The Foundry Pour

Malik Osei: You can probably hear the furnace. That roar has been constant since I arrived at seven. This is Field Notes, I'm Malik Osei, and I'm in a bronze foundry on the morning of a pour, standing on a sand floor between the crucible furnace and a row of moulds. Bastien Morel is the foundry master. Bastien, what are we casting today?

Bastien Morel: A pair of door handles for a library, about sixty centimetres each, and a series of twelve small plaques for the same building. Altogether a hundred and forty kilos of bronze in the crucible. Silicon bronze, mostly copper, three percent silicon, a little manganese.

Malik Osei: How long has the metal been heating?

Bastien Morel: Since five thirty. It needs to reach about eleven hundred and fifty degrees for this alloy. We pour at a little less, maybe eleven twenty, depending on the thickness of the pieces. Thin plaques want it hotter, so the metal doesn't freeze before it fills the mould.

Malik Osei: You keep looking into the crucible rather than at the thermometer.

Bastien Morel: The thermometer confirms. The eye decides. When it's ready the surface goes very bright, almost like a mirror, and when you skim off the dross it sounds different. Heavier. After thirty years you just know.

Malik Osei: Let's look at the moulds. They're boxes of dark sand, clamped shut, with a funnel in the top.

Bastien Morel: That's the sprue, where the metal goes in. And there are risers, little chimneys, which give the air a way out and feed the casting as it shrinks. Bronze shrinks about one and a half percent as it cools. If the riser isn't big enough, the shrinkage happens inside the handle and you get a hole you only find when you machine it.

Malik Osei: What goes wrong on a day like this?

Bastien Morel: Damp, always. One wet patch in a mould and the metal turns it to steam instantly and spits. That's why nobody stands over a mould when we pour, and why I distrust any mould that has been in the yard. These were dried overnight with a gas torch.

Malik Osei: Everyone has gone quiet and put on face shields. Here comes Ines with the crucible on the overhead hoist.

Ines Vaz: Crucible's up. Bastien, handles first?

Bastien Morel: Handles first, then the plaques while it's still hot enough. Steady, Ines, keep the stream unbroken. If you stop and start you trap air.

Malik Osei: A long orange rope of metal going into the first mould. It took about eight seconds. Now the second.

Bastien Morel: Good. Both full, the risers came up. Now we wait four hours before we break them out. The sand comes off, the sprues are cut, and then the long part starts: two days of chasing and polishing for each handle. The pour is the exciting forty minutes. The work is the rest of the week.

Malik Osei: Bastien Morel and Ines Vaz, thank you. From the foundry floor, this has been Field Notes.