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Chapter 4

Fluxing the Melt


          In metallurgical circles, “flux” is defined as “a substance that can be added to a molten alloy to entrain impurities in a fusible mass, making them easy to remove”. When we dig up an ore out of the ground and process it, there are invariably problematic impurities carried along with it. The nature of these impurities will vary from ore to ore, but the general concept of using a flux to combine with these impurities to form a fusible slag, allowing their easy removal has value throughout the industry. Fluxes have been used for millennia to purify ores and metals, and slag heaps dating two thousand years before the birth of Christ are known.

          The use of a flux to purify metals is a simple, brute force chemical separation. As with any separation process, fluxes can be alkaline (e.g. calcium carbonate), acidic (e.g. silica) or neutral (e.g. calcium fluoride). What kind of flux gets used depends on the nature of the ore, its impurities and the requirements for the separation. Silicate fluxes are commonly used throughout the metal industry, but have little application for lead processing because their melting temperatures are much too high.

          Fluxes can also be oxidizing or reducing, and can be used to selectively remove a targeted impurity by oxidizing it or reducing it. Oxidizing fluxes include the various peroxides (lead, manganese and sodium are the most common), and nitrates (sodium and potassium) which are used in refining precious metals. True reducing fluxes are few in number, but include compounds like sodium or potassium cyanide; however their danger and cost limit their use to high return processes like refining precious metals. Although not strictly satisfying the formal definition of “flux” (since they don’t form a fusible slag) there are a number of reducing agents that are also useful in processing metal alloys. Such reducing agents would include coke, coal and charcoal. We will return to this concept of using a reducing agent to process bullet metals shortly.

          Perhaps the most commonly encountered use of flux would be in welding and soldering. Here the “impurity” is the inherent oxide coat on the metal being worked and the purpose of the flux is to remove this oxide coat to expose a bare metal surface. Molten metal (e.g. solder, or molten steel) wets the surface of bare metal much more effectively than it does an oxide coat, allowing for more intimate contact between the molten and solid metal phases. Therefore, the soldered or welded joint is much stronger if a flux is used to remove the oxide coating.

          The important thing to recognize is that all fluxes are not born equal. Just because something is used as a flux in one application, doesn’t mean it will have any value whatsoever as a flux in a different application. For example, a calcium carbonate flux used to remove silaceous impurities from iron ore would be useless for removing calcium from lead battery plates. A flux is used to effect a chemical separation of specific contaminants from a specific metal (or alloy). As such, it must be tailored to fit the metal, the impurities and the temperature of the process in which it is being used. Just because a material shows up in a can with “Flux” printed on the label doesn’t mean it will perform the separation you are asking of it.

          A related concept used in the metal industry is that of the “cover material”. A cover material forms a physical barrier between the surface of the melt and the atmosphere. Molten metal is hot, and hot metal oxidizes more rapidly than does cold metal. Since the rate of oxidation of the molten alloy will be proportional to the amount of surface area exposed to the atmosphere, the cover material effectively inhibits the oxidation of the molten alloy. The cover material can be something as simple as an inert atmosphere (e.g. argon or nitrogen), a liquid pool (e.g. molten paraffin on top of lead) or a floating layer of solid material (e.g. granular clay, aka “kitty litter“). In each case this cover layer forms a physical barrier between the molten metal and the oxygen in the atmosphere, thereby preventing the combination of the two. Some cover materials (e.g. charcoal) also serve as a sacrificial reductant and react with oxygen, essentially forming an oxygen depleted zone immediately above the barrier layer.

          OK, so much for the definitions and generalities, what do we want to accomplish by fluxing our bullet metal? What are we asking our flux to do for us? To answer these questions, let’s review a little basic chemistry first (I promise to keep this relatively painless). The elemental state of a metal is that in which it has it’s original compliment of electrons, it is neither positively or negatively charged. This is also referred to as the metallic state. Removal of one or more of those electrons is called oxidation, and the most common form of oxidation is for a metal to combine with oxygen (hence the name). Addition of one or more electrons is called reduction, so if we have a metal oxide and want to get back to the metallic state, we must reduce it and we do this by adding some material that can give up electrons easily. Different metals undergo oxidation with varying ease. By placing the metals in descending order of reactivity, we obtain what is called the “activity series” (also called the "electromotive series"). Those metals high on the activity series are easily oxidized, while those lower on the activity series are less easily oxidized. Of importance to the current discussion is the fact that calcium, magnesium, aluminum and zinc are all fairly high on the activity series (i.e. easily oxidized), while lead and tin are much lower (less easily oxidized, or conversely, their oxides are more readily reduced back to the metallic state). This difference in reactivity can be exploited to effect the desired separation. When a metal is oxidized it forms a positively charged ion (called a “cation”). These cations can be bound by negatively charged ions (called “anions”). OK, now that didn’t hurt much, did it?

Remove impurities from lead -- Ah yes, “impurities”! That wonderful catch-all heading that encompasses everything except the desired metals. If we want to effect an efficient separation we need to know what these impurities are, which depends heavily on the source of the lead. Battery plates are commonly contaminated with calcium; some kinds of wheelweights contain small amounts of aluminum; Babbitt metals can have zinc or copper; and range scrap can have a little of all of the above in it (not to mention dirt and gilding metal jacket material). The good news is these impurities are all electropositive metals, that are more easily oxidized than is lead (i.e. they are higher on the activity series) and the oxidized metals are all Lewis acids meaning they can be entrained in a sorbent matrix that has suitable anionic binding sites for them. We want to accomplish this without removing any of the tin, antimony or arsenic present in our bullet metal (WW alloy, linotype, etc).

Reduce tin -- Tin helps to keep surface tension and viscosity of the alloy down so it can fill out the mould cavity properly. If the tin metal gets oxidized to tin oxide, then it is no longer soluble in the melt (oxidized tin is insoluble in lead and forms a “skin” across the surface) and thus is no longer able to impart its desirable qualities to the alloy. Therefore, we want our flux to be able to give up electrons and reduce any oxidized tin back to the metallic state to keep it in the molten alloy. 

Prevent oxidation -- Ideally, the flux material could also be a cover material and form a barrier layer to protect the molten metal from subsequent oxidation, thereby maintaining optimum casting properties throughout the course of the casting session. We also want to prevent the oxidation and loss of arsenic. Arsenic oxides have very high vapor pressures and are readily lost through evaporation, not only depleting the alloy of a potentially valuable component (arsenic allows the alloy to be heat treated, if desired), but also creating a significant health hazard to the caster. A reducing cover material prevents this loss.

So, in summary, the job description of bullet metal flux is to remove, reduce and protect.

          OK, so how do all the different materials that have been used to flux lead alloys work, and which ones work best for the bullet caster? Pretty much everything that smokes, fizzles, pops and burns has been used to flux bullet metal. What do each of these candidate fluxes offer and how do they work?  Or do they?

          One of the more common classes of “flux” (the quotation marks are being used here because these materials don’t form a fusible mass and hence don’t fully satisfy the formal definition of “flux”) described in the older cast bullet literature are the various oils (e.g. used motor oil, vegetable oil, etc.) and waxes (e.g. paraffin, beeswax, etc.). Whoever came up with using used motor oil to flux his lead pot was either a lifelong bachelor, or must have liked sleeping on the couch, ’cause that CAN’T be a good way to make points with one’s Better Half! Aside from smoking like a chimney and stinking to high heaven, used motor oil also has the disadvantage of being a source for contaminating metals  (ferrous alloys, aluminum alloys, bearing metal alloys, even magnesium depending on what motor it came out of). Oiled sawdust was another popular choice in years gone by. It would have suffered from many of the same smoky, stinky drawbacks that used motor would have. Let’s all do ourselves (and our families) a favor and just scratch those two off the list….

          Various waxes have also been used to clean bullet metal. Most commonly these have been paraffin, beeswax, various forms of tallow, or even lard. These have the advantage of being cheap, universally available, and working reasonably well (depending on the alloy). These materials are very good at satisfying two out of the three selection criteria for bullet metal flux in that they are excellent reductants and can reduce any oxidized tin present, and they can be used in sufficient quantity to form an excellent barrier layer, thereby preventing any subsequent oxidation of the alloy. Unfortunately, they offer no means for removing any Ca, Zn or Al impurities. If one is working with a relatively clean source of bullet metal (e.g. linotype or foundry metal), then the waxes can serve admirably in this capacity. However, if one is using a dirtier source of lead (e.g. range scrap, battery plates, or WW alloy), then there are probably better choices. Then there is also the minor issue of distraction; using lard as a cover material makes the lead pot smell like a deep fryer. To this displaced Southern Boy, the odor of fried chicken coming from the lead pot makes it difficult for me to concentrate on the matter at hand. One should not be licking one‘s fingers while casting bullets….

          One of the materials that is currently sold as bullet metal flux includes pine rosin. While pine rosin smells nice (it makes the lead pot smell like a pine campfire) and does a reasonably good job, it operates pretty much the same way that the oils and waxes discussed above do, and is therefore limited in its ability to remove detrimental impurities.

          Some of the commercial fluxes on the market today contain boric acid, borax, or other borate containing materials (e.g. Marvelux). These materials are fluxes in the true definition of the term since they melt to form a borate glass which entrains any oxidized materials and extracts these contaminants into the molten glass phase. These fluxes have the significant advantage of being smoke-free and odorless. They are also extremely effective at removing contaminants. This is because the borate anion binds all metal cations and extracts them into the molten borate glass. Unfortunately, this includes any oxidized tin, and so the alloy is depleted of this valuable component. The borate fluxes do nothing to reduce the oxidized tin, nor do they protect the melt from further oxidation. You’ll note that this behavior is exactly opposite to that of the waxes, described above. 

          Is there anything that combines these two modes of operation so that we can get all three of the desired attributes? Fortunately, there is. What’s more, you probably already have a pile of it in your shop. It’s good ol’ fashioned sawdust (hold the motor oil, thank you). The benefits of sawdust are that it‘s a sacrificial reductant that can reduce any oxidized tin back to the metallic state, and it‘s cheap enough that the caster can use enough to form an effective barrier layer to protect the alloy from subsequent oxidation. What’s more, as the sawdust chars on top of the melt, it forms activated carbon, which is a high surface area, porous sorbent material that has a large number of binding sites capable of binding Lewis acid cations like Ca, Zn and Al. So it not only keeps the tin reduced and in solution, but it effectively scavenges those impurities that raise the surface tension and viscosity of the alloy (Al, Zn and Ca), keeping the alloy in top shape for making good bullets. Vigorously stirring in a heaping tablespoon of sawdust into a pot full of bullet metal does a fine job of conditioning and protecting that alloy. Sawdust doesn’t really qualify under the formal definition of “flux” as it doesn’t produce a fusible slag, but it does very cheaply and very effectively accomplish the three primary goals that we set out for cleaning up bullet metal. Reduce, remove and protect, sawdust does it all!


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