Copper Head 
If the furnace you are proposing to use, is made of refractory cement, you are not going to want to allow any charcoal to come into direct contact with the refractory cement as it will probably cause it to crack, flake and eventually cause holes in the refractory, which over a period of time will eventually cause the furnace itself to crack and start to fall apart. Also, bits and pieces of whatever you are using for fuel, coal, wood, etc, will start to stick to the refractory. Each time I melt in my furnace I put a piece of cardboard under the crucible so that it burns under the crucible, creates ash and prevents the crucible from sticking to the furnace bottom. You might be able to do something similar. I'm just not sure.
If you are interested in a comparison between the two, charcoal or natural gas, you can check this link out.
Charcoal VS. Propane as fuel
You are probably still going to want to use a crucible, and I believe if I am not mistaken, you can use a metal crucible of the correct type so you don't have to incur the costs involved with buying new crucibles. The type I use cost me around $80 for each, and only last 10-20 times depending on how aggressive the flux I am using is. And these are suppose to be the best on the market for my application. So be glad you don't have to use those types.
If you are looking to do this on the cheap and easy, you might want to look around for someone who has an old habachi. You can easily turn it into a charcoal foundry with very little trouble, you would just simply purchase the clay bowl to burn your charcoal in and place that in the habachi. The habachi would retain the heat, without getting too hot on the outside. Just make sure you get one made of the correct type of clay so that it will not crack once you get up to heat.
Another easy and quick method would be to again purchase the clay bowl made for charcoal foundries, then purchase refractory brick and build your oven by hand. In ancient times in both the Middle East and Asia, a charcoal foundry was made each time it was needed by building up mud/clay bricks and filling the gaps with mud/clay to seal the entire thing in, leaving the proper holes for ventilation and even being able to attach a bellow. This is how the first steel was made to create weapons like the Samurai swords of ancient times.
Scott
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