The deterioration of specimens in natural history collections is a major concern of curators. Malacological collections, i.e., those containing molluscan shells (including fossils), are of special concern, since the specimens are composed largely of calcium carbonate (CaCO3), a compound that can deteriorate in the presence of acidic vapors. There are two main dangers related to the presence of volatile acids in collections: Byne’s disease (also called Bynesian decay) and pyrite disease (also called pyrite decay or pyrite oxidation).
Byne’s disease is seen in disfigured shells covered by a thin white granular layer of efflorescing salts and was initially associated with bacterial action, hence the name. However, further studies revealed its real nature as a simple physicochemical reaction related to high relative humidity and inappropriate temperatures in collections: volatile acids corroded the carbonatic shells and fossils, thus causing irreversible damage. Curiously, although historically it was never related to bacterial or fungal action, curators commonly refer to the oxidation of pyrite in collections as ‘‘pyrite disease.’’
These ‘‘diseases,’’ however, are not as extensively known as one would expect, especially in developing countries, where many collections unfortunately have less than adequate storage environments. This led us to write a paper explaining these “diseases” and offering methods to prevent or mitigate their action.
Bynesian decay is the result of volatile acids, mainly acetic and formic acids, reacting with the calcium carbonate of shells and some fossils. These acids result from the decomposition of wood and other components of the collection’s storage media, such as paper, cardboard, cabinets, cases, drawers, and paints and varnishes. The liberation of acidic vapors by these materials and their reaction with the calcium carbonate of shells are greater in high relative humidity and high temperature. The reaction with the shell produces salt, which deposits on the shell’s surface as a whitish encrusting layer, like on the figure below.
Dealing with affected specimens: The damage is permanent and irreversible: the calcium carbonate is consumed by the acids, producing one or more salts, carbon dioxide, and water. The affected specimen must be immediately (and carefully) cleaned by simply brushing the specimen gently under running water to remove all the salt. Other ways proposed in the literature for dealing with affected shells besides cleaning them with water must be disregarded, since they are ineffective or even harmful; these include employing Vaseline, alcohol, antiseptics, caustic compounds, heating, boiling, freezing, and microwaving.
Preventive methods: The most efficient method for avoiding Bynesian decay is making sure that all specimens are stored in an archivally sound environment, with proper storage and temperature/relative humidity conditions, allowing preservation for a virtually indefinite time. Therefore, storage materials that are not of archival quality must be avoided, and environmental conditions such as relative humidity and temperature must be carefully controlled.
Some molluscan fossils contain pyrite (FeS2), either in their fosssilized shells or in the rock matrix surrounding them. Despite the name, pyrite decay can also happen with fossils that contain marcasite, a polymorph of pyrite. The pyrite in the fossils in oxidized, liberating sulfuric acid. This acid then goes on to corrode the specimens, much like what happens in the Bynesian decay. The occurrence of pyrite decay can be easily identified by the odor of acid inside the collection’s cabinets and cases. Detecting an oxidized pyritic specimen is easy because of the rust color and the white, yellow, or brownish powder (oxides or iron sulfates) covering the specimens. Other specimens that are affected by the sulfuric acid vapor show a white encrustation (generally calcium salts), as in Bynesian decay.
Dealing with affected specimens: Pyrite decay actually poses two problems (in both cases, the effects are irreversible): (1) if the pyrite is oxidized, the fossil containing it will be compromised or destroyed; (2) the sulfuric acid vapor produced by the oxidation may attack other shells and carbonatic fossils in the collection, in a manner similar to Bynesian decay. Affected specimens of the last case may be treated in the same way as Bynesian decay (as described above), but the former case requires more attention and special treatment; so, please, refer to the full paper for this methodology (link at the bottom of the page).
Preventive methods: The first step is controlling relative humidity in collections storage; the second is isolating any fossils containing pyrite in separate cases (preferentially with silica gel or other desiccant).
It is hard to come up with a universal solution for all the problems a collection might face, especially to reconcile the different ideal requirements of relative humidity and temperature in order to avoid both Bynesian and pyrite decay. Since the damage caused by these phenomena is irreversible, the best solution is to avoid them in the first place. First and foremost, the collection must be kept in conditions of low relative humidity (preferably between 45% and 50%; never lower than 40% and never exceeding 55%) and appropriate temperature range (between 16ºC and 21ºC); this can be easily achieved with HVAC systems and the use of common desiccant materials such as silica gel. Ventilating the collection from time to time is also a good preventive measure. The combined use of hygrometer, thermometer, and pH indicators is recommended to monitor, respectively, relative humidity, temperature, and acidity levels, so it can be easily seen when the conditions in the collection are not appropriate. Other precautions include the use of materials that do not liberate volatile acids with time, such as steel cabinets painted with electrostatic powder coating. Finally, specimens that contain pyrite must be kept preferentially in closed containers.
The preceding text is a very abridged version of the main paper about this research: CAVALLARI, D.C.; CUNHA, B.R.; SALVADOR, R.B. 2014. Dangers to malacological collections: Bynesian decay and pyrite decay. Collection Forum 28: 35-46. [PDF]
Please refer to it for a more complete account and methods to prevent and mitigate the Bynesian and pyrite decays.