Weight: 2- 3 kg
The great helm covered the whole head including the face. It was the heaviest, most protective type of helmet of its time.
Narrow ‘sights’ allowed the wearer to see out, and sets of small cut-outs or ‘breaths’ made it easier to breath.
Large pieces of iron or steel were expensive and difficult to make in the early fourteenth century, so helms were built out of several smaller pieces riveted together.
The great helm was worn over a padded cloth arming cap and usually a mail coif. Sometimes a close-fitting metal skull-cap was worn under or over the mail, with the great helm then placed over top of all of these other pieces. This meant that the head was very well protected, by several different layers. This was one of the secrets to good, reliable armour.
Most soldiers fought in open-faced helmets. For them being able to see and breath well was much more important than the danger of getting hit in the face. But knights fighting on horseback with lances had to have better face protection- the spear of another charging knight, driven through the face, could be instantly fatal.
Great helms could also be worn on foot. The knight could rely on his superior fitness and training (and often his personal bodyguards), to make up for his lack of good vision.