The Samuel F. B. Morse statue in Central Park: scientific study and laser cleaning of a 19th-century American outdoor bronze monument

Abstract

Many of Central Park's bronze statues were coated with lacquer as a protective measure between the late 1970s and early 1990s. In several cases, these coatings outlasted their performance life and were no longer adequately protecting the sculptures. In 2010, a cyclical campaign to remove and refurbish the coatings provided an opportunity to closely assess the condition of de-coated surfaces and to develop suitable, sustainable treatment approaches. The Samuel F. B. Morse statue, created in 1870 by Byron Pickett, commemorates the 19th-century painter and inventor known for the telegraphic code that bears his name. Initially, samples from this statue were collected on the field and analyzed with various techniques to characterize the degradation products that were heavily affecting its surface and the organic coatings used to protect the bronze. Various corrosion species, mainly sulfates and chlorides, were identified by X-ray powder diffraction (XRD), while pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) revealed that the organic coatings were based on mineral wax and acrylics. These results informed a decision to relocate the statue to a controlled workshop space to aid in the manipulation of the large figure and optimize cleaning by laser ablation. Treatment was monitored through three campaigns of analysis using portable X-ray fluorescence (pXRF) spectroscopy, performed on select locations, to characterize the extent of corrosion across the sculpture's surface and monitor changes on the target areas throughout the cleaning. Following the initial phase of treatment, pXRF showed the substantial removal of iron-and lead-rich compounds from the brownish-toned coating and atmospheric pollution deposition. In the subsequent treatment phases, variations in the relative amounts of sulfates and chlorides were noted and further investigated through the examination of a cross section by means of scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS). Based on the findings of iron-containing pigments in the organic coating, a decision to apply a new, similar, translucent, brown-toned coating containing such coloring materials was made to achieve the desired visual nuance for the finish and provide additional corrosion protection to the bronze. Scientific analysis on the Morse statue has helped characterize its materials, methods of fabrication, and current conditions on a microscopic scale; in addition, it has guided the means and methodologies of treatment and relays essential information useful for future preservation.

Corrosion Behaviors of Outdoor Bronze Sculptures in an Urban–Industrial Environment: Corrosion Experiment on Artificial Sulfide Patina

“…In particular, by conducting corrosion experiments on artificial patina, standard data relating to individual corrosion products such as copper oxide (cuprite) and sulfide patina (brochantite), which are typically observed during the corrosion of outdoor bronze sculptures, were obtained. However, multiple corrosion products are detected in actual outdoor environments [33][34][35][36]; occasionally, other pollutants such as soil minerals, soot, and ashes may also be detected.…”

Corrosion Behaviors of Artificial Chloride Patina for Studying Bronze Sculpture Corrosion in Marine Environments

“…Particularly, an artificial patina corrosion experiment was conducted to obtain standard data on corrosion products such as copper trihydroxychlorides, which are identified in the corrosion characteristics of outdoor bronze sculptures. However, according to several previous studies [4,[37][38][39], products of multiple corrosion are commonly detected in outdoor sites, sometimes in combination with surrounding soil minerals and contaminants, such as soot and ash.…”

Cleaning iron rust compounds from cotton textiles: application to Qing Dynasty armor

“…There are numerous ways of removing rust corrosion products, such as mechanical [10], chemical [11], and laser methods [12,13]. Physical and laser methods are safe and effective for removing rust compounds on the surface.…”