Early Bronze Casting...

from a private e-mail :

I was reading with interest Anders Soderberg Viking Bronze experimental bronze age casting, ... I see you had made a few pointers on the subject on one section so I was wondering if you could throw light on the subject, he says that he has 2 hearths one to melt the bronze and one to warm the moulds,  I want to know how the second hearth is heated to warm the moulds? is it connected to the first hearth with another pipe and just hot air passes through to warm them or is there a fire in the second hearth ? 

Aaron

I would never attempt to contradict anything Anders might say on this topic! He most certainly has far more experience, and has researched and done considerable practical work.

Drawing from my own (limited) experience with historic methods, but also relying on some years working in industrial casting... 

The pre-heat temperatures are significantly lower than what is required for melting the bronze itself. Both in terms of speed of heating, concentration of heat and total top end temperature.
So a simple charcoal fire - without air addition via bellows, is quite functional.
Thus the second hearth.

Concentrating heat, especially with the rapid consumption rates for charcoal, is why there needs to be a special set up to the fire on the melting crucible side.

Bronze Casting experiment - Kevin Jarbeau of DARC, 2001. Bellows nozzle at left. Open topped crucibles being slowly moved inwards to heat. One prepared mould seen to two the right.

There are three reasons you pre-heat your moulds:

1) In lost wax, to completely burn out the wax master pattern (or any other organic materials)
This requires at least 80 C or so

2) To completely remove any water remaining in your mould material.
Time can be a factor here, but again 80 C will certainly do this

3) To pre-heat the mould block so the molten metal does not cool faster than it can run into your pattern.
This element is a variable. The freezing point of your chosen metal is one consideration. How viscous your specific alloy might be when fluid. Just how much detail is in your pattern is another. What kind of temperatures your mould material can endure.

When I was working in fine detail castings (dental) the most typical pre-heat for the modern investment moulds (using lost wax) was roughly 2/3 of the melt point of the metal involved. The moulds were first heated slowly, then left for a while at a lower temperature (roughly 150 C) - the first to remove any water, the second to burn away the wax.

Since the clay itself Anders is using for a mould material could actually start to melt at temperatures close to the required melting point of his bronze alloy (likely about 1000 C), it is obvious he is using the lower temperatures of a 'plain' charcoal fire for pre-heating his moulds. (The composition of the crucibles is usually different than that used for the moulds themselves - just because of this endurance requirement.)