Under Construction August 2011
There is a wide variety of metal artifacts in Alaskan museum collections. Tlingit copper daggers and carved silver jewelry, Siberian Yupik lead pipe bowls, Russian axe heads, aircraft engines and maritime artifacts are but a few examples.
2. POSSIBLE CAUSES
The most common white stuff we have seen on Alaskan metal artifacts is corrosion. The most common white corrosion products are found on zinc (so-called white rust), lead, pewter, tin, and aluminum. Also seen on metals are polishing residues, which can cause corrosion stress cracking in brass if those polishing compounds contain ammonia. Those polishing residues often have a greenish white look and are in the crevices or carving details of the metal. Metals in association with leather sometimes corrode due to the oils present in the leather. Lead corrodes easily in the presence of certain pollutants, particularly volatile organic acids that may come from other collections artifacts or poor collections storage materials. In fact, a fresh lead coupon in a jar with suspected off-gassing materials is part of the so-called “Oddy Test” for poor storage materials. If white stuff appears on the lead, the material in question may be releasing pollutants. Siberian Yupik pipes are typically made with lead and many in the Alaska State Museum collection are corroding. The often have small leather or cloth pouches attached, and those sometimes have white crystalline formations on the outside. We have tested one for lead, and come up negative. However, the tobacco mixture sometimes contained birch tree fungus ash, assort of a potash. Potash is chemically rich in potassium, and we did get a nice peak for potassium on our XRF analysis so possibly those white crystals on the pouch are related to the tobacco that was kept inside. Some Tlingit rattles have been found to be filled with small pellets of lead shot to make the rattling sound. Aleut kayak models in our collection have kayaker hands made of lead, and those have corroded aggressively, including after treatment with a corrosion inhibitor called Incralac and coating with Acryloid B-72. Remember that lead corrosion is poisonous, so wear gloves when handling. We have also seen the drawer runners on some of our collections cabinets corroding, but have had trouble determining what the white corrosion product is and why those drawer runners are corroding. We don’t want that white powder getting on our artifacts. Similar white powder has been seen on aircraft engine parts in the collection, but those parts are likely made of aluminum. An interesting white metal issue we have not observed but may be possible in some collections is tin pest. Tin usually appears as its stable beta form allotrope, but at low temperature pure tin can transform into another less stable form and become white and crumbly. Tin must be fairly pure in order for this to happen, however, so it is rare. According to information added to Wikipedia in the past several years, the temperature needed is usually below 56F and may need to be sustained for over a year. Tin solder on cans, uniform buttons, and medieval organ pipes are cited as examples. Different metals in contact with each other may experience galvanic corrosion. In this case, one metal acts as an anode and the other as a cathode in the presence of an electrolyte. Zinc and aluminum for example, are often used intentionally as a sacrificial anode to corrode preferentially to more noble metals they are protecting. This situation can happen accidentally as well.
Selwyn, Lyndsie. (2004) Metals and Corrosion: A Handbook for the Conservation Professional. Canadian Conservation Institute. Ottawa.
Tennant, Norman H, Brian G. Cooksey, David Littlejohn, Barbara J. Ottaway, Steven E. Tarling, and Martin Vickers. (1993) “Unusual Corrosion and Efflorescence Products on Bronze and Iron Antiquities Stored in Wooden Cabinets.” In, Conservation Science in the UK: Preprints of the Meeting held in Glasgow, May 1993. Editor: Norman H. Tennant. James & James Science. London. Pp. 60-66.
Ellen Carrlee’s notes: there are many conservation articles on obscure and strange corrosion products on metals. The small amount of sample usually available and the range of possible corrosion products makes identification of the corrosion product challenging and pinpointing the cause even more difficult.
4. EXAMPLES IN ALASKA