Several people I know are making or remaking blacksmiths forges from the medieval period. Now obviously there are some changes in precise equipment used and various other things, but the designs have ended up fairly similar, due to the need to have portable forges.
The base is usually, but not always, a wooden box, because what is more authentic than wood, and can be easily worked to your desired shape. The interior can be filled with fire bricks, insulation or sand, topped off with something clay like, which can resist heat and yet not look wrong. This photo is of an iron box, although the interior of it and other bit look authentic enough, I’m not so sure about using a box.
So, now to the sources.
Firstly, we have Theophilus, circa 1122.
He seems to suggest using two wooden boards, upright, at right angles to each other, with the space inbetween and up them filled with clay:
“Then take some freshly dug clay, neither kneaded nor mixed with water, and start by putting a little of it into this space and compact it well with a round piece of wood; then put in some more and ram it down again. Continue in this way until two thirds of the space is filled, leaving a third empty. Then take away the board in front, and with a long knife trim the front [edge} and the top of the clay flat and smooth. Then strike the clay hard with a slender piece of wood. After this take some clay that has been kneaded and mixed with horse-dung, and build up the first and a hearth for it. Coat the wall [of the building] also so that it will not be burnt by the fire. Pierce the clay through the hole at the back of the board with a slender piece of wood. Build all smith’s forges in this way.”
When we turn to the 16th century, Biringuccio has a few pages on the art of the smith who works in iron, it is not his personal speciality or area of knowledge, so whilst he describes some secrets, and artistic descriptions of the work carried out, there isn’t anything directly related to forges.
The nearest we can get is with the descriptions of how to make various sorts of hearths for melting bronze in. These are functionally equivalent to forges.
For instance, “Then it is filled with clay, very well pressed and beaten in, and a hollow is dug out in the middle as deep and wide as you think will contain the material that you wish to melt. Having prepared the bottom and made a hole as an exist for the bronze, and having put in the iron pole, cover it all very well with ashes that are moistened with water in which salt has been dissolved. Then back it, and putting the bellows in their places where you have set the tuyeres, proceed a you did when you melted with a hearth.” (Page 289 of the Dover paperback edition)
Biringuccio does love his salt water moistened ashes as a final outer layer over any clay lining, and it does work well. Well burnt ashes are by their nature very refractory and survive a fire well.
Unfortunately I haven’t found much about any surviving medieval forge hearths. There are a few mentions in archaeology journal articles but they are hidden behind paywalls.
So it would be interesting to compare the performance of hearths built out of clay and coated with ash, or clay with an outer layer of horse dung mixed in. The horse dung would hold it together better as it dried, and give some porosity after being fired, which might help it be more insulating. On the other hand that might make it a little more fragile and prone to damage when moving things about in the fire.