blacksmiths use to purify the metals they are trying to join. It is an ancient and diverse tool that is useful in a variety of practices.

Regardless of what specific aspect of metallurgy you hope to practice, a flux will serve as a helpful purifier; sometimes, it will even help with heat transfer.

The etymology of “flux” can help explain more deeply what a flux is.

Flux is derived from fluxus, which is the Latin word for flow. (If you ever wondered where the saying “influx” comes from, it shares this same root.) 

Flux was synonymous with the flow because the product that results from adding a flux to the smelting process is an odd viscous concoction of ash, impurities, and flux.

Smelting was the original use for flux, though today, the uses are far more diverse.

Whatever you choose to use a flux for, “flow” will still be an essential part of what it does.

A flux is useful because it can flow in and out of small places and, on a molecular level, clean up the metal touched as it moves about. 

Composition of a Flux

All fluxes, regardless of function, consist of four elements.

  1. Activator- This is the part of the flux that will expose the unoxidized metal. For highly corrosive activators, the activators will be active at low temperatures. Less corrosive activators will only become active at high temperatures.
  2. Vehicles- The flux’s vehicle has three purposes: it will serve as an oxygen barrier, carry away impurities, and distribute the heat. This vehicle is what gives a flux its consistency; thus, it will manage all of the aspects to its physical state.
  3. Solvent– This is a smaller part of the flux that the heat should dry out before the flux is ready to use. It helps facilitate joint processing and disposition. 
  4. Additives– The effect of the additive changes depending on which additive the flux uses. Some use thickeners, stabilizers, antioxidants, or other additives to suit the specific needs.

Uses for a Flux

Flux is commonly used for three things.

  • To remove impurities.
  • To prevent the formation of oxides.
  • To disperse heat during high-temperature welding.

What property you need your flux to provide will determine which flux you should select. 

There are so many types of fluxes that instead of attempting to name them all, I’ll discuss the various uses for a flux.

Hopefully, by learning what a flux can do and why it does it, you’ll be able to determine the best one to fit you and your job. 

Removing Impurities

In smelting, flux is added to the smelting furnace to remove impurities from the material being smelted.

In this process, the most common fluxes are inorganic chlorides such as limestone or fluorite.

When these materials are added to the furnace, they pick up the impurities from the metal you intend to smelt. The resulting mixture of impurities and ash is referred to as “slag.”

In the same vein, fluxes can remove impurities from nonferrous metals.

Alternatively, you can use a flux to add desirable trace elements into a metal. For example, you can use flux to add titanium into metals. 

Preventing Oxidation/Removing Oxides

In high-temperature metal joining (such as welding or soldering), fluxes serve a dual purpose.

For one, they coat the material and prevent oxygen contact. Thanks to this oxygen barrier, your welding will not oxidate.

It is easier to join two metals when you can attach them to each other’s clean surface instead of one that has oxidized. 

In high-temperature metal joining, fluxes also can help wet the metal. They will remove oxides that already existed in the metal.

There are a few simple steps to access both of these properties of fluxes. 

  • To start, you simply need to apply a room-temperature flux to the metal. 
  • Then, as it heats up, the flux will become a strong reducing agent. This will allow it to remove oxides and serve as an oxygen barrier. 

Heat Dispersal

The least common use of a flux is to distribute heat in the area that is being joined. It is useful, though, because as it becomes hot, the heat will transfer more evenly across the entire joint.

Soft Soldering

You can also use a flux for soft soldering. You will, however, need different types of fluxes for these different temperatures.

High-temperature joining requires an inorganic substance while lower heat joining will use an organic substance. 

The organic substances for soft soldering will be combined with another element that is sticky to help the flux serve its purposes without needing to reach high temperatures.

Since it is the high temperatures that transform fluxes into reducing agents, this addition is necessary at low temperatures.

Tips & Tricks for Using Flux

While fluxes are always a useful tool to facilitate metal joining, there are a few simple tips that will help you get the most out of your flux.

Learning a few tips and tricks will also explain some of the common mistakes people make when using a flux, and therefore make it easier for you to avoid these traps.

Heat Correctly

As discussed earlier, fluxes contain solvents. They are useful for facilitating the joint processing and disposition but can also cause problems. 

Be sure that you heat your flux fully and properly as these solvents are meant to dissolve in the preheating process. When that process is done incorrectly, some of the solvents might remain.

This will cause problems once you start to weld as the particulates of solvent will react and cause splatters of solder paste or molten solder.

Use Caution

Be very cautious about what type of flux you are using. Acid fluxes contain chemicals that are harmful to humans.

It is important to take care when handling these as fluxes bind easily to organic material (like your hands).

Wear Protective Gear

Wear protective gloves when using a flux. Molten metal will not bond with skin, but molten flux is like glue; it will adhere to you instantaneously and be difficult to remove.

As a result, molten flux will feel extremely hot even if the metal is at a higher temperature.

Research Flux Use Through History

Check out the history of fluxes. Metallurgy is an ancient art, and fluxes have always been an important aspect of the trade.

If you are interested in learning more about fluxes, try starting at the beginning.

For instance, did you know that the ancient Egyptians used borax not only as a medicine and to aid in the process of mummification but also used it as a flux?

Here is another interesting article concerning the history of metallurgy in South America. It tells of how Incas used fluxes, or similar processes, to coat alloys in gold and silver.

When the Spanish came, they were seeking gold and often mistook these masterfully plated alloys as pure gold or silver, totally deceived by the clever craftsmanship. 

The more you become familiar with the fascinating history and varied uses of fluxes, the more you’ll get a feel of what you need for your project at hand.