The Oddy Test – What Works and What Doesn’t

The Oddy test was first proposed by Antony Werner in 1973. In 1975, Andrew Oddy, who worked in the British Museum’s scientific department at the time, developed it into a replicable testing method to detect volatile contaminants (VOCs) presumed to offgas from materials used for preparing display cases. The Oddy test would then be used to exclude risky materials from display case design. (Oddy 1975) The Oddy test is a corrosion test in which contaminants induce corrosion in three indicator metals (silver, copper and lead) exclusively via the gas phase at a relative humidity of 100%. The condition at which the indicator metals can be evaluated is reached after a treatment time of 28 days at 100% relative humidity and 60°C.

The testing method is described in a 1979 publication by Lee and Thickett, in which the authors expressly point out that: “The following recommended method should be followed exactly”. According to said method, two grams of sample material are cut in small pieces and placed on the bottom of a clean glass container, such as an Erlenmeyer flask. The purified indicator metal is suspended on a nylon thread above the sample, and, in order to adjust the relative humidity, a glass vessel filled with distilled water is also placed on the bottom of the flask, which is then hermetically sealed with an appropriate plastic or glass seal. After that, thermal treatment is conducted over the predefined treatment period of 28 days in a drying cabinet at a constant temperature of 60°C. An improved and simplified procedure is found in a more recent publication by Robinet and Thickett (2003), and this is also the version taught by HTW (see References) in training courses concerning the Oddy test.

Despite the fact that the authors dictate strict adherence to test design and methodology, not least for the purpose of comparability of findings, about 20 different protocols are currently in practice, most of them conducted at museum laboratories or by freelance conservators. As a consequence, it is important to stress that results that have been obtained and occasionally published are neither comparable nor reproducible and in many cases fraught with considerable inaccuracies.

The fact that the Oddy test as a corrosion test of test metals is considered extremely unsecure is essentially due to the following facts:

1) Corrosion of test metals at 100% humidity is substantially different from corrosion under “normal” humidity conditions (50% RH).

2) Corrosion of test metals in direct contact with tested materials is substantially different from corrosion due to a mere gas phase reaction with the substances (VOCs) offgassed from these materials.

3) Corrosion of test metals is crucially dependent on how the surface of the metals has been prepared for the test. This includes the precision of both how the surface is ground and how it was subsequently cleaned. In any case, for comparability of results and interpretation of any corrosive change in at least one of the test metals to be possible, the indicator metals must be ground and cleaned with absolute accuracy. To this end, Lee and Thickett’s directions as well as those from other publications (Zhang et al. 1994, Robinet and Thickett 2003) apply.

4) Corrosion of the three test metals, silver, copper and lead, is in most cases due to three substances (VOCs) offgassing from tested materials: hydrogen sulfide or other volatile sulphides (in the case of silver), acetic acid (copper) and again acetic acid (lead), albeit the exact mechanisms of each individual corrosion process are hardly assessable on visual inspection in the case of mixtures of outgassed contaminants.

5) In addition, intensive color changes occur at the surfaces of copper and lead due to the formation of oxide layers, which are manifest in the distinct obscuration of the metal surfaces.

6)Comparison of the occurring changes in the metal surfaces according to steps from “no corrosion” via “slight corrosion” to “strong corrosion” is subject to the observer’s subjective impression, therefore it cannot and must not be regarded as a reproducible and unambiguously interpretable scientific measurement result for assessing tested materials.

7) Sampling, manipulation and storage of the materials to be tested in workshops and laboratories can substantially affect the test result, which, in case of storage, is due to contaminants present in the air that the materials may absorb.

Accordingly, the Oddy test is an insufficiently precise analytical instrument to interpret the causes of corrosion phenomena or discoloration of indicator metals. Its use has been, and still is, a topic of controversial debate (Grzywacz 2006), despite its broad use in museum contexts. The test is only and exclusively suitable in detecting whether contaminant gases are emitted from certain materials under the respective experimental conditions. It cannot be used to inform about any deducible risks affecting other materials which are in any contact with the test materials during prolonged periods of time. Direct transfer of data is only permissible, and with reservations, for the test metals proper and among them, fairly realistic for silver (turning black by forming silver sulphide in the presence of contaminant gases containing sulphidic sulphur) and lead (forming white films by generating basic lead carbonate and/or lead acetates upon release of acetic acid). More details on lead corrosion in the presence of acetic acid is found in Tetreault (1998).

More recent studies from the years 2003 and 2011, all listed in the References section, relate to slightly more precise testing methods, such as an optimized Oddy test (Robinet and Thickett, 2003) and a modified design with more precise analytics (Strli? 2011). Especially the method proposed by Strli? seems to be suitable to allow more accurate statements on the level of risk for cellulose-based materials, however, this test requires a significantly higher degree of analytical effort.

To examine cellulose-based materials like paper and cardboard that are used for permanent storage of cultural property, Strli?’s work provides a number of indications, even though these are in need of further verification. It is known that gaseous contaminants have influence on the resistance of paper, while the exact risk level depends on the paper composition and the specific contaminant compounds that are present. What can be said, though, is that gaseous acids have considerable influence, while aldehydes, as they are able to oxidize into acidic components, may also cause paper degradation. Acetic acid is emitted by lignin-containing papers as they age, but based on current research results, (Di Pietro and Ligternik 2012, Potthast et al.) their occurrence has little effect on the stability of paper. In addition to acetic acid, formic acid and other compounds, whose impact is still hardly assessable, can be present as well. (Volland et al., in print) What needs to be noted above all is, of course, that all boards and paper based artifacts offgas acetic acid during ageing that accumulate when they are encased in containers.

It is remarkable how enclosure materials (DIN/ISO 16245:2011-04) for gaseous contaminants and their accumulation in containers remain disregarded. Collections could consider modifying the respective standards because it stands to reason that in addition to boxes and assembling materials, all of which emit acetic acid and other substances at least to a limited extent, collected artifacts within containers are capable of emitting acetic acid as well. To what extent these accumulated acetic acid concentrations could have resulted in damage is hard to assess, but if the damage were overwhelming, this would be very clearly visible in numerous collected objects, particularly if they have acid-labile colorations.

What museums should also contemplate is whether the standard for enclosure materials for collected items (DIN/ ISO 11799: 2005-06) should be upheld in its current version, in which the threshold value is set to < 4 ppb for the concentration of acetic acid and to < 4 ppb for formaldehyde, although currently produced research results reveal that the detrimental effect of acetic acid on cellulose is on the low side.

Literature appendix:

DIN ISO 16245: 2011-04: Information und Dokumentation –Schachteln Archivmappen und andere Umhüllungen aus zellulosehaltigem Material für die Lagerung von Schrift- und Druckgut aus Papier und Pergament. Deutsches Institut für Normung, Berlin: Beuth.

DIN ISO 11799: 2005-06:Information und Dokumentation – Anforderungen an die Aufbewahrung von Archiv- und Bibliotheksgut. Deutsches Institut für Normung, Berlin: Beuth.

DI Pietro, G., F. Ligterink, F.: The limited impact of acetic acid in libraries and archives. In:Indoor Air Quality 2012, 10th International Conference Indoor Air Quality in Heritage and Historic Environments “Standards and Guidelines”, Book of Abstracts, London: UCL Centre for Sustainable Heritage, 2012, 19

Green,L.R., Thickett, D.: Testing materials for use in the storage and display of antiquities: A revised methodology.  Studies in Conservation 40 (1995): 145-152.

Grzywacz C.,:Monitoring for gaseous pollutants in museum environments. Los Angeles: The Getty Conservation Institute, 2006.

Hochschule für Technik und Wirtschaft Berlin: Eine Weiterbildung zum Indikatortest nach Oddy. http://weiterbildung.htw-Berlin.de/SysBilder/File/HTW_Weiterbildung_Oddy_2013.pdf

Lee, L.R., Thicket, D .:Selection of materials for the storage or display of museum objects. London: Britisch Museum Occasional Paper 111.

Oddy, A.: The corrosion of metals on display. Conservation in Archaeology and the Applied Arts,(N. S.Brommelle und P.Smith  Hrsg.), London: IIC (1975): 235-237

Potthast, A., Kostic, M., Jeong, M-J., Becker, M., Zweckmair, T., Ahn, K., Bohmdorfer, S, Rosenau, T.:  Hydrolysis and surface modification of paper during aging.  in European workshop on cultural heritage preservation EWCHP 2nd, EWCHP, Oslo, 2012.

Robinet, L., Thickett, D.:  A new methodology for accelerated corrosion testing.  Studies in Conservation 48 (2003): 263-268

Strli?, M., Kralj Cigi?, I., Možir;A.,de Bruin; G.,  Kolar, J., Cassar, M.: The effect of volatile organic compounds and hypoxia on paper degradation. Polymer Degradation and Stability 96 (2011): 608-615

Tetreault, J., Sirois, J.,Stamatopoulou, E.: Studies of lead corrosion in acetic acid. Studies in Conservation 43 (1998): 17-32.

Volland, G., Hansen, D., Knjasev, V., Meyer ,F.: The „Schinkel’s Legacy“ Project at the Kupferstichkabinett / Schinkelmuseum Berlin. Subproject: Air Quality in Warehouse Storage Cabinets – Cause and Effect. Restaurator 34, No.3, Restaurator, in Druck.

Zhang, J., Thickett, D., Green L.: Two tests for the detection of volatile organic acids and formaldehyde. Journal of the American Institute for Conservation 33 (1994): 47-53