I wanted to make a coal forge for quite some time now, and (everything has a bad and a good side to it) now in lockdown-mode I had the time to do it. It took me three days, mainly because I wanted to allow the concrete to cure adequately between steps. It’s a simple construction from what I had on hands, now I’ll let it sit for some days before gently firing it for the first time and see what happens.
The steel tube (40mm dia.) had 5mm holes drilled in it to serve as an air outlet. You can attach a real blowing fan, a hairdyer (my version) or even bellows to it to supply oxygen for your coal fire.
It is vital that the brick and the concrete/mortar are as dry as possible before firing it for the first time to prevent almost immediate cracking. Best lay out your construction before actually building it, so you won’t have to ponder on how you’ll arrange everything while building, wet your stones and surfaces that will be mortared thoroughly and just begin! Give it time to begin curing for about 30 minutes before using a wet sponge to wipe and swab the excess mortar and inevitable mess-up off the bricks. Then let dry and cure for at least a week.
I highly recommend using fireproof / refractory cement. Although small, the forge will ideally be able to reach temperatures of around 1100 °C (2010 °F) in the center, so normal mortar will almost certainly crack and break.
It’s a pretty self-explanatory construction, so the images will most probably tell you everything. I’ll report here on progress (hopefully…)