Hot and heavy

I am on light duties this week. A few adjustments to the creatures, then a trip to the art foundry to discuss them being turned into bronze.

I used to do a lot of bronze casting some years ago - I built two foundries from scratch - but I was more interested in the process than the finished article. These days it is the other way round.

It is very easy to kill yourself and those around you during the pouring of bronze. All bronze casters examine and maintain the roofs over the furnaces meticulously, especially in weather like this. One drop of water in the hot crucible can cause an explosion which will send gobs of molten metal raining down on you and, when it hits your skin, it sticks fast and does not stop sinking in until your body has taken the heat out of it.

If you were to pour a crucible of molten bronze into a tub of water, the water explodes with the force of TNT, weight for weight. That's 100lbs of water = 100lbs of TNT. It is - like the bomb - the hydrogen atoms in the water which cause the event.

There are usually three people involved in the procedure of pouring molten bronze. Everyone else stays well out of the way.

The pouring is rehearsed an choreographed down to the last detail, around three times if there are any unknowns. Firstly, the crucible is loaded with ingots of cold metal and put into the furnace. The lid is closed and the furnace is fired up. If you have about 200lbs of metal in the crucible, the metal can take about one to one and a half hours to reach pouring temperature, depending on the furnace.

After about half an hour, gas or paraffin-fired furnaces begin to breath. The noise from the furnace is an extremely loud roar which has to be shouted over, and it increases the already dramatic sense of anticipation. As the metal goes molten, the burning of ambient oxygen becomes rhythmical and the furnace takes in a deep breath, then exhales again like a sleeping giant. It carries on breathing like this until it is switched off, then an ominous hush falls on the studio, with only the tinkling and shifting of cooling components to magnify the silence.

The casting crew talk in calm, soft voices - if at all. They usually do not need to talk until the very end because they are practiced and rehearsed. One goes to the furnace and opens the lid. Another approaches with a long metal rake and throws a handful of dry oak chips into the metal, so that the rogue gasses are expended by burning them. The rake has been pre-heated to avoid any shock reactions between hot bronze and cold steel.

Then the third approaches with the first and both are either end of a huge pair of scissor tongs with half-round jaws. They carefully place the jaws around the crucible and slowly lift it out of the furnace. Tungsten crucibles become soft and sticky at that heat, and it is quite easy to crush them with the tongs, sending 200lbs of molten bronze washing over your feet and legs like thin treacle.

They are covered in fire-resistant jackets, aprons, hoods, hats, spats and visors. In my day the clothing was hide, mainly so you could smell burning leather if bronze landed on you, giving you a few seconds extra to react. It is very hot inside all that - it is very hot outside too. Within seconds, the visors mist up with sweat, and the two pourers rely on the third 'dross-raker' to warn them of any trip-hazards they may encounter whilst walking backwards carrying 200lbs of liquid metal.

The 'shank' is the pouring tool and is a six or seven foot length of steel with a handle at one end and a ring in the middle. It rests on bricks as it waits for the crucible to be placed in the middle ring. Once that is done, the tongs are put to one side and the pourers brace themselves to pick up the extra weight of the shank as well as the bronze.

They then pick up the crucible in the shank and walk to the moulds, which have been baking in a low-fired kiln overnight. No body runs, even if they are capable of it. They position themselves over the moulds and carefully aim the steady flow of bright pink metal into the largest hole in the top of the moulds. Because the metal is glowing brightly, you can see it through the mist of your visor, but you may not be able to see when the mould is full as the little tips of excess metal rise to the tops of the smaller holes. The third who is holding back the dross on the top of the metal may have to be asked. "See anything yet?"

The volume of metal has already been calculated, so the pourer knows roughly when the excess should start to appear in the small riser holes. If they think that it is taking too long, this could be an indication that the mould has split and is sending rivulets of molten metal across the floor - often behind you before they appear. "See anything yet?!" His voice trembles with his arms and legs.

This happened to me once, and the first we knew about it was when the concrete floor started exploding behind us as the bronze flowed over it, sending molten metal raining down on us like bird shit. Even - or especially - during disasters, you follow reverse procedures calmly and methodically. You carefully put the crucible back into the furnace, close the lid - and then you can run!

Having stripped off a couple of layers of leather, we lay on the grass outside, drenched in sweat and utterly exhausted. Someone said, "You should never, ever use flint as an aggregate in the concrete of a foundry floor..."