petro,
I would suggest both. If you are using pre smelted bullet alloy metal ingots, lead, antimony etc, you can just drop that in the lead casting pot and melt it down, it will be clean enough and you won't need to flux alot , that has been already done at the source.
But if you are going to try to save a few bucks and experiment with wheel weights,or range salvage lead, then you need to use a propane fired or other powered smelter.
It is used to process rather dirty alloy into clean ingots , it will usually mean a lot of fluxing SMOKE AND FUMES, and is best done out side or with plenty of ventilation.
This process of smelting in the outdoor pot then if bringing in the CLEN ingots to drop in your casting furnace, is the most common way done.
Also pre- cleaning of your salvage lead, getting rid of dust and grit before it is introduced to either the smelting or casting furnaces is a good idea, can just be a simple a using a collander to sift out the unwanted debris, seperating stick ons, cutting the steel clips from the wheelweights, or picking out jacketed bullets from the range lead collected ,to be smelted as a batch at another time.
You can melt small batches with a large casting furnace, and the bottom pour is an advantage.
check out the casboolits site for more info.