So today we're going to be talking about copper and bronze. So we have to think about this a little bit. What is the difference between copper and bronze? Well, copper is this material, and it's sort of a reddish material, and it's an element on the periodic table. Bronze is this material, and it's an alloy. It means I've mixed my copper with something. So they're two different materials but have similar properties to begin with. So, let's talk about them. What are some of the properties of copper and bronze? Well, it's obviously metallic, and turns out that copper's pretty malleable. It can bend fairly easily. It's opaque. It's electrically conductive. In fact, it's extremely conductive, both electrically and thermally, so it will conduct heat. It's shiny and it's fairly hard. So these are properties of copper alloys and copper itself. All right now the melting point of pure copper is around 1100 degrees C. It's fairly high. It was named after where it came from, which is Cypress. And as I said, bronze is an alloy. Now, bronze typically is an alloy, and in ancient times, it was an alloy with either arsenic or tin. And it comes from the Italian word bronzo, which stood for bell metal, and at melting point it's actually lower than copper, so this melts at a lower melting point than pure copper. So why do we have a Bronze Age? Well the earliest story of copper involved sort of this role in serendipity. It turns out that copper is actually probably one of the easiest metals to reduce from its oxidized state or its sulfidized state. And so that process is called smelting, and it turned out that copper was very easy to smelt. And so the way it was discovered is probably by accident, but it turns out that there is a green material called malachite that if you heat it up in a reducing ambient like with some carbon as you'll see in the demonstration, you can actually turn that malachite into copper. So we've been talking about copper and bronze, and one of the key questions would be how do you actually make the stuff? This is a sample of malachite which is a copper carbonate sample. And this was one original ores that was used to make copper and bronze. Now the key to make a copper ore is that this, because it's a copper carbonate we have to get the carbonate out of it or the CO3. And the way we do that is we're gonna heat this up. Now, if you heat up Malachite, then you can drive off the CO2, but that leaves you still with copper oxide. And so to reduce that copper oxide, you have to have a form of carbon monoxide. And so what we're gonna do is we're gonna use this crucible which I've crudely made out of a piece of charcoal. When we heat that up, the charcoal is gonna actually decompose and form carbon monoxide. And that carbon monoxide then is going to help reduce the remaining copper oxide to just pure copper, or in this case, probably a copper with some impurities in it, whatever was in the malachite. So let's go ahead and try this. So what I've done is, I've taken a piece of charcoal, carved up a little notch in it, and I've taken some of this malachite and flaked it off. So now I'm gonna take a torch and start heating it up. And so, to do that, I'm gonna have to get this sample very, very hot, so we'll put on a glove to protect my hand. And we will start heating it. Now you can see fairly quickly that what happens is that the malachite is turning pretty quickly into a reddish material. It usually takes a few seconds, or a few minutes actually, for this to turn. There it goes, it's starting to react. You can tell it's reacting because it actually start to flow a bubble. There it goes. So now you can see in there, there's a little puddle, and that puddle is actually copper. So I managed, with just heat and charcoal, to turn malachite into copper. So we'll let this cool for a second, and then I'll pull it out and you can actually see that it's metallic. So we're gonna take some tongs now that it's cooled, and see if we can get it out of this. There it comes. There's our little ball of copper. You can sort of see on tha back side of it. That it's that coppery color. I could take this now with a hammer and beat it out and it would be a piece of metal. Okay, so here's our sample of copper that were just made in our crucible, and I'm gonna put it on top of this piece of steel and see if I can actually hammer it out. So you can see that I'm actually now got a piece of malleable copper. I had to knock off some of the carbon. All right. And so, now you could beat this into whatever it is that you wanted to make, say a plow or a scythe or something that could harvest wheat. And that enabled you then to have hunter gatherers settle down and start living in one place because they could cultivate wheat, all thanks to this one little observation. So the demonstration that you saw is all about how you can turn malachite into copper. Typically, it takes about 140 pounds of wood to make just 20 pounds of charcoal, which makes one pound of copper, so, it was a resource intensive method of making copper. And so, typically, that was a strong incentive to recycle copper in ancient times. It was one of the reasons why we actually don't find a lot of artifacts is because they were reused. Now, in the smelting process, it's really critical that you have something that can reduce the oxide or sulfide of the copper back down to make copper. And so typically the most common agent we use is carbon monoxide that comes from a burning of charcoal. All right, carbon monoxide wants to be carbon dioxide, and so by mixing your copper ore with a charcoal or carbon source then you would make the monoxide and you would reduce it back down to copper. Of course, that means a Copper in a natural state wants to be in some oxidised or sulfudised state, and so copper is inherently unstable with time. And that's a corrosion property of copper that you need to understand if you're going to use copper in some given applications, is to recognize that it does want to go back to it's oxidized state. Now, why do we call it the Bronze Age and not the copper age? Well, copper itself has a hardness of about 80 megapascals, all right? Bronze on the other hand, has a hardness of about 170 megapascals, and its strength is about three times stronger. So the question obviously comes up is, why is bronze stronger than copper? So in order to understand that, what you have to understand is something we haven't talked about yet, and that is how does a metal deform? So when a metal wants to deform, what happens is, when I take this piece of metal and I bend it, all right? What I'm actually doing is I am passing what we call a dislocation through the material. Now what is a dislocation? The usual analogy is, if you're at your roommate's place and they're moving into their dorm and they say hey, help me lay out this carpet I got. All right, and you unroll the carpet and you put it on the floor. Now there's two ways, the carpet's not going to fit right on the floor and you need to move it. There's two ways to move it. You can either grab the edge of the carpet and yank it, right, and try to pull it place. Now if it's a really big carpet, that's really hard to do. All right, a much easier to do it is actually if you're smart, you'll just put a little ripple on the carpet and then you push it with your feet all the way down to length of the carpet. When you get to the other end, the carpet will have moved. That little ripple is called a dislocation. And in metals what happens is that's how they want to plastically deform is that dislocation wants to move through the material from one side to the other and as it's doing so, I'm actually deforming the material plastically. So, if you wanna make something harder, what you have to do is figure out how to slow down those dislocations, those little ripples that are trying to run through the metal. Now, there's a couple ways you can do it. The simplest way to do it is what's called work hardening. That means that what I'm gonna do is I'm gonna hit it with a hammer or I'm gonna bend it a bunch of times, and if I bend this thing back and forth like this a whole bunch of times, the metal actually becomes harder and harder and harder to bend. So for example, I can demonstrate it with this piece of wire. It is that I've take this piece of wire and I bend it right, and I bend it back and I bend it again, you'll see that it's actually very hard to bend it in the same place. That part has become hard. And so now, it wants to bend over here. And if I keep bending it, the bend actually propagates down. So, it's very hard to bend the material in the same place, over and over again because it's becoming hard. I've got lots of dislocations piling up. They get tangled, and that makes it stronger. The same thing happens when you cold roll copper. It will increase its tensile strength. Now, how else could you strengthen it? Well, in addition to work hardening it, you can actually add an impurity. What an impurity does is it will actually slow down that dislocation. It gets into the lattice, and it messes it up and it makes it hard for that dislocation to propagate through the material. So, when I make a bronze, I've added an impurity. Now, early on, the most common impurity was arsenic and then later on, they use tin. In order to understand how much arsenic you could add, you really need to understand what the phase diagram of the material looks like. Now, we haven't talked about phase diagram, so I'm gonna mention it briefly here. A phase diagram is where I'm trying to figure out how much material I can dissolve into another, and the classic case is iced tea, right? You wanna make iced tea, you're gonna have to figure out how much sugar you can dissolve in it. And here in the south, we dissolve a lot of sugar in our iced tea. So the trick is not to get your iced tea cold and add your sugar. The trick is to take your iced tea while it's hot, add your sugar, and then cool it down. So the solubility of sugar in water is higher, at higher temperatures, all right? So, in order to do that, you can see this phase diagram shows that, as I increase the temperature, the liquid lot region, which is where my iced tea plus sugar is solubilized, actually increases with sugar as the temperature goes up. And so that's what a phase diagram tells you is how much you can dissolve one material into another, and what phases it will form as you cool that material. So here is the copper arsenic bronze phase diagram. And what you'll notice is, ignore all this lower stuff, what you can notice is that on one edge I have pure copper, and on the other edge I have pure arsenic. As I add arsenic to my copper, what's happening is the melting point is going down. All right, and that was actually very important because it meant that I could actually melt my bronze at a lower temperature which made it more accessible. All right, now that's if I add arsenic. The same thing happens if I add tin, all right. I can add tin, and my melting point goes down. The thing that you'll notice is that blue line indicates where the solubility is. That's how much tin I can dissolve into my copper, like I was dissolving sugar into water. And what it's telling me is I can actually dissolve a lot more tin into my copper than I could arsenic. So, that was a real advantage to tin. The other thing is that tin is non-volatile. Arsenic, when you heat it up, will form a gas, right? So, if I added a pound of arsenic to my copper to make a bronze, well I might wind up with a half a pound of the arsenic coming off as gas. So I never knew how much copper and arsenic I had in my mixture, right? And so it was very difficult to make a reproducible, lots of bronze. The second challenge of course is that all that arsenic coming off is toxic, right? And so typically people that worked with arsenic had a lot of nerve damage from all this arsenic. And so we, for example, depict ancient metalworkers as oftentimes being physically handicapped from all the arsenic that they were inhaling. So, tin was obviously better, it's less toxic. You put a pound of tin in your melt it's gonna be a pound of tin in melt, so you know how to reproduce it. The problem is tin was a lot harder to find. So when the alloy copper with tin it increases its strength. Here is a graph showing you how the tensile strength increases in copper or bronze as I add more tin. What you'll notice is it increases it up to a point, but then if I add too much tin, the tensile strength drops precipitously, right? And that's because I've entered another phase of bronze that's not desirable. So understanding how much tin was critical if you were an ancient metal worker. Now then in terms of when it got started, the Bronze Age in the near east happened around 3300 BC. That's when we started to see people actually developing and fabricating bronze, and it spread to Europe by 3000 BC, and China was 3000 BC, India was 3300. So it was very ubiquitous in that whole part of Europe and Asia. It didn't actually spread to Japan and Korea until about 500 BC, and it didn't even hit Peru until 100 AD. So each reason had its own separate Bronze Age. So what were the impacts of Bronze, right? Well, the critical thing was up until then you had stone tools, stone implements. So if you wanted to be growing a crop of wheat, and harvest that wheat, you had to use a stone, or clay scythe, right, to harvest the wheat. And that was very heavy, very cumbersome, and often very brittle, and it would break. And so the invention of a bronze that could cut through and harvest meant that I could harvest a whole lot more wheat with that. And that accelerated our ability to actually settle down and quit being hunter gatherers. There was also a really interesting implication. This copper needed to be alloyed with tin. The question is, is where do I get the copper, and where do I get the tin? They don't come from the same areas. And so interestingly enough, a lot of early bronzes were made in the Mesopotamia area of of the eastern part of mediterranean. However, the copper was coming from southern Italy, and the tin surprisingly was coming from England as far back as 2000 BC, which meant that the impact of bronze was enormous on trade. So you can see, for example, in this map where the, this region shows you how the area's developed, and there was a diffusion of metallurgy, and it started very concentrated in what is present day Iraq, in that area of the world, and then spread out from there, both to the east and the west. And it didn't actually get into Britain until a fairly decent amount of time afterwards. Now, like I said, the important aspect of this thing is where was the tin mine. Surprising that the Phoenicians would actually go all the way to England to get the tin, and it meant that you had to establish trade routes. And what was critical about this as you'll find out in the reading is that these trade routes were important, not only for the standpoint of making bronze, but also set up a political structure. So you had to have the ability to ensure that you had a safe travel of the material in order to ensure that you had a supply of copper and tin to make the bronze. We learned quite a bit more about copper and bronze with the discovery of Otzi. Otzi was a Chalcolithic mummy that was found in 1991 in the Northern Alps when you had an ice was actually melting. He'd been under ice for about 5000 years. He lived 3300 BC, and what was interesting about it was he was found carrying a copper ax, and that copper ax actually had some arsenic in it. And he had high levels of copper and arsenic in his hair. And so we believe that he was actually from Italy, crossing the Alps, and he was probably worked in the smelting process because of the amount of arsenic that was found in his hair. Early bronzes were quite large,in fact in 1000 B.C. there was a pair of pillars in King Solomon's temple that were 39 feet tall, enormously tall pieces of bronze. However, they no longer exist, and the reason is because every time someone pillaged or conquered that area, the first thing they would do is take all the bronze, cut it up, melt it down, and make the next thing. And so, see, you don't have a lot of ancient bronze because it was very valuable. The bronze enabled a lot of technology. So, for example, it enabled casting technologies for production of vessels, arts, tools and weapons. So, people learn how to make molds and then pull the bronze into the mold, and then, they could mass produce things like scythes or swords or axes or knives. The ability to actually do that was instrumental in further development of civilizations. Here's an example of a mold from Ireland that was found and was developed somewhere around 900 BC. And it just shows you a bronze sword that came out of that mold, and so the ability to pour it was critical. This was one of those metals that had the strength necessary to actually perform functions like this, but also had a low enough melting point that you could pour it. For example, you'll find out when we talk about iron, that iron had a much higher melting point and made it very difficult to pour.