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Episode 6 - The Big Smelt

Gold Smelting
by Mike Leahy

Why do we need a furnace?
Furnace features
Why are bellows so important?
Did we have problems?
How hot do we have to go?
Creating a carbon arc
The result

The Challenge
Mike and Kate making furnace The challenge was to obtain pure gold, free from contaminants, and make it into a souvenir. The final product has to include the little nugget we were given, the alluvial gold panned from the river, and the gold we extracted from the mountain rock.

Smelting is a method of obtaining metals from a mineral-bearing ore, and dates back to pre-historic times. It usually involves the reduction of metal oxides (the ore) to metal (reduction reactions are those which take electrons away from elements, reduction being the opposite of oxidizing reactions), and the formation of non-metal oxide waste known as slag. It is a chemical process and involves much more than simply melting the gold. It sounds like alchemy, and it nearly is.

Mike and Jonathan building furnaceWhy do we need a furnace?
We decided to use a furnace. This is how metal has traditionally been smelted, and it's the easy option because we can follow plans that people have used for centuries to smelt iron.

But why do we need a furnace? Couldn't we just use a bonfire? No, because metal ores need to be heated to very high temperatures to produce pure metals — temperatures much, much higher than open fires can usually produce. Iron melts at 1800°C (3270°F) and gold melts at 1062°C (1943°F). Because iron has a higher melting temperature than gold, we hope that, even with our limited resources, we can get a furnace originally used to smelt iron to work our gold.

A furnace also helps maintain certain chemical conditions during smelting. For example, iron smelting benefits from a reducing atmosphere (an atmosphere low in oxygen). We used a furnace design that has been used throughout known history. It was a rough variation of a historic design known as a Bloomery Shaft Furnace, which was used to smelt iron many centuries ago.

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Furnace features
Furnace fireInsulation helps prevent heat loss. We used a double skin design and clay-lined brick walls in order to make sure that the high temperatures we created wouldn't be lost to the atmosphere. Even though we expected to hit over one thousand degrees Celsius (almost two thousand degrees Fahrenheit) inside the furnace, the outside shell never even felt warm to the touch. In addition, we built the furnace on foundations of dry sand to prevent heat loss to the ground. Water or wet sand would be good conductors, allowing heat to escape from the furnace. Air and dry sand are good insulators that prevent heat loss.

Fuel is important. We chose to use charcoal rather than wood because charcoal is mostly pure carbon, which provides the correct chemical environment, and reaches high temperatures when it is burned. The temperature at which charcoal burns depends on what wood it is made from. If it has been prepared from good hard wood, it should be reasonably easy to achieve 600-1500°C (1110-2730°F). Charcoal is a labor-intensive fuel to make, however, and it takes 22 pounds of hardwood to make only 2 pounds of charcoal.

Team with their furnace We also need plenty of oxygen, but it has to be in the right place. Chemically speaking, a reducing atmosphere - one low in oxygen or filled with hydrogen — is needed to smelt most metals successfully. In our furnace this was dependent on the presence of carbon monoxide. We can tell that carbon monoxide is present by the color of the flame at the top of the furnace. We need to look for a blue flame, which would show us when we had supplied enough oxygen to allow a hot enough flame - but not so much that we prevent the production of carbon monoxide (with a surplus of oxygen, only carbon dioxide will be formed).

The right balance is necessary. High-temperature fires need both oxygen and the fuel to burn well, but with too much oxygen we won't have our reducing atmosphere. We can't monitor this exactly with the equipment on hand. Since a shortage of carbon monoxide means a little waste would be left in with the gold, but a shortage of oxygen would mean low temperatures and no smelting at all, we had to strike a compromise.

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Why are bellows so important?
The key to supplying oxygen in roughly the correct amount is in using a bellows (a mechanical device for blowing air into fires). But the job is not as straightforward as it might look. We need the bellows to suck air from the atmosphere and blow it into the furnace. If air is blown into the furnace and then sucked back out, no oxygen would be added to the fire. Worse still, we would suck hot air into the bellows and melt it or even set it on fire. To solve this problem, we made a one-way valve.

Sketch of bellows and one-way valve

Did we have problems?
Yes. The first problem was when the valve fitted to one of the bellows got stuck and a hot coal was sucked into it, setting fire to the leather. The bellows also got very hot. We got around this by cooling the pipe joining the bellows to the furnace with water. By and large, however, the bellows held up very well, especially considering the length of time that they were used.

Kate and Mike using bellowsHow hot did we have to go?
We needed a high enough temperature to melt the gold, plus a little more. Metals melt at different temperatures. For example, lead melts at 328°C (622°F). If the gold is pure we need over one thousand degrees Celsius (1943°F) to melt it. That's a pretty high temperature! We will have to monitor it somehow to see if we can get there with the limited technology (and knowledge) available.

We made a simple thermometer by placing small pieces of lead, aluminum, brass and copper on a piece of brick. These metals melt at increasingly higher temperatures. By taking the thermometer out and looking at the metals to see which ones had melted, we would know the temperature. However, the thermometer was bulky and couldn't be positioned in the center of the furnace next to the gold, so Jonathan double-checked the temperature by lowering a copper olive (a component used by plumbers to join pipes, etc.) into the center of the furnace with a piece of steel wire. The copper melted; that meant we were in business.

So how did we do? Well, we definitely reached a high enough temperature, and we did manage to smelt the gold, but sadly it was very difficult to make even one nugget, which was what we wanted./p>

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Creating a carbon arc
Sketch of carbon arcJonathan decided to create a carbon arc. This is a method in which an electric current is forced to jump from one electrode to another in a bright arc similar to lightning. When this happens, very high temperatures (up to 5500°F) are generated, which will be more than enough to melt our now purified gold. We used a car battery to supply the electric current and took some carbon rods from a big flashlight battery (the old fashioned kind of flashlight battery, with two springs as terminals and containing carbon rods, and not AA-type batteries) to make the electrodes. Jonathan held the electrodes each side of our gold, struck an arc, and melted it into one odd shaped nugget.

Was this the end of our problems?
Well, almost. During the carbon arc procedure, pits were created on the surface of the gold and the crucible was broken. I wanted to cheat and borrow some gas welding equipment from a farm workshop in order to melt the gold and smooth out the pits. Another option was to fire up the furnace again, but there really wasn't time, so instead Jonathan opted to polish the gold.

Gold pendantAnd the result?
A pendant, which looks a bit like a whitebait - a kind of food that the New Zealand west coasters are passionate about. Or is it a shrimp? Or is it a spermatozoa? We'll leave it to your imagination!

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Photo: Rough Scientists at work
Metal Detector Interactive