Understanding lost-wax casting

[Eleatic Guest]

I don't quite understand why there are so many steps necessary. Check out this image sequence. Why isn't the liquid bronze simply cast into the mold of picture 2 instead of enclosing the wax apple in a new clay mold (pic 4)?

[PhilusEstilius]

I think that if you were to pour 1200 degrees molten bronze into a rubber mold you would simply melt the mold.
The whole system is where one has to make an investment mold of plaster or clay around the wax copy that is made from the artistic piece that produces the rubber mold, when the investment mold is made around the wax copy all the wax has now to be melted out of it.
Then the molten bronze at 1200 degrees can now be poured into the plaster or clay mold that can stand the high temperature.
When the casting has been done the investment mold now has to be smashed to get out the cast bronze piece which is shown with all its sprues that have to be removed and the finished piece cleaned up.
The rubber mold can now be kept and wax copies can be made from it whenever new bronzes are needed, but new investment molds must always have to be made around these wax copies, hence the name lost wax

[M. Demetrius]

You have it right, Brian. There are a few modern silicone-based mold materials that can be used a few times for low-melting point metals, like lead and tin, but they degrade very quickly.

Almost all castings are done by the method you describe, from detailed rings to bronze statues. The wax and the investment plasters are disposable, expendable materials, just part of the price of making the final bronze. Very detailed copies can be made in these silicone molds, using vacuum pumps to remove all the air around the original, even tiny surface features are reproduced.

And high pressure or vacuum is used to force all the air out of the investment to make an accurate copy there. This helps reduce finish/polishing work after the metal is cast and cooled, and the sprues are removed.

[Eleatic Guest]

I think that if you were to pour 1200 degrees molten bronze into a rubber mold you would simply melt the mold.

Obviously, the ancients would have used clay for the mold instead. Then bake it and then pour the bronze into it and that's it.

[PhilusEstilius]

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[attachment=3840]DSC01803Small.JPG[/attachment]
Here is an original beltplate I had up until recently that shows just how the casting was done, this one I would think was made in a two piece mold for on the reverse there is a square piece of the sprue at the centre of the plate.
The decoration was made in the lower half of the mold and the integral pins at the back would have been created as air vents when the bronze was poured.
This is of course not lost wax but more a direct method for producing plates.

[The Dioskouri]

Not to nit-pick, but so it's clear, bronze melt temperature is a lot higher than 1200 degrees. It's melt temperature is approximately 1836 degrees with a casting or 'liquidus' temperature about 100+ degrees higher.

Because the metal would cool too rapidly into a cold investment mold, the plaster/clay mold must also be brought up to heat. Usually around 1200 degrees for a bronze pour. This may be where the confusion lies.

Cheers

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