So, after a couple of previous attempts, and a slightly larger foundry - a 12 litre steel bucket instead of the 10 litre one I had before, with a better placed air intake, I have great success melting aluminium cans quickly and I even got it hot enough to melt some copper tube.
The larger bucket means a larger cavity can be formed inside, which allows more charcoal to fit around the crucible with the lid down. The result being a very hot fire concentrated around the crucible, which melts the cans almost instantly with a hiss and a flash of flame. The melting point of aluminium is 660.3 C, and it was clearly well above that.
The melting point of copper is 1085 C, and this wasn't as easy to achieve. I could get the copper to start to glow and become soft, but it wasn't quite melting so I loaded as much charcoal as possible in, turn up the hair dryer fan to high and this did the job, the copper tube melted into the crucible.
The copper was glowing in the muffin pan after pouring it in, but quickly cooled to a black colour.
The cavitities and imperfections in the copper you can see in the the first photo were from a lot of little coals which had been blown into the crucible with the blower turn up high, but resulted in a really interesting effect.
The crucible I was using is made from graphite which stands up to the heat I can generate.
The top of the copper dried black, maybe due to oxidation as it is exposed to the air.
Two new "muffins" complete.
Overall I am very happy with the newly constructed mini foundry, see my earlier post about this for the original instructions. Once I have accumulated enough material I will start experimenting with green sand casting. It is amazing what you can do in your back yard with a little help from YouTube!