Thee staff of the Curator of Ship Models at the Naval Surface Warfare Center has long experience in observing and treating the deterioration of exhibition ship models. We maintain the U.S. Department of the Navy's ship model collection containing over 1900 models built between 1813 and today.
Museum professionals call the self-deterioration of objects "inherent vice" and some amount of decomposition is expected to be seen in all things.(18) General observation has shown that older ship models made using a limited variety of materials are less susceptible to inherent vice than newer models which employ a mix of many types of commercially available products. For new models, it appears that if deterioration has not been observed within the first five to ten years, and if the climate is not altered, the model probably will be relatively stable for many future decades.
We have found that even a low level of acetic acid inside an exhibit case can be detected by the human nose. When the display case is opened, the inside smells like vinegar. The human nose can detect the vinegar smell with a concentration perhaps as low as half a part per million. This amount also seems to be the lower concentration threshold for acetic acid to damage lead. The coincidence suggests that if an exhibit case interior carries even a slight vinegary smell, then acid is present in a harmful amount.(19)
We have noted that thin pieces of lead, such as moldings made from toothpaste tubes cut into strips, corrode faster than more solid shapes. For example, model anchor flukes tend to show the effects of corrosion before the arms or shank. In general, lead corrosion is first observed along the thin edges of parts. This is probably because of the large ratio between the surface area of sheet stock, and thin edges, to the total volume of the piece.
When we started our investigation we had long stopped using lead parts in new models and repairs. We now use parts made from lead-free britannia metal. Britannia looks and behaves similarly to lead. It is commonly called "pewter" today and originated in the nineteenth century as a popular pewter substitute when the ill-health effects of genuine pewter (much of which contains lead) was discovered.(20)
We decided to artificially create a corrosive micro-environment for lead parts so that we could observe the process. We employed a surplus ship model dust cover 20-inches long x 12- inches wide x 8-inches high (50.8 cm x 30.5 cm x 20 cm) made from 3/16-inch (4.8 mm) thick plexiglass and set it on an unpainted plywood sheet. Inside we placed two cereal bowls each filled with a few ounces of household white vinegar (labeled "5% acetic acid") and a paper towel wick. From our tackle box of old and reclaimed fittings, we selected about a dozen old lead items, none of which then showed any signs of corrosion. The fittings were unpainted, from unknown sources, and at least 20 years old, probably older. We arrayed them in various locations within the case and the entire setup was placed near a window facing south.
All of the fittings were observed to tarnish darkly first, then eventually form a light surface coating of white powder. The powder increased in thickness and then showed small surface eruptions (blooming) as more of the metal was consumed. Some parts corroded faster than others. The first white corrosion was seen on two parts after only seventy-two hours in the setup. Parts positioned in areas of the case occasionally struck by direct sunlight corroded faster than parts in other areas possibly because the sun's warmth accelerated the chemical process.(21) The parts continued to corrode when the bowls of vinegar had been removed from within the display case.
Impurities in Lead
We originally started our investigation of lead corrosion on the wrong track. In a casual discussion in 1980, an employee of the Model Shipways Company suggested that their lead castings were made using "type metal". We thought what he meant was most likely expended metal type from printing presses. An examination of literature showed that type metal should contain mostly lead and some measurable amounts of antimony, tin, and perhaps copper.(22)
Based on our experience and bolstered by observations made during the simple experiment described in the previous section, we knew that under seemingly identical conditions, some lead parts corroded faster than others. We surmised that lead corrosion was triggered by "impurities" such as antimony or tin in the lead used in the castings.(23) We were wrong.
Recent testing done by the NSWC Materials Laboratory for us indeed confirmed that there were minute amounts of antimony and tin and other metals in some lead ship model castings which corroded, but the amount of lead corrosion appeared in positive proportion to the purity of the lead used in the fitting. In other words, the purer the lead, the more readily the part was affected by acetic acid.(24) Contrary to our first thoughts, antimony, copper, and tin in lead castings apparently tend to retard or reduce the formation of lead carbonate.
Finally, our general experience over a two-decade period is that lead fittings on models displayed in plexiglass (cast sheet acrylic) exhibit cases corrode more rapidly than those displayed in glass cases. Oddly, our office seems to be the only museum group actually experiencing accelerated deterioration of lead objects under acrylic. While polycarbonates have been rated as non-producers of acetic acid, there are some current conservational concerns about acrylic sheet. We cannot yet explain what causes what we surely see, and more study needs to be done.