Charcoal is a very acceptable fuel for forging, however I would us "lump" charcoal not briquet charcoal.
Briquet charcoal has a lot of foreign material in it. Since raw charcoal is relatively expensive they add ground limestone which is cheap and a chemical compound to improve combustibility. I am sorry I don't remember the exact compound but I knew 20 years ago and if I remember correctly it is a chemical that we don't want near hot steel. My first reaction it contains sulfur but I am not certain.
You can see the limestone, as ash, remaining after the fire is out. Sometimes I have had briquets look nearly whole after they are burnt due to the quanity of limestone ash.
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