Copper-based alloys... bronzes and brasses etc.
Posted: Mon Nov 06, 2017 8:41 pm
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.
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.