Melting gold using resistive heating.

[Amol Gupta]

L

I think in most small furnaces the coil does not touch the crucible, it's mounted on the inside of the insulating surround, with a small air gap, and so it heats the crucible through radiation. That way the crucible can be removed for pouring without disconnecting or damaging the wires. There are quite few videos on youtube for DIY mini furnaces and they all seem to do it that way.

You are right the 'air gap' is the only reason why such furnaces need to be air tight.

 

[haveagojoe]

You are right the 'air gap' is the only reason why such furnaces need to be air tight.

I don't think so, because it's mostly radiative heating rather than conductive or convective. It's the infra-red glow of the coils which heats the outside of the crucible. I think minimizing the airflow around the outside of the crucible would help a bit and allow some convection, but making it completely airtight would cause problems with expansion and in the worst case could cause an explosion. Certainly the internal chamber of the crucible should not be completely airtight.

 

[Amol Gupta]

You ne

I don't think so, because it's mostly radiative heating rather than conductive or convective. It's the infra-red glow of the coils which heats the outside of the crucible. I think minimizing the airflow around the outside of the crucible would help a bit and allow some convection, but making it completely airtight would cause problems with expansion and in the worst case could cause an explosion. Certainly the internal chamber of the crucible should not be completely airtight.

You need it to be tight enough to retain all the heat produced by the coils.

 

[haveagojoe]

You need it to be tight enough to retain all the heat produced by the coils.

There will inevitably be some loss of heat, but insulation is the most important factor.

I am playing around with the numbers, it's actually quite difficult to make an electric furnace which is so small, it gets easier as the size increases.

 

[Amol Gupta]

There will inevitably be some loss of heat, but insulation is the most important factor.

I am playing around with the numbers, it's actually quite difficult to make an electric furnace which is so small, it gets easier as the size increases.

Here a link to a video where guy tries to melt metal in a microwave, the idea of using a microwave is a bit outlandish but the insulation he has built looks very neat for this application.

 

[haveagojoe]

Here a link to a video where guy tries to melt metal in a microwave, the idea of using a microwave is a bit outlandish but the insulation he has built looks very neat for this application.

Yes there was a discussion about microwaves recently on another thread, I think its a good idea for small amounts but others were not convinced

 

[Amol Gupta]

Yes there was a discussion about microwaves recently on another thread, I think its a good idea for small amounts but others were not convinced

I really liked the insulation part apart from it there were a lot of overhead for something that can be done fairly simply.