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Figure 4: Bright Orange Florescence Characteristic of Shellac Figure 4: The Dull Green Fluorescence of Aged Shellac is (Long Wave UV)

Savage Coating: NHHC Conservators Team With USNA to Solve a History Mystery

By Kim Roche, NHHC UAB Archaeology and Conservation Laboratory Intern

The remains of Royal Savage, approximately 50 timbers and 1300 artifacts, were excavated in the 1930s and brought to the Naval History and Heritage Command (NHHC) Underwater Archaeology (UA) Branch in July 2015. Most of the Royal Savage artifacts were removed from Lake Champlain more than 80 years ago and received only minimal treatment which resulted in compromising a large percentage of the collection. The iron artifacts, in particular, displayed physical stress and an unidentified surface coating. Beneath this coating, conservators observed active corrosion on the surface of the iron and determined that identification and removal of the coating would be necessary in order to access and treat the corrosion and preserve the artifacts.

Thanks to support from the Office of Naval Research through the Naval Research Enterprise Internship Program, a research project commenced in May 2016 at the UA Archaeology and Conservation Laboratory (ACL) to identify this unknown coating and determine the most effective way to remove it from the affected artifacts.

The Scientific Process

The project began with an overall survey of the artifacts to assess their state of preservation, degree of corrosion (including the presence of any active corrosion) and physical characteristics of the coating. The survey results revealed that 383 out of 459 iron artifacts in the collection were coated with what appeared to be the same unknown material.

Sample Analysis and Testing Techniques

Following detailed condition assessments and documentation, several samples of the coating were collected from different artifacts. These samples were taken to the US Naval Academy in Annapolis, MD where they were examined using spectroscopic and elemental analyses. Fourier transform infrared spectroscopy (FTIR) and X-ray fluorescence (XRF) were conducted at the Naval Academy Chemistry Department, and ultraviolet (UV) imaging was completed at the ACL. These analytic techniques indicated that the coating was likely shellac, a natural resin which also appeared aged and degraded.

FTIR is a spectroscopic technique that analyzes a sample under a specific wavelength range in the infrared region of light. Different bonds within a sample absorb this light at unique wavelengths, and so the resulting absorbance spectrum produced by FTIR (Figure 1) provides information on the types of bonds present in the sample material. The bonds, manifesting as “peaks” on the absorbance spectrum, can be used to help identify a particular material or its class (such as a wax, protein or carbohydrate). In this case, the absorbance peaks produced by FTIR were consistent with the bonds present in shellac.

Figure 1: FTIR Spectrum of the Coating Material (Courtesy US Naval Academy)
Figure 1: FTIR Spectrum of the Coating Material (Courtesy US Naval Academy)
Figure 2: A Sample is Placed on the FTIR Plate for Spectroscopic Analysis
Figure 2: A Sample is Placed on the FTIR Plate for Spectroscopic Analysis

XRF was used to help identify certain elements that might be contained in the still unidentified coating, which could have posed a health risk to conservators. An XRF mosaic was compiled of multiple points on a sample artifact that presented with the coating. The results of the XRF analysis did not indicate significant levels of any hazardous or toxic elements.

Figure 3: XRF Mosaic of an Iron Artifact with Shellac (Courtesy US Naval Academy)
Figure 3: XRF Mosaic of an Iron Artifact with Shellac (Courtesy US Naval Academy)

UV light was used to aid visual assessment of the condition of the shellac.

Initial findings

Generally, shellac fluoresces a characteristic bright orange color under UV. However, in this case the coating on nearly all of the artifacts fluoresced a dull green color indicating that the shellac was old and extremely degraded. As natural resins age, they increase in acidity which poses a threat to the stability of the iron. This is relevant when related to the rate of corrosion of metal, which increases in acidic conditions. Furthermore, the coating had a matte appearance rather than a glossy sheen which also supported this conclusion.

Figure 4: Bright Orange Florescence Characteristic of Shellac (Long Wave UV)
Figure 4: The Dull Green Fluorescence of Aged Shellac is visible on an Iron Fastener (Long Wave UV)
Figure 4: The Dull Green Fluorescence of Aged Shellac is visible on an Iron Fastener (Long Wave UV)

 

Shellac removal and treatment

The poor condition of the aged shellac made its removal much more challenging. As shellac ages, it becomes more polar which requires aggressive solvents in order to remove it. After a series of solvent tests, a solution of 50% ethanol and 50% acetone was determined to be the most effective means of removing the shellac. However, due to its aged condition, significant contact time between the shellac and solution was needed to swell the resin. This was achieved by using a gel as a supporting agent which provided an increased contact time. After the ethanol/acetone gel swelled the resin, it was removed with ethanol and cotton swabs. A brush was used to remove the coating from deeper crevices in the artifact. In addition to the gel, a solution of 2% w/v sodium hydroxide (NaOH) also effectively removed the shellac.

Assessment

The two methods of removal described above should be used in conjunction so that the coating is removed from the numerous artifacts in the safest and most efficient manner. Following the complete removal of the coating, conservators will be able to proceed with treatment of the iron, properly mitigate the corrosion, and preserve the artifacts for the future.

Figure 5: The Gel is Applied to the Shellac (Long Wave UV)
Figure 5: The Gel is Applied to the Shellac (Long Wave UV)
Figure 6: The Shellac Has Been Removed After Application of the Gel (Long Wave UV)
Figure 6: The Shellac Has Been Removed After Application of the Gel (Long Wave UV)

 

UA wishes to thank Professors Joseph Lomax and Wayne Pearson at the US Naval Academy Chemistry Department for their assistance in completing this analysis.

Stay tuned for updates.