A Cast Bullet Guide for Handgunners 04

7/27/2019 A Cast Bullet Guide for Handgunners 04

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From Ingot to Target: A Cast Bullet Guide for Handgunners

Index of Additional Glen E. Fryxell Shooting Articles

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 dont 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, doesnt 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 doesnt 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
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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 theatmosphere, 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, lets 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 its 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 metaloxide 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 didnt 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 activityseries) 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.


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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 dont form a fusible mass and hence dont fully satisfy the

formal definition of flux) described in the older cast bullet literature are the variousoils (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 CANT be a good way

to make points with ones 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. Lets 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 probablybetter 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 ones 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 truedefinition of the term since they melt to form a borate glass which entrains any oxidized


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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 allmetal

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 furtheroxidation. Youll 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. Whats more, you probably

already have a pile of it in your shop. Its good ol fashioned sawdust (hold the motor

oil, thank you). The benefits of sawdust are that its a sacrificial reductant that can

reduce any oxidized tin back to the metallic state, and its cheap enough that the caster

can use enough to form an effective barrier layer to protect the alloy from subsequent

oxidation. Whats 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 onlykeeps 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 doesnt really qualify under the formal definition of flux as it doesnt

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!

Table of Contents

Continue to Chapter 5 - Cast Bullet Lubrication

Index of Additional Glen E. Fryxell Shooting Articles

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A Cast Bullet Guide for Handgunners 04