Pickelhaube, Sheet Steel

As the war progressed, Germany's leather stockpiles dwindled. After extensive imports from South America, particularly Argentina, the German government began producing ersatz Pickelhauben made of other materials. In 1915, some Pickelhauben began to be made from thin sheet steel. However, the German high command needed to produce an even greater number of helmets, leading to the usage of pressurized felt and even paper to construct Pickelhauben.

Source: Wikipedia > Pickelhaube