While many people wouldn’t immediately think of metal as a primary helmet material, few other materials have been so extensively employed in helmets. Early metal helmets were designed for military use. Bronze helmets dating back to the fourth millennia BC have been unearthed in archeological digs. Ultimately, bronze, brass, iron and steel would all be employed to help soldiers deflect rocks, swords, hatchets, arrows and musket balls.

Naturally, there are downsides to wearing a metal bowl on your head. Poor ventilation, and excess weight are all disadvantages of the material. Compared to modern materials, metal’s ability to attenuate impacts is also lacking. By the 1700’s metal helmets had fallen out of favor as high-powered rounds from rifles could easily penetrate steel helmets.

Metal helmets, however, made a return with the outbreak of World War I. Though still ineffective at shielding their owners from rifle shots, steel helmets were then useful in protecting fighters from a new threat—artillery shrapnel and falling objects. While most armed forces no longer use metal-shelled helmets, it was only relatively recently that metal helmets were replaced with non-metal, composite models. The steel M1 helmet, for example, was worn by members of America’s armed forces until 1985.

These days, metal is rarely used in helmet shells and is largely relegated to the buckles and snaps that help secure some helmet straps.

Most trees have an outer layer of cork bark. The cork oak, or Quercus suber, however possesses a much thicker band of cork than most tree species. This cork layer is composed of water-resistant cells that separate the outer bark from the delicate interior bark. The cork protects this species of oak from the fires, droughts and temperature fluctuations that are common Mediterranean countries such as Algeria, France, Italy, Morocco, Portugal and Spain.

Cork was used to pad helmets during the 1800s. You can think of cork as a precursor to the collapsible foam liners in modern helmets. Cork could be found in many “pith helmets” as a readily available substitute for the actual pith, a material primarily harvested from vascular plants native to Asia.

Cork is rarely incorporated in helmets today, though it does have a few interesting things going for it as a material. Cork is lightweight, rot resistant and impermeable to liquid, which is why it has been the preferred means of sealing wine bottles for more than 400 years now. That said, the labor costs alone (it must be removed from trees, boiled and shaped) make it an unlikely choice for helmet liners today.