This review paper offers a summary of historical techniques and materials used to produce plaster casts and to treat their surfaces, as found in the historical literature. It reviews a selection of manuals, treatises, and recipe books on the art of the cast makers as well as a selection of patents related to the manufacturing of casts and the coating of their surfaces. The introduction to the review shows how relevant archival, historical, and technical investigation demonstrates that nineteenth century replicas are works of cultural significance in their own right, by means of a synthesis of background information. This paper also offers a brief introduction to plaster moulding and casting as well as a summary of historical recipes on how to treat the surfaces of plaster casts to change the surface properties or their appearance. This review aims therefore to provide a summary of materials used by the nineteenth-century plasterer, and technical information for the use of conservators, researchers, and curators who engage in the care and study of historical plaster casts. Examples from the Victoria and Albert Museum's collection support the information found in the historical literature.

Surface decontamination is of general concern in many technical fields including optics, electronics, medical environments, as well as art conservation. In this respect, we developed thin copolymer networks covalently bonded to flexible polyethylene (PE) sheets for hydrogel-based cleaning of varnished paintings. The syntheses of acrylates and methacrylates of the surfactants Triton X-100, Brij 35, and Ecosurf EH-3 or EH-9 and their incorporation into copolymers with acrylamide (PAM) and N-(4-benzoylphenyl)acrylamide are reported. Photocrosslinked polymer networks were prepared from these copolymers on corona-treated PE sheets, which can be swollen with aqueous solution to form hydrogel layers. The cleaning efficacy of these PE-PAM hydrogel systems, when swollen with appropriate cleaning solutions, was evaluated on painting surfaces in dependence of the PAM copolymer composition and degree of crosslinking. Specifically, soil and varnish removal and varnish surface solubilization were assessed on mock-ups as well as on paintings, indicating that even surfactant-free cleaning solutions were effective.

Previous works of the authors have presented the changes in the optical, mechanical, and chemical properties of the oiled areas of the supports that occur upon ageing due to oil-binder absorption in works of art on paper and printed material. In this framework, transmittance FTIR analysis has indicated that the presence of linseed oil induces the conditions to promote the deterioration of the oil-impregnated areas of the paper supports. However, the analysis of oil-impregnated mock-ups did not provide detailed information about the input of linseed oil formulations and the different types of paper support on the chemical changes that occur upon ageing. This work presents the results of ATR-FTIR and reflectance FTIR, which were used for compensating the previous results, proving indications on the effect of different materials (linseed oil formulations, and cellulosic and lignocellulosic papers) on the development of chemical changes, thus, on the condition of the oiled areas upon ageing. Although linseed oil formulations have a determining effect on the condition of the oiled areas of the support, the paper pulp content appears to have an input to the chemical changes that occur in the system of paper–linseed oil upon ageing. The results presented are more focused on the oil-impregnated mock-ups with cold-pressed linseed oil since results have indicated that this causes more extended changes upon ageing.

This work shows the impact of one Er:YAG laser pulse to both the surface and bulk of aged dammar varnishes. Tests were performed in fluences ranging from 0.56 to 2.4J/cm² and pulse durations of 100 and 300μs. Temperature changes upon irradiation were examined by (a) live recordings with an infrared thermal camera, and (b) determining the optical properties of the varnish, which allowed for estimates of the linear absorption coefficient and the temperature rise associated with the laser irradiation. Transmission and Fourier Transform Infrared (FTIR) spectroscopy studies captured increased energy transmissions through varnish films, as well as registering a reduction of hydroxides and carbon–hydrogen bonding as a function of fluence, respectively. FTIR and Differential Scanning Calorimetry (DSC) experimentally determined the absorption coefficient and the specific heat capacity of dammar films, and an overview of laser spots was carried out with Scanning Electron Microscopy (SEM). Evidence on the varnish response to Er:YAG laser irradiation is provided.

A multi-analytical study was designed to characterise historical coated plaster surfaces. The method was applied to investigate the surface coatings of the nineteenth-century plaster cast of the tombstone of the Presbyter Bruno that belongs to the Victoria and Albert Museum collection. At first, selected samples of the object were examined with Visible Light Reflectance and Ultra-Violet Fluorescence Optical Microscopy (VLR- and UVf-OM respectively) and Scanning Electron Microscopy (SEM) demonstrating a consistent stratigraphy featuring a bulk, an interface and an uppermost layer. The latter layer appeared to consist of an aged coating and dirt. Overpainted and repaired areas of the object generated samples that had additional layers on top of the aforementioned stratigraphy. A layer that seemed to be an additional surface varnish or a coating that had not been absorbed to the bulk has been observed in a couple of samples. Elemental characterization was carried out with energy dispersive X-ray spectroscopy (EDS) and further analyses were performed with X-ray diffraction (XRD) and Fourier-transform infrared (FT-IR) spectroscopy with focal plane array (FPA) imaging which confirmed that the bulk of the object is made of gypsum plaster containing mostly silicate and carbonate inclusions. Gas chromatography/mass spectrometry (GC/MS) and pyrolysis-GC/MS with extraction methods based on n-propanol followed by pentafluoropropionic anhydride (PFPA), tetramethylammonium hydroxide (TMAH) and 3-trifluoromethylphenyltrimethylammonium hydroxide (m-TFPTAH) were performed to detect organic media. The results suggest that the organic medium used for the surface coating is a diterpenic resin that contained silicon, aluminium and traces of other inorganic elements. The organic medium of overpainted areas was based on alkyd resins and the in-paints were characterised as a blend of silicon and barium at varied concentrations. This multi-analytical approach can generate a better understanding of manufacturing, component materials and conservation issues of coated plaster objects.

Four nineteenth-century casts of the decoration on the north side of the exterior of the apse of the Notre-Dame Cathedral in Paris are held in the plaster casts collection at the Victoria and Albert Museum in London. The casts were manufactured by two different nineteenth-century workshops, one run by Jean Pouzadoux and the other by Auguste Malzieux. After an assessment of the condition of the casts, a scientific analysis allowed the characterization of the manufacturing materials and subsequent conservation treatments aimed at ensuring the stability of the casts and removing dirt and grime from the casts’ surfaces. Optical microscopy of the samples taken from the casts allowed the stratigraphy to be studied, which largely consisted of gypsum plaster and a coating layer (oxidized diterpenic resin or shellac) containing silicon and aluminium partially diffused in the porous substrate. These materials were identified by a range of techniques, including X-ray diffraction, scanning electron microscope–energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, and gas chromatography/mass spectrometry. The conservation works returned stability to the panels for redisplay in the galleries and achieved a closer comparative study between the two workshops. The two sets of panels showed numerous differences in manufacturing processes that corresponded to their observed deterioration.