Recently, while re-installing one of the exhibition cases of Islamic arms and armor, conservators noticed bright green corrosion around the copper rivets on one of the helmets. What was causing this corrosion, and could it be stopped?
The helmet engraved with floral patterns (WAM 51.1) was made sometime in the 16th century in Turkey or Persia (modern day Iran), and was purchased in 1911 by Henry Walters. A circular mark cut into the proper right side of the helmet indicates that it was once part of the Imperial Arsenal of the Ottoman Empire in Istanbul.
The steel helmet has a series of copper rivets at the front, sides, back, and around the lower edge. Though these rivets no longer hold anything, originally they would have served to attach other steel pieces to the helmet, including a nasal (nose guard), a short projecting brim in front, hinged cheek guards, and a hinged nape (neck guard) in back. Very few helmets of this type survive with all the attachments intact. One rare example is found in the Metropolitan Museum of Art.
To address the corrosion on the copper rivets of the Walters helmet, conservators first examined the helmet inside and out. The helmet felt greasy to the touch, and exhibited a hazy greenish visible fluorescence under long-wave ultraviolet radiation, suggesting that an old oil coating was present. Though weapons and armor have traditionally been oiled to prevent them from rusting, oils oxidize over time, becoming increasingly yellow or even brown. They may also break down into smaller, more reactive molecules, such as stearic acid, which can attack copper, forming bright green waxy copper stearates like the corrosion on the rivets.
Armed with this knowledge, conservators and curatorial staff decided to remove the aged oil coating. During the course of cleaning with solvents on small cotton swabs, conservators discovered traces of gilding scattered across the surface, suggesting that the helmet originally may have been gilded overall.
As a result of examination and treatment, the cause of the copper corrosion has been identified and removed. The elaborate engraved design is now more visible, and evidence of original gilding has been uncovered.
Stop by the Islamic arms and armor gallery on floor 3A to see the helmet engraved with floral patterns and to learn more about other rare helmets and weapons.