Edit from the future
I seem to have forgotten to finish this, but there's some decent material here so I'll publish it anyway.
This is another one from the collection of projects which I seem to keep stabbing back at again and again. I've made all sorts of forges, mostly from mixtures of Plaster of Paris and sand in flower pots. This design was conceived one day when I found myself in excess of bricks from an old garden feature.
My design was essentially as follows: a stack of bricks gives the forge some height, bringing it up to around my waste of the way. This is hollow, like a chimney, allowing air to be forced up into the coals, and a place for ash to fall out. More bricks then form the bed of the forge across the gap, with a gap at the back for air to be forced upwards. Finally a ceiling of bricks protects me from the coals and the coals form the wind. I'd light a fire inside it, let it burn down and add coals, pushing it to the back to allow air to be forced in, superheating it. This creates a temperature gradient, potentially useful when forging. My hot air gun acted as the air source, although I did try making a pair of bellows (read on).
I initially just threw the bricks together dry, but of course the forced air leaked. I resolved to the temporary solution of mixing a large batch of mud and using that as a mortar to hold my bricks together nice and airtight - resulting in a surprisingly solid structure. The only other airflow issue I encountered (at this point) was the air would blow straight back out of the large hole into which I inserted the hot air gun. Simple plug of mud and a hole in it proved a quick and dirty fix for this experiment, and airflow was good.
After firing it up it worked well, although airflow perhaps could be a bit better getting me better temperatures. I tested it by heating and smacking an old alan key. I attempted the classic leaf keyring, but sadly the material was too small to effectively work with my crude hammer and I-beam anvil. I had some spare re-bar, so I planned to reattempt it with this tomorrow.
The next morning...
The next morning with the help of my brother, I attempted to get the forge set back up. I found however the airflow channel was blocked with ash and small bits of coal that had fallen through. This was the main issue with this design. Perhaps some mesh could have aided this issue, but I found not the motivation to complete the project today and so deconstructed the forge, to remove the blockage and see what had fallen down into the cavity - a bunch of half-burned coal, ash and bits of mud mortar.
Overall I'm pretty pleased with this experiment. The only other issue is that the air can decompress in the big cavity, so a pipe to deliver the air would be more ideal. Another stepping stone on the journey to the ultimate forge.
This was a quick build, and worked reasonably well. I sawed up an large drawer front that had seen better days into two large planks and a shorter section for the front. This was attached to one of the large bits at right angles, and to the other large section with hinges. A hole was drilled as the outlet and I then used thick blackout blind fabric to form 'bag' of the bellows, which worked well in place of the more traditional leather.
It didn't work this project far enough to be used - I had leakage issues, and would have liked to add a spring to push the bellows back apart, but didn't have one on hand. A one-way valve for an inlet would also probably be needed if I manage to fix leakage issues.