Bronze and Brass and Copper, oh My!
What IS Brass and Bronze, how are they made, and what was used in period? Probably one of the most common questions I get, and also…probably the most complicated.
Why? Well…partially because in period they often used the words brass and bronze interchangeably, so research is difficult. Also…when you are looking at extant pieces, to see WHAT metal used….they are often called Copper, or “a copper alloy” Meaning…some kind of brass or bronze. Why? Mostly because they don’t have the time and money to break down and test EVERY single buckle or button pulled from an archeological site to see metal content (Being that they have millions of said buckles and buttons). So they describe the item as copper or copper alloy, which it is.
Does this mean all these buttons and buckles listed as copper, or copper alloy are Copper? No, it doesn’t. In truth, pure copper was not as commonly used for that as brass or bronze. Bronze was most common cast for your buckles, buttons, belt mounts and day to day items and such. Obviously when you get into specific jewelries you have other metals that are more common, such as gold and silver (though it was VERY common to cast items in bronze and silver or gold plate them)
First, lets talk about what the difference is between Brass and Bronze. They are both an alloy of copper…typically with about 90-95% copper, mixed with either Tin (to make Bronze) or Zinc (to make Brass)
Note that Pliny the Elder (Gaius Plinius Secundus, AD 23 – August 25, AD 79), who wrote Natural History said that Bronze, which he referred to as Aurichalcum, could be find as a True Mineral (not made as an alloy) This obviously is not true, and was stated so by Vanoccio Biringuccio in his book of Priotechnia (16th Century).
“It is the opinion of some concerning both of these that they are true minerals because Pliny in his Natural History calls it Aurichalcum and says that is has its own ore; he does not say, however , where it is found, and I have not heard from anyone else that it has been found anywhere, and , surely, if it had been found when he wront, it would also be found today.”
In the case of Zinc, in period it is usually referred to as Calamine. The name Calamine came from the Belgian town of Kelmis, whose French name is La Calamine, which had a very large Zinc mine which was active in the middle ages.
Theophilus, in On Divers Arts (First published in 1150AD) talks about Calamine:
“ There is also found a kind of stone of a yellowish color, sometimes reddish, which is called calamine. This is not broken up but, just as it is dug out, it is put on closely stacked, thoroughly blazing wooden logs and is burned until it is completely red-hot. After this the stone is cooled and broken up very small and mixed with charcoal dust. Then it is allied with the aforesaid copper in a furnace built in this way.”
So the process of alloying was kinda hit and miss. Their measurements were not exact, and as a result the percentages of Copper to Zinc or Tin varies from batch to batch. Interestingly enough, when buying modern Brass or Bronze, you get MANY different mixtures of alloys depending on who you buy your metal from.
Giringuccio talks about having witnessed the process done by masters:
“The Masters whom I saw had made in a large room a furnace, much longer than it was wide, and built with a certain kind of stone that by its nature resisted conned firing without melting and burning up. Where the fire entered the furnace it was almost entirely open. The body of the furnace was half or more underground; the vault was low; at the top and bottom there were everywhere little air holes; and above in the vault there were two square opening through which the crucibles containing the copper to be colored could be put in and removed. These were then close with little fitted clay shutters. The crucibles were made of Valencia clay, or they were brought ready-made from Vienna; they were very large and I believe that those which I saw were about two-thirds of a pound, and I understand they had a capacity of fifty or sixty pounds of metal.
For this process they placed in each one of the vessels twenty-five pounds of German rosette copper broken in pieces, and they filled up the rest to within two rita of the rim with powders of a mineral earth, yellowish in color and very heavy, that they called Calamine. The rest of the empty space in the crucible they filled with powdered glass. Then they put the crucibles in the vault through the above-mentioned openings and arranged them in pairs on the bed at the bottom. They then applied a melting fire for twenty-hours and after this time they found the material entirely fused, and the copper, which was red before, had become a smooth and lovely yellow, almost like 24-carat gold in color.”
(Note: Biringuccio refers to covering this with powdered glass. I believe he is referring to what was called Glass Salt or Sal Alkali. Salt was often used as a flux, instead of Borax, to prevent oxidation)
In Theophilus’ book, On Divers Arts, he also talks about making of Brass and Bronze:
“And when the crucibles are red-hot take some Calamine, about which I spoke above, that has been [calcined and] ground up very fine with charcoal, and put into each of the crucibles until they are about one-sixth full, then fill them up completely with the above mentioned copper, and cover them with charcoal. From time to time poke the holes below with a slender hooked stick, so that they may not become blocked and also so that thrashes may come out and more draft may enter. Now, when the copper is completely melted, take a slender, long, bent iron rod with a wooden handle and stir carefully so that the Calamine is alloyed with the copper. Then with long tongs raise each crucible slightly and move them a little from their position so that they may not stick to the hearth. Put Calamine in them all again as before and fill them with copper and cover them with charcoal. When it is once more completely melted, stir again very carefully and remove one crucible with the tongs and pour everything into little furrows cut in the ground. Then put the crucible back in its place. Immediately take calamine as before and put it in, and on top as much of the coper as you have cast as it can hold. When this is melted as before, stir it and add calamine again and fill it again with the coper you cast and allow it to melt. Do the same with each crucible. When it is all thoroughly melted and has been stirred for a very long time pour it out as before and keep it until you need it.”
As you can see…not very exact. The results would vary greatly, as do the metal content of our extant pieces. It was found that alloying with Tin, rather than Zinc, made the alloy much stronger. Thus did that become the more common alloy method, though the processes are the same for Zinc/Brass and Tin/Bronze. This is one of the primary reasons that museums don’t bother a great deal with breaking down metal content of averages pieces. They ARE simply an alloy of copper… how much? It will change from piece to piece.
TYING UP LOOSE ENDS
A couple of things to add in closings. First is to address word origins. They started using the word brass around the 10th Century.. Bras, Braes, and Bres Copper. Before that Pliny used the word Aes, which we see Theophilus use (He WAS, after all, a Benedictine Monk). The word Bronze, though, wasn’t used until the early 18th Century, first by the French.
Around 1300AD they started using the word Latten (From old French, Laton) I’ve heard reference from people that Latten was used to specifically mean a certain brass, specially for making bells. This is actually not true. The word is used to describe BOTH brass and bronze, and is used for those alloys used for most anything. Not just casting, but also sheet metal and wire and such.
Last, Pliny refers to Aurichalcum. This actually came originally from the Latin word Orichalcum, which means “Mountain Copper”. The Romans transliterated it as “Aurichalcum” which they took to literally mean “Gold Copper” (because of its coloring) While the word Aurichalcum is used throughout history, it is not as commonly used as the other words mentioned above.
Last, I want to talk about LEAD. Yes, that very bad word LEAD. While lead is NOT found naturally in Copper, as Theophilus thought:
“It cannot be gilded, since the copper had not been completely purge of lead before the alloying.”
The fact remains that Lead is still VERY common in the metals we use today. PLEASE…trust me when I say, you do NOT want to cast metals that contain Lead! It just isn’t worth it. Stay away. How common is Lead in casting metals today? I find that somewhere between 60-70% of the Brass and Bronze alloys that are sold online today (from common places like Otto Frei and Rio Grande) have lead. PLEASE look at your alloy contents before buying.
Further…do NOT cast random metals that people give you or that you “Find” in second hand store, or brass from old handles and such. These almost ALWAYS contain Lead.
This was a long one folks. And I still feel like there is so much more to write. I think I’ll take a break from this for now, and revisit as time goes by. There is SO much more I could drill down on this stuff…but…it all takes time…