Copper-based alloys... bronzes and brasses etc.

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Will Whitfoot
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Copper-based alloys... bronzes and brasses etc.

Post by Will Whitfoot »

I've been doing this stuff a long time, and it took me a while to wrap my head around the terminology used with copper-based alloys... especially confusing: the technical vs common usage of the term "bronze". I have a whole shelf of metallurgical reference books here, from my dad's library (he was a professional metallurgist). So perhaps I can answer some questions. But I can anticipate a few of them, because they were the ones I had.

Metallurgically speaking, "bronze" refers to any and all alloys in which the primary component is copper. There are hundreds of named types, including binary, tertiary, and even quaternary types.
"Brass" is a sub-term that refers to a bronze in which the only secondary element is zinc. (typically 15% to 40%, with name changes about every 5%)
"Commercial bronze" (like, what you can buy from Online Metals) is actually a true brass with a low (10%) zinc content.
Classical bronze (what we think of historically) is an alloy in which the secondary element is tin. Tin content in classical bronze usually ranges from about 10% to 18%.
"Billon" is the term for a silver bronze, typically 80% copper and 20% silver, that can be "blanched" or treated to deplete the copper at the surface and appear silver.
Nickel-silver is a nickel bronze tertiary alloy with no silver in it at all... it's simply intended to approximate the color of silver. Typically 16% to 18% Ni with some zinc will whiten the copper to look more or less like silver.
Cupro-nickel is a nickel bronze binary alloy with 25% Ni and the balance copper, used to make US 5-cent pieces. Nickel has the property that its neutral gray color will overwhelm the color of the alloy.

So it can become extremely confusing trying to order metal for projects! Sometimes the dealers don't actually know what alloy they're selling. It is quite difficult to find true tin-bronzes! The primary usage today seems to be cymbals. I've occasionally been able to find some here and there.

Almost all copper alloys display a eutectic behavior, in which the addition of alloying elements lowers the melting point of the alloy. So bronze melts lower than copper. Pure copper is actually rather difficult to cast... the addition of tin or zinc lowers the melting point and increases the fluidity of the melt. Interestingly, silver and copper both have very similar melting points, but mixing them together lowers the melting point until the eutectic, or lowest point is reached at 72.5% silver... which just happens to be the content of Mexican silver coins!

Hopefully that little primer has helped some people through the brass-bronze confusion.... let me know if there are any other points of difficulty. It's nothing to be embarassed about... there are whole books on this stuff and it can be very confusing even to metallurgical professionals.
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Re: Copper-based alloys... bronzes and brasses etc.

Post by Peter Remling »

Interesting read. Thanks for the education !
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Re: Copper-based alloys... bronzes and brasses etc.

Post by Iodo »

Good summary, I had to research all that for a project on alloys 2 years ago, in my chemistry A-level, it wasn't easy :lol:
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Re: Copper-based alloys... bronzes and brasses etc.

Post by Manveruon »

Hah! I didn't know any of that at all, so it was extremely enlightening! Thank you for the little lesson!
With that in mind, can you tell us anything about the history of such alloys? As in, when did the more "brassy" alloys start to come into play? You mention that classical bronze seems to be primarily copper and tin, but what about some of the more varied types? Any ideas?
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Re: Copper-based alloys... bronzes and brasses etc.

Post by Elleth »

fascinating..

I know I've been a little disappointed in the brassy-yellow color of Shapeways "bronze" - I bet that's why.
Billon sounds fascinating - is it used much these days at all?
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Re: Copper-based alloys... bronzes and brasses etc.

Post by Kortoso »

Thank you for this1
I understand that in casting brass, you have to make allowance for the zinc funes, as it's quite toxic.
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Re: Copper-based alloys... bronzes and brasses etc.

Post by Will Whitfoot »

I'd need to do more detailed research to be sure... but just off the cuff:

Zinc was not known as a separate metal until the late 1700s. There are indications that it was thought to be a "kind of silver". Here in the Ozark Mountains of Northern Arkansas and Southern Missouri there are all kinds of legends of lost silver mines.... none of which exist. But there are numerous zinc deposits and tales of "Ozark Silver", and even (zinc) silver bullets (as in the Lone Ranger). It appears that brass was developed more or less independently in several areas of the old world. My guess is that where copper was being smelted, people would throw other heavy rocks in the mix... including sphalerite and smithsonite... which would give the copper a desirably golden color. The ancients called it "aurichalcum" and thought it was a "kind of gold".

Yes if white smoke is coming off the melt, it is a zinc alloy. The common alloy "Commercial Bronze" at 10% Zn is probably what is being used when the term "bronze" is tossed around casually.

Billon was used as a coining metal from Roman times into the Renaissance. Turns out that you can go as low as 20% silver... and the silver forms in microscopic blebs of eutectic surrounded by a matrix of copper. If you make blanks of this material and cook them in a fire, the copper will oxidize. If you then boil them in a bath made from sea water and the dregs from winemaking (aka tartaric acid or "cream of tartar") it will dissolve the copper oxides away leaving a "foam" or crust of eutectic silver. If you then strike the blank, the porous foam is compressed and appears just like pure silver! This "blanching" process was used primarily to debase the currency by making coins that appeared to have more silver in them than they actually had! Billon is seldom used today. We used it to make the silver "talents" for Patrick Rothfuss' THE KINGKILLER CHRONICLE series. But the process was pretty brutal. My skin did eventually heal! There's a reason that minter's assistants in ancient times were often slaves!
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Re: Copper-based alloys... bronzes and brasses etc.

Post by Kortoso »

Very interesting!

There's also an alloy of copper and gold called by the "Easterlings" shakudo.
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Re: Copper-based alloys... bronzes and brasses etc.

Post by Will Whitfoot »

Yes I've played with Shakudo also. I have a few grams of leftover scraps. (We made the gold marks for Patrick Rothfuss' Kingkiller Chronicle from Shakudo) Typical composition ois 94% copper, 5% gold, 1% silver. The raw metal just looks like copper... nothing special about it. But it takes an unusually brilliant plum colored patina, either with heat or with an alchemy involving daikon radishes and salt. The biggest problem I had with it was its brittleness... it did not forge or roll well.
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Re: Copper-based alloys... bronzes and brasses etc.

Post by Elemmakil »

This is a very useful little thread, by the way. I'd always wondered what "nickel silver" is - thanks for the clarification.

As an aside, I seem to recall that medieval brasses tended to range from 10-30% Zn (corresponding to the "Commercial Bronze" you mentioned all the way to most commonly available brass alloys, which tend to have more Zn)

I have some commercial bronze sheet in the shed, along with some phosphor bronze which actually has tin in it, and is a reasonable approximation of known Bronze Age alloys. This last is intended to make bronze scales for an Egyptian scale harness - I have a bunch raw cut into rectangles, but not trimmed nor having the central rib embossed into them, or holes punched. The stuff is *EXPENSIVE* though, at least to be able to make a whole harness.

Have a bunch of Si Bronze, which is strong, and doesn't look bad (distinct from brass), but is very a-historical (high Cu content, with some Si and other trace stuff IIRC)
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Re: Copper-based alloys... bronzes and brasses etc.

Post by MiketheBlacksmith »

I would like to add a little information for those who would like to make items out of these metals.Tons of these materials have moved through my shop. For a simple definition Bronze is copper/tin Brasses are copper Zinc alloy. Yes other metals are in the mix to give different properties to the brass/bronze.

If you want to machine such as drilling or turning 360 brass is the stock all others are compared to. All the alloys I used were easy to solder. From lead alloy solders to high temp silver brazing will work depending on strength you need/application. Copper can be TIG welded.

The stock ,bar,rod sheet and plate at one time was readily available in different hardness. Full soft,half hard etc. to fit the operation you want to preform. The softer the temper the easier it is to bend/form under pressure. Dead blow mallets or heavy rawhide mallets do a good job shaping these metals. The metals work harden but you can anneal them back to a softer state with a torch. At one time I had three wholesale distributors within 45 minute drive. Any shape/size in stock .Those days are long gone.

If you want to drill these metals you need to modify the cutting lip on twist drills. You grind a small 90 degree flat on the cutting edge of the drill. This way you can drill through without the bit catching and pulling through the hole.Also works on plastics.

Finishing these metals are yet another subject. They will discolor from handling with no finish applied and care in the process is needed or finger prints will appear under the finish.
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