2015 Copper Alloy Treatment Survey

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2015 Copper Alloy Treatment Survey (CATS)

A small informal survey of archaeological conservators undertaken in Summer 2015 revealed that further investigation of treatment protocols for archaeological copper alloys is necessary as professionals currently working in the field need more concrete knowledge regarding the efficacy of corrosion inhibitors and coatings.

The survey asked respondents to describe their treatment protocols for copper alloy archaeological materials, and asked questions about long-term storage, condition checks, and retreatment. Questions were distributed to a group of archaeological field conservators by email. The following is a summary of the initial survey results.

See also: Conservators Converse Blog post: “Treating Archaeological Copper Alloys on Site: A Survey on Current Practice” Anna Serotta, Eve Mayberger, and Jessica Walthew. February 2016.


Scope of survey

There were 24 full responses from colleagues and input on individual questions from a handful of others.

  • Collectively, respondents have worked on approximately 50 different sites. The majority of these sites are in the Mediterranean, North Africa, or the Middle East (Greece, Cyprus, Italy, Turkey, Jordan, Israel, Egypt, Sudan, and Syria), but respondents have also worked in Pakistan, Mongolia, Peru, Panama, Chile, and on various sites in the Continental US. Almost all of these sites are terrestrial, although several respondents have worked on metals from underwater/shipwreck sites as well.
  • The condition of metals on all of these sites is of course extremely variable, but all respondents reported unstable copper alloys and bronze disease outbreaks on at least one of the sites they worked on.

Cleaning

Most respondents use predominantly (or exclusively) mechanical cleaning methods for corrosion reduction.

  • Respondents generally avoid wet cleaning, except for the use of ethanol and/or a mixture of ethanol and water in combination with mechanical cleaning.
  • Most respondents avoid any sort of chemical cleaning, although some mention doing so in the past.
  • A minority of respondents did report using one or more of the following chemical treatment methods (predominantly for coins): Rochelle salts, alkaline glycerol salts, EDTA, formic acid, ion-exchange resins, Calgon, electrochemical and/or electrolytic methods; these methods are generally followed up with rinsing and mechanical cleaning.

Desalination

Most respondents do not soak their metals to remove soluble salts. This seems to have been a more common practice in previous years; several respondents mentioned discontinuing previously established soaking procedures on their sites.

  • Some respondents said that they only soak metals after chemical treatments to remove residues.
  • Several respondents questioned the efficacy of soaking to remove chlorides and mitigate bronze disease; since corrosion products like nantokite are not water-soluble, it is unknown what is actually being removed with soaking. In addition, there was some concern that soaking could actually have adverse effects on condition by exposing the metal to moisture and promoting chloride corrosion growth. The time-consuming nature of this treatment was also mentioned as a factor against its use. It can also be challenging to obtain enough deionized water for desalination.

Corrosion Inhibitors

Most respondents reported occasionally or regularly using benzotriazole (BTA) as a corrosion inhibitor


Summary of BTA application protocols reported:

  • BTA is generally applied by immersion (whenever feasible)
  • Roughly half of the respondents who treated with BTA immersed in a vacuum desiccator. Availability of equipment and stability of the artifact were considered in deciding whether to immerse in a vacuum.
  • Those respondents who reported the specific concentration all used 3% in ethanol; one responder mentioned the use of a brush application of 10% BTA for particularly concerning chloride-driven corrosion


Immersion time varied considerably. Reported immersion times ranged from 15 minutes to several days.

  • Overnight, 12-24 hours, or 24 hours were the most commonly reported immersion times
  • Several respondents mentioned research supporting the optimal effectiveness of immersion for one hour
  • Several respondents have tried using 0.1M BTA + 0.01M AMT, as reported by Golfomitsou (ref 1); those who tried it generally did not notice much of a difference between this treatment and treatment with BTA alone


Additionally, a couple of respondents mentioned testing other corrosion inhibitors: e.g. cysteine, or carboxylic acid-based treatments (ref 2)

  • Several respondents only use corrosion inhibitors only when an object cannot be placed in a desiccated environment; otherwise, only preventive methods are used.
  • There were concerns raised regarding both the efficacy and the safety of BTA. (i.e. safety both during application and also safety concerns for people handling the artifacts). On one occasion concern was expressed about BTA interfering with future analysis.
  • The importance of rinsing in ethanol after treatment to remove excess BTA was mentioned. One respondent reported the development of a BTA-copper chloride complex within 24 hours.

Coating

Many respondents reported occasionally or regularly coating their copper alloy objects.

  • Paraloid B-48N was the most common material used, but many people also reported using Paraloid B-72, Paraloid B-44, and Incralac; there was one report of the use of cellulose nitrate.
  • When mentioned, factors influencing the choice of coating material included: availability, Tg, and whether or not solvent toxicity was a problem (pertaining to Incralac).
  • Many respondents did not indicate the method used for coated, but those who did generally reported coating by immersion; a couple of respondents reported two applications of the coating material.
  • Several respondents do not coat their metals and expressed some concern about the creation of microclimates under the coating film that would encourage further corrosion. A couple of respondents coat only in specific circumstances: when metals will be displayed or when consolidation is required.

Storage

Over half of the respondents store metals in silica gel at some or all of the sites on which they work.

  • Several respondents used the RP system and Escal bags (ref3) for long-term storage.
  • Most of the respondents who use silica gel recondition it annually. One respondent reported reconditioning based on indicator color change. A few respondents who use silica gel report having no annual access to metals after treatment.
  • A couple of the respondents who do not use silica gel or other desiccated storage reported environmental conditions that were dry enough not to warrant micro-climates.
  • Some respondents expressed apprehension about using silica gel when yearly access for reconditioning was not guaranteed; these respondents are concerned that housing with unconditioned silica gel will cause greater problems than housing without silica gel. One respondent suggested that reconditioning yearly may not even be enough.

Re-Treatment

Over half of respondents were able to survey their metals to check stability.

  • Some reported doing this regularly every year or every other year. Some respondents reported having too many metals for annual survey, so partial surveys were done, or more random checks, depending on time constraints and when metals are accessed by researchers.
  • Some respondents reported using silver oxide for treating bronze disease outbreaks; others reported using the same methods used for initial treatment. Several respondents questioned the efficacy of silver oxide and expressed concern about its implications for future analysis.

References

Golfomitsou, Stavroula and John Merkel. “Understanding the efficiency of combined inhibitors for the treatment of corroded copper artefacts.” METAL 07 Proceedings of the Interim Meeting of the ICOM-CC Metal Working Group (5): 38-43.

Gravgaard, M. and J. van Lanschot. 2012. “Cysteine as a non-toxic corrosion inhibitor for copper alloys in conservation.” Journal of the American Institute for Conservation 35 (1): 14-24. http://www.tandfonline.com/doi/abs/10.1080/19455224.2012.681618

Mathias, C., K. Ramsdale, and D. Nixon. 2004. “Saving archaeological iron using the Revolutionary Preservation System.” Proceedings of Metal 2004, National Museum of Australia Canberra ACT, October, 4-8 2004: 28-42. http://www.nma.gov.au/__data/assets/pdf_file/0020/346034/NMA_metals_s1_p3_saving_archaeological.pdf