Surface micro-profilometry for the assessment of the effects of traditional and innovative cleaning treatments of silver

Abstract

In the complex procedure for the conservation of a work of art, the effective monitoring of the surface treatment is an important step in order to establish a conservation protocol. The evaluation of the effects of the cleaning requires the inspection of the outermost layer with complementary diagnostic techniques to obtain information on both materials and surface morphology. In this interdisciplinary work, a custom optical micro-profilometry device based on laser conoscopic holography was used for testing the performance of different mechanical procedures for the cleaning of silver alloy objects. Tarnishing is the main alteration phenomenon for silver alloy artworks. Even if such modification does not affect the long-term preservation of the objects, it heavily influences aesthetical features, making the removal of tarnishing one of the most important conservation treatments of the case. The mechanical cleaning procedure based on abrasive powders suspended in a liquid matrix is of widespread use. Recently, a novel dry cleaning procedure based on the use of erasers was proposed as alternative method and tested for both the efficacy of tarnishing removal and effects on the surface appearance using optical microscopy and scanning electron microscopy. However, the employed laboratory techniques cannot provide an objective measure of the roughness at micrometric scale over the sample. In this work, the effects on the surface structure of traditional and innovative silver treatments was investigated with multiscale profilometry, using scanning conoscopic holography and atomic-force microscopy. 3D surface metrology was carried out using standard areal amplitude parameters and a multiscale approach. Preliminary results are presented, addressing two aspects: 1) the validation of laser optical micro-profilometry for the diagnostics of silver objects, which are challenging due to the highly specular surface; 2) the validation of the dry-cleaning treatment based on rubbers, by adding to the previous study the complimentary analysis of the micro-surface. The results obtained on silver mockups confirm that the proposed dry cleaning methods produce surface variations less or similar to traditional methods. The optical micro-profilometry results are of particular significance since this technique is portable with wide field capability, thus allowing an in-situ use and a real monitoring of actual artwork surfaces.

3D scanning of daguerreotypes

“…Conoscopic holography has been successfully applied to measure metal surfaces and evaluate the results of different cleaning methodologies. The signal-to-noise (SNR) ratio, provided by the instrument driver as a percentage value, was used to evaluate the quality of the output of the probe [23,24]. The authors [23] developed a routine to optimize the performance of the conoscopic holography probe for the multi-material characterization, demonstrated on a Russian icon.…”

“…The signal-to-noise (SNR) ratio, provided by the instrument driver as a percentage value, was used to evaluate the quality of the output of the probe [23,24]. The authors [23] developed a routine to optimize the performance of the conoscopic holography probe for the multi-material characterization, demonstrated on a Russian icon. By using the SNR value as threshold, a stack of acquisitions with different laser settings is combined into a single optimized surface map.…”