The iron chair was a torture device, very similar to a gridiron. The victim was placed on several strips of brass over an open flame, and slowly roasted alive. There were other variations, such as: "[culprits] were tied to an iron armchair, then slowly pushed nearer and nearer to a blazing fire."[1][2][3]
Another version of this chair was even more diabolical due to the addition of hundreds of sharp spikes which lined the back, seat, arm rests and leg rests. The victim would be placed into the chair and then iron restraints would be tightened, forcing the spikes deep into his/her flesh. The chair could then be heated by lighting a fire underneath.