Steve,
If at all possible, hook up with the blacksmith group, or a smith who uses coal. Starting and running a coal forge is not quite the same as making a camp fire.
Your forge is fine as far as depth goes.Most table forges are only a square table with 1" side walls.A portable farriers/blacksmith forge is about 3-4" deep at most. The green coal is filled to the edges, and heaped toward the center. The fire is kept in the center over the tuyer/tuyere - pronounced either too-yair (correct), or tweer (Americanized) - The blast ( air from the blower) is only run to light the forge and to raise the coal heat when heating and forging. Don't crank it all the time or you will eat through a bag of coal fast. Get a small watering can to dampen the coal around the center. This is necessary to keep the fire from spreading out of the center and to aid in the green coal coking up to made it ready to forge with. On that subject, you don't actually forge with coal, but with coke you make from the coal. This is why the wet coal is piled around the center to "coke up". Coking is the process of driving off the volatile gasses and tars from the coal ( what smells) and leaving behind the purer carbon to burn. All this is nearly impossible to explain in print but makes perfect sense when seen in person, which is why you need to visit a blacksmith or bladesmith who is experienced at coal. The coal you want is soft low sulfur coal. Pocahontas #3 is the preferred forging coal, IIRC. The coal should be purchased already in small pieces ,or broken up into gravel and dust size. Anything bigger than a peanut is too big. A single bag of coal will last a good length of time when the fire is made and run right. Keep the coal in a covered metal trash can.
Natural wood ( lump) charcoal is also great to forge with.....and smells a lot better.
Additional note - Shutting down the coal forge is as important a step as starting it. If you finish the session by coking up a batch of green coal, and then shut the damper, you will have coke to start with next session. This will get you going faster and cleaner each time you start the forge up. Clean the ash trap and clinkers to avoid them clogging up things.
Final note - Adding a liner of fire clay to the forge will make it last longer. It isn't necessary if you will be only occasionally using the forge, but will keep things cooler and extend the life. Fire clay is purchased from a blacksmith supply, or dug out of the field as red clay.You have to mix it with sand ,ash, lime, and other things to get the right formula, so using a ready mixed product is easier. You could use Satanite mixed 50:50 with sand just as well. Castable refractories (Mizou,etc.) are perfect right out of the bag. You want to made "ring" of clay,creating a small pit about 1" wider than the tuyer, and about 2" tall. The walls should slope toward the center on the inside, and toward the edges on the outside. Sort of like a very shallow volcano. This controls the fire from spreading and keeps it directly over the tuyer. You can coat the bottom of the forge out to the rim ,too, but this is not needed, and can cause rust out if you live in a wet environment.
You heap up the coal around the fire pit and push it in as it cokes up and as needed to maintain the fire. The fire pit also makes HT in a coal forge much better, as keeping the heat concentrated in the "Tunnel" or "Cave" ( the blacksmith will show you how to create these).
Stacy
