This is a sideblast forge designed specifically with charcoal fuel in mind. I made this forge from recycled two by fours and scrap ends of 1x6 pine. On the inside it is 18 inches square. The bottom of the forge is lined with one inch of clay with the remainder being filled in with wood ash from the wood stove. I capped the wood ash with 4.5 by 9 inch wood stove fire brick, which created a 9by9 inch fire pot in the center of the forge--being lined with wood ash I can reduce the size of the fire pot to what ever I need, and for the type of work that I do at home I rarely need anything larger than 9 inches. The blast pipe is 3/4" black pipe off of a burn pile that I frequently pick through. The forge is portable and breaks down to fit in the back seat of my car, I lay the shelf over the top of the fire pot to protect it while in transport.
I have used both coal and coke in the forge, but the slag soaks into the lining which then has to be redone every couple of days or so (slag builds up and eventually clogs the blast pipe). Relining isn't that big of a deal, but during the summer months I'm not burning wood so my lining material some what precious during the summer. I built this forge in less than three hours (you can use clay but it will be much heavier and the fire pot will not be adjustable)