Characterization and Identification of Varnishes on Copper Alloys by Means of UV Imaging and FTIR

Abstract

The application of varnishes on the surface of metal objects has been a very common practice since antiquity, both for protective and aesthetic purposes. One specific case concerns the use of tinted varnishes on copper alloys in order to mimic gilding. This practice, especially flourishing in the 19th century for scientific instruments, decorative objects, and liturgical items, results in large museum collections of varnished copper alloys that need to be preserved. One of the main challenges for conservators and restorers deals with the identification of the varnishes through non-invasive and affordable analytical techniques. We hereby present the experimental methodology developed in the framework of the LacCA and VERILOR projects at the Haute École ARC of Neuchâtel for the identification of gold varnishes on brass. After extensive documentary research and analytical campaigns on varnished museum objects, various historic shellac-based varnishes were created and applied by different methods on a range of brass substrates with different finishes. The samples were then characterized by UV imaging and infrared spectroscopy before and after artificial ageing. The comparative study of these two techniques was performed for different thicknesses of the same varnish and for different shellac grades in order to implement an identification methodology based on simple non-invasive examination and analytical tools, which are accessible to conservators.

Identification of Coatings on Persian Lacquer Papier Mache Penboxes by Fourier Transform Infrared Spectroscopy and Luminescence Imaging

“…But Kaman oil had a green color and a milky color in UVL1 and UVL2 respectively. Previous studies have also reported an orange fluorescence for shellac [48][49][50]. In UVIBL, shellac and Kaman oil appeared in dark blue and light blue colors, respectively.…”

The Cleaning of Corroded Lacquered Brass with Complexing Agents: A Comparative Study

“…The varnish was prepared by dissolving 112.5 g of orange dewaxed shellac (Kremer Pigmente, Aichstetten, Germany) in 500 g of 96% + 2% MEK ethanol (Kremer Pigmente, Aichstetten, Germany) [40]. The exact procedure for the lacquer preparation has been described in a previous publication [42]. The orange shellac varnish was dewaxed by filtering the solution on 602H filter paper.…”

“…The pictures were taken in manual mode, ISO 200, and f/11 aperture. Calibration of the pictures was performed according to the procedure which has been described in a previous publication [42]. L* a* b* values of three spots (11 px × 11 px) were then recorded for each coupon in Adobe Photoshop using the color sampling tool to compare the color of the fluorescence of each varnish.…”

Investigating five totalizing counters manufactured by the Alphonse Darras company of the CNAM collections in Paris

“…The UV Innovations Target was used to calibrate the color of the pictures in Adobe Photoshop software. The pictures were taken in manual mode, ISO 200, and f/11 aperture; the target (low fluorescence side) was photographed with 1 s exposure time, whereas exposure was adapted for the objects and reference samples (15 s) according to a former study [22]. White balance correction was performed on the pictures in Camera Raw using the Adobe Photoshop Setup and Capture Workflows recommended by the calibration target manufacturer [23].…”

“…Two lacquers based on the same recipe for precision instruments were produced with orange shellac in flakes [24]. The exact procedure for the preparation of shellac lacquers has been described before [22]. The coatings were applied to CuZn37 satin finished coupons (60 mm × 60 mm × 3 mm) by manual dip-coating.…”