Created a small forge by casting gfrc around firebrick. Placing the firebrick at a 75 degree angle (with respect to the forge bottom) allows one to pile coal up and over the sides of the forge for longer firings. Bottom of the firebox is a firebrick drilled for use as a tuyere. Used a nifty trick for forming a cavity. Between the bottom firebrick and the air inlet, placed a section of eps foam sttripped of the plastic film. Once the gfrc was cured, used acetone to dissolve the eps, leaving a uniform 3/4" deep space. This space serves as a manifold, allowing the air to evenly distribute across the tuyere. Aplied a parge coat of refractory mortar, then left the unit to cure up for a few days. This Saturday, fired up the forge. First test was a low temp firing, more to cure the parge coat and to check the systems. After burning for about 30 mins, I tossed in about 3 lbs of green coal. Let the impurities burn off, then cranked up the air. Before long was able to heat a railroad spike to a nice soft yellow. Works better than expected!!