Smelting Bronze

This time we made bronze straight from ores of copper and tin!  

After melting store-bought brass, we wanted to see if we could reach the temperatures needed to smelt bronze. Smelting is the process of extracting metals from ores as naturally found.

Now, bronze is an alloy traditionally made of copper and tin, around the range of 90% copper and 10% tin, depending on what you're after. For example, bell bronze (for casting bells) is more like 80-20. My friend and neighbour Alex Moyle makes some of his sculptures with silicon bronze (which confusingly has no tin in it, but ~97% copper and 2% silicon), like his latest of Pam McConnell:

 

At the Pam McConnell Aquatic Centre

Copper can but does not usually appear in nature as a ready-to-go, orangey-brown metal; it is more commonly found as an ore. That is, a rock with a higher-than-normal amount of copper mineral within it. One common copper ore is malachite, which looks like this:

photo by JJ Harrison

The copper in malachite is copper carbonate. Years ago we made homemade copper carbonate, but this time we used store-bought stuff. We put 40g of it in a baggie.

Then, following a recipe from Caveman Chemistry, we needed 8g of tin (20% worth). Tin never appears in pure form in nature, but instead is found as an oxide in the mineral cassiterite. So we added tin oxide to the copper carbonate.

Then, in order to lower the melting point of the copper, we added 24g of soda ash, as a flux (so the ratio is 5 copper, 1 tin, 3 soda ash). We mixed this all up and put in our crucible. 

Next, we covered it all up with charcoal, to create a reducing atmosphere because ores are smelted by reduction. As Wikipedia explains: "The carbon source acts as a chemical reactant to remove oxygen from the ore, yielding the purified metal element as a product."

We sealed it all up with some clay to stop outside oxygen from oxidizing the metal,

and put it in the furnace where some charcoal was already burning.

We covered it completely with more charcoal, and within less than an hour, the pyrometer already read 1072º C. The more we use it, the more we are impressed by this little furnace - it works incredibly well and largely on its own. 

I mean, while the furnace heated up, we sat around and had coffee under the grapevine. Hardly what you imagine when you picture metalworking...

Shortly after reaching a maximum temperature of 1110º (!), we refuelled with more charcoal to make sure it would stay around 1000-1100º C for a while.

Another 45 mins or so and we figured we were all right, and let the fire die down. Early that evening we took out the crucible.

The clay was totally cooked onto the crucible but because the crucible had cracked it all came apart easily. Some alluring gleaming bits of metal at the bottom...

I hacked away at it to extract the metal, and sure enough, but still amazingly, found a few pieces of bronze that are just beautiful.

It felt like magic. We took green and white powders and heated them up and they turned into solid yellow and orange metal. Suddenly, for a second, I understood that everything was possible, because everything can change.

Of course, I know this can all be explained by chemistry, but this was an intuition that came from somewhere else. As Silo said in a public talk in 2007 in Punta de Vacas, Argentina:

"Let us not forget the short phrases that have emerged from our interior, let us not forget the occurrences that have come to us suddenly, let's not leave aside making notes of those truths that we have been able to intimate as we have seen them briefly dance before us.... These phrases, these occurrences and these dancing truths are inspirations that we are ready to be thankful for and are inspirations that invite us to go further on in our experiences..."

Among the crucible contents, there were also a few crusty, almost-spherical chunks that came out. 

When I split them open, they looked like copper in colour. Perhaps the tin oxide and copper carbonate hadn't mixed sufficiently, so we smelted straight copper? It's not entirely clear.

In any case, we put in the ingredients for bronze, and we got bronze!

Next up, making moulds for bronze casting.