Preserving metallurgical artifacts with temporary climate control solutions is vital to minimizing conditions that cause damage, such as corrosion, tarnishing and pitting. Since metals have differing valences, their storage requirements also differ, making climate control technologies essential to their preservation.
Types of Metal Corrosion
With the exception of gold, all metals and their alloys start to corrode the moment they’re exposed to a new environment. Metals naturally convert into a more stable compound based on the amount of oxygen and moisture in an environment. This is why quickly changing the environment of a newly excavated metal artifact may do more harm than good.
Ferrous and Non-Ferrous Metal Corrosion
Often occurring with base metals like iron, lead, tin and steel, ferrous corrosion refers to oxidation. When exposed to humidity, ferrous metals rust. Less noble metals, such as copper alloys also corrode, but not as easily.
Some non-ferrous metals tarnish instead of oxidize, like silver. Others, like aluminum, harden on the exterior.
When two different metals touch or are near each other in a marine environment, they create a galvanic cell. The cell couples because of an ion-conducting electrolyte that changes the number of electrons and ions in a metal. The result is electrochemical corrosion. For this reason, curators do not store different types of metal together.
Also referred to as hydrogen corrosion, anaerobic corrosion sometimes occurs spontaneously in water that’s depleted of dissolved oxygen. Anaerobic corrosion also refers to corrosion that occurs when metal is exposed to sulfate-reducing bacteria in water or soil. The bacteria consume a metal’s cathodic hydrogen, which forms when metal is exposed to moisture.
Temporary Climate Control for Metal Artifacts
As a ferrous metal, iron is most stable in an environment with a constant temperature and relative humidity levels of 12 percent or less. You may also store the metal in an oxygen-free environment with a desiccant. Because iron expands and contracts with different temperatures, climate control may be necessary with seasonal changes in temperature and humidity. Coating iron with an acrylic lacquer or microcrystalline wax further protects it from environmental elements.
Cupreous metals include brass, copper and bronze. Museum staff need to take particular caution when cupreous metal alloys contain tin or lead. Before storing cupreous metals, sealing them with microcrystalline wax or clear acrylic lacquer creates a barrier between the object and the moisture in the air. To create a stable environment, store the artifacts in an environment with a constant temperature and a relative humidity of less than 55 percent.
The ideal relative humidity level for the preservation of silver is 50 percent or below. If the silver is not on display, wrap it with a tarnish inhibiting cloth so it absorbs tarnish-causing pollutants. Then place the wrapped silver in a Mylar or clear polyethylene bag. Lacquering silver to protect it is effective when a professional applies it correctly. Waxing, however, is not advisable because the results are inconsistent.
Lead, Tin and Lead Alloys
Lead, tin and lead alloys, such as pewter, benefit from a microcrystalline wax coating to protect them from elements and moisture in the environment. Store the artifacts away from items that produce corrosion-causing organic acids, like wooden shelves. Further, protect the metals by placing them in polyethylene bags or sealed containers and keeping them in a room with a relative humidity of 55 percent or lower.
When storing and displaying metal artifacts, a lack of temperature and humidity control puts the objects at risk for permanent damage. With the help of a humidity sensor, museum staff will know when seasonal changes necessitate the use of climate control technologies, which Polygon can provide. The technologies also help improve the indoor air quality, so dust and other pollutants are less likely to harm artifacts. Polygon also offers climate control solutions when metal objects experience water damage, as the technologies create an environment that promotes safe, fast drying. Contact Polygon to learn more.
[Photo from A.Davey via CC License 2.0]