Within the armouring community there is an idea that is taking foothold. This idea is that some armour parts, such as helmets and breastplates, were not only raised from a uniform sheet of metal, but also, an maybe more commonly, by forging out a thicker plate to obtain the final product of desired thickness. This is backed by medieval visual sources and documents. Today I tried this technique myself, with the aid of a striker and a specially designed sledgehammer. I started forging from a round slab of steel, 13 mm thick, 17 cm in diameter and 1.7 kg heavy. I started alone, with a simple dishing hammer. After a while I realized that I would need a helper, as dishing 13mm of steel to 2 mm on my own, would be too big of a task.
So I forged a sledgehammer, 3 kilos heavy with the "beak" 23 cm long, to allow the striker to reach deep parts of a helmet/breastplate. The results were very encouraging. Having a second person helping with the hammering steeply increased the efficiency of the squashing technique, where the material while being forged on the surface of the anvil forces the surrounding material to move, thus making the whole plate deeper and deeper.
The inside of the would-be helm looks remarkably similar to some museum examples, where there are plenty of hammer marks, showing that those examples were indeed forged mainly from the inside, rather than raised from a flat sheet of steel from the very start.
I'm finally making some solid progress on the forge I designed this spring. Currently I'm working on the bellows, and today I'm more specifically working on the valve system. The smaller valve at the mouth of the bellow will prevent air to be sucked from the fire pot and also stop any sparks from entering the bellow itself and damaging it. The design of the bellows is taken directly from the "De Re Metallica" by Georgius Agricola, written in the XVIth century. http://www.gutenberg.org/files/38015/38015-h/38015-h.htm
For the helm I'm using a few surviving examples held in various museums in Europe. They are all fairly large and reach down the shoulders, so I'll be modifying my design to make it more suitable to be used on foot. So I'm making it more compact, reducing the height and the width so it rests solely on the head, allowing for more movement. Right now the helm weights a whopping total of 5 kilos, which I think will be reduced to 4.5 as I work more on it.