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Amazing Facts

The Parylene Project: An Update

The Parylene project was announced in the February 1989 issue of the CCI Newsletter, and you may have been wondering what has become of it since. Although a number of publications have reported some of our results, 1-7 all the information has not yet been released. But what have we found?

In a nutshell, we have observed that Parylene deposition can indeed strengthen fragile materials unobtrusively and in a manner unequalled by other processes. It can save items that would otherwise be utterly lost. It can be used in unusually small amounts and under rather controlled conditions compared to other consolidants. Thus, very little material is actually added by the process. The problem for conservators, however, is that application is irreversible. So, under what circumstances can it be used? To answer this question, I will briefly review and discuss the progress made in each of the three parts of the Parylene project.

Coating diorama specimens, such as these birds, with Parylene gives them better protection in travel, makes them much easier to clean, and increases their useful display life.

The Fossil Forest

The primary aim of the project was to preserve delicate leaf and cone specimens from the fossil forest on Axel Heiberg Island in the Canadian Arctic. This has been successfully accomplished. With the assistance of Margaret Morris, a conservator on contract at CCI, we treated a substantial collection for the Canadian Museum of Nature, and smaller quantities for the Universities of Saskatchewan and British Columbia. The results were generally successful and have been published,₄; as was a description of the storage method for the Canadian Museum of Nature's cones.₇;

Applications of the Technique to Artifacts and Specimens

This aspect of the project involved the cooperation of a number of museums and archives from Quebec, Ontario, Manitoba, Alberta, Saskatchewan, and British Columbia as well as from other countries including Germany and the USA. A great many items have been treated with Parylene, and the results have been examined and commented upon by those who supplied test samples. All their comments and data are included in a report, which is still incomplete. Some parts of it, however, have been published, 1,3,5,6 and others, such as the evaluation of Parylene-coated textiles, are available upon request.

We have found a number of instances where this process can be of great use. The gross morphology of natural history specimens is important; hence, Parylene is useful. In simple terms, it's nice to be able to show insect specimens with the legs, wings, antennae, etc. still attached. We successfully consolidated various plant specimens, birds, insects, and a wide variety of marine items from crustaceans to sponges. The possibility of irreversibly damaging something unique is less of a concern for natural history specimens because museums rarely rely on single specimens. There are also many aspects other than just morphology that museums may wish to preserve. No single technique suffices — or has ever sufficed — to preserve all of these facets. Thus, using a range of preservation techniques is a sensible approach and one into which Parylene can be fitted without ethical difficulty.

Another area of potential use is with diorama display items. Not only does Parylene allow delicate items such as flowers or leaves to be prepared for such use, it also allows them to be cleaned easily by washing them with hot soapy water or even with commercial dry-cleaning agents. Coated specimens are also more likely to survive the stresses of being in travelling exhibitions.

Parylene has found application for artifacts that are to be used for didactic purposes or that are reaching the desperation stage. Examples of the latter are vestigial archaeological textiles, charred fibre samples, and degrading and crumbling 20th-century materials such as foam rubbers (for example, in radio equipment, puppets, and dolls).

One rather interesting use shows some of the unique potential of Parylene. Recently, Parylene was used to aid in the reversal of another treatment. In the Historic Resource Conservation Laboratory of the Canadian Parks Service, Environment Canada, Ottawa, conservators wished to remove alum during the reconservation of some wooden artifacts. Attempts to soak out the alum in water were unsuccessful because the degraded wood ended up as pulp. Clifford Cook, of the Historic Resource Conservation Laboratory, had the idea of using a Parylene coating to keep the wood intact during the soaking period. This worked very well: the thin Parylene coating allowed the alum to diffuse out, yet held the wood together. Thus, irreversible Parylene deposition allowed another "irreversible" process to become reversible. (Heaven only knows what this means for our conservation codes of ethics, which are rather dogmatic about reversibility!)

Another possible application is in consolidating 19th-century red-rotted ledger bindings, which was achieved without coating the paper inside. This process could enable the salvage of such bindings, which are now written off when re-binding is carried out.

In the course of our study, we did a few successful consolidation tests with burnt paper for Don Purdy of the RCMP forensic laboratory. Then, on March 10, 1989, an Air Ontario Fokker F-28 crashed in Dryden, Ontario. Shortly thereafter, CCI received a request from the Canadian Aviation Safety Board (now the Transportation Safety Board of Canada) to freeze-dry the soaked and charred logbooks from the plane. Parylene coating proved very useful; it strengthened the burnt pages sufficiently for the books to be separated page-by-page, and permitted a high recovery of evidence that apparently proved useful in the enquiry.

Aging Properties of Parylene

The third aspect of the project has been to characterize the aging properties of Parylene films. Initially, we conducted a thermal study to measure stability in the absence of light. Initial results, which were published in 1991,² predicted an extremely long period of stability of the order of thousands of years. More recent longer-term studies at lower temperatures (unpublished) predict somewhat shorter useful lifetimes. What this may mean in practice is not yet certain, but at the moment we think that it is unlikely to alter current criteria for the adoption of Parylene in conservation.

Light aging experiments are also in progress. Early studies showed that Parylene could be degraded by light, and that Parylene N was much more sensitive than Parylene C. The effects of aging were primarily physical; there was only very slight yellowing. Films embrittled and fractured more easily after exposure to high light intensity. This work continues, and we hope to be able to determine quantitatively how long and under what conditions it is possible to expose Parylene to light.

The Future

Generally speaking, the project has been a success. The position now is that the Parylene coater — which will permanently reside at CCI — is available for tests for any Canadian museum and for treating artifacts or specimens as required. If you wish to try coating a test object or if you simply want to know further details about the process, please get in touch with David Grattan at CCI.

Acknowledgment

We thank Bruce Humphrey of Speciality Coating Systems, Union Carbide Corporation, for his assistance during all phases of this project.

References

David W. Grattan, "Parylene at the Canadian Conservation Institute," Canadian Chemical News, vol. 41, no. 9 (October 1989), pp. 25-26.
D.W. Grattan and M. Bilz, "The Thermal Aging of Parylene and the Effect of Antioxidant," Studies in Conservation, vol. 36 (1991), pp. 44-52.
David W. Grattan, "Parylene at the Canadian Conservation Institute - An Initial Survey of Some Applications," in: Preprints of the 9th Triennial Meeting of the ICOM Committee for Conservation Conference, Dresden, Germany, 26-31 August 1990. Kirsten Grimshaw, ed. Los Angeles: ICOM-CC, 1990, pp. 551-556.
D.W. Grattan, "The Conservation of Specimens from the Geodetic Hills Fossil Forest Site, Canadian Arctic Archipelago," Geological Survey of Canada Bulletin 403: Tertiary Fossil Forests of the Geodetic Hills, Axel Heiberg Island, Arctic Archipelago R. L. Christie and N. J. MacMillan, eds. Ottawa: Energy Mines and Resources, 1991, pp. 213-227.
David Grattan and Margaret Morris, "The Potential of Parylene for Consolidating Natural History Specimens," Natural History Conservation, Issue 6 (September 1991), pp. 4-7.
Helen D. Burgess and David W. Grattan, "The Conservation of Parylene Coated Books and Papers," Sauvegarde et conservation des photographies, dessins, imprimés et manuscrits, Actes des journées internationales d'études de l'ARSAG, Paris, 30 septembre au 4 octobre 1991, pp. 231-242.
David Grattan and Margaret Morris, "Tray with Polyethylene Foam Grid Separators," Storage of Natural History Collections: Ideas and Practical Solutions. Carolyn L. Rose and Amparo R. De Torres, eds. Washington, D.C.: SPNHC, 1992, p. 199.