Materials and techniques for coating of the nineteenth-century plaster casts. A scientific and archival investigation of the Victoria & Albert museum cast collection.

Risdonne, Valentina (2022) Materials and techniques for coating of the nineteenth-century plaster casts. A scientific and archival investigation of the Victoria & Albert museum cast collection. Doctoral thesis, Northumbria University.

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Abstract

This thesis presents the first interdisciplinary and multi analytical study of the historical coatings of nineteenth century plaster casts of the Victoria and Albert Museum (V&A) collection. The findings presented herein are anticipated to support the decision-making for the care, preservation and conservation of these objects.

Seventeen historical manuals and fourteen patents were reviewed to understand the nineteenth-century practice for coating plaster casts. More than eighty organic and inorganic materials were mentioned in over a hundred recipes retrieved in the literature. When specified, the purpose of the application of the coating was to ensure a desired appearance and/or improve the durability of the object by hardening and waterproofing the surfaces.

A selection of twelve nineteenth-century casts produced in different European workshops was analysed. A multi analytical approach was designed to observe the stratigraphy – by Visible Light Reflectance (VLR) and Ultra-Violet fluorescence (UVf) Optical Microscopy (OM) - and characterise the inorganic - by Scanning Electron Microscopy-Energy - dispersive X-ray spectroscopy (SEM-EDS) and X-Ray Diffraction (XRD) - and organic components – by Fourier-Transform Infrared spectroscopy (FT-IR), Gas Chromatography/Mass Spectrometry (GC/MS) and pyrolysis GC/MS. A complex layer system and evidence of coatings’ diffusion into the bulk were consistently identified. The bulk was made of calcium sulfate dihydrate (CaSO4·2H2O) and mineral carbonate and clay inclusions. In most of the analysed casts, the coating contained aluminium, silicon and an organic medium that was either aged diterpenic or shellac resin.

To study the changes on coated plaster surfaces upon ageing, reconstructions made of gypsum plaster coated with linseed oil, shellac, barite (BaSO4) and beeswax were made, based on the analysis and the literature review. The reconstructions were aged using selected conditions of temperature, humidity and light. The aged samples were soiled and cleaned using a novel cleaning method. The effects of ageing were evaluated with visible and UVf photographs, SEM, High-Resolution 3 Dimensional (HR 3D) surface measurements, glossimetry, colourimetry and FT-IR. Linseed oil and shellac penetrated the pores, barite chemically modified the surface by interacting with the calcium sulfate, whereas beeswax formed a film on the plaster surface. Polyolefin-supported polyacrylamide ultra-thin hydrogel films were tested for the removal of soil from the reconstructions and proved effective, in particular on the film-forming coatings.

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