Nikki van Basten, Ulysses Jackson, and Rachel Rivenc
Abstract
Outdoor painted sculptures are constantly exposed to aggressive environments and therefore highly prone to surface damages. Surface damages are not only disfiguring but will also accelerate degradation mechanisms and in the long-term will lead to invasive and costly repainting campaigns. Local retouching can be an efficient solution to postpone these campaigns when paint losses occur on outdoor painted sculptures, as a temporary measure to restore the aesthetic integrity while preventing further damage to the exposed substrate. Carrying out unnoticeable repairs on flat monochromatic surfaces can be challenging, as is finding materials with the desirable handling properties. A wide range of materials and methods for priming, filling and retouching metal painted surfaces were investigated at the Getty Conservation Institute.
The study builds on previous research, in which primarily industrial paints were tested for retouching. While industrial paints designed for the outdoor are undeniably the more durable option, they are costly, difficult to procure in small quantities, and can be hard to apply and manipulate to obtain the desired appearance (gloss and color). At the Getty Conservation Institute, artist’s- and conservation paints usually employed indoors were tested instead, since they offer a number of advantages including their ease of handling (i.e., application, manipulation) and their availability in small quantities at low costs.
The main focus of the study was to test a number of additives to enhance the longevity of the paints investigated. In consultation with paint manufacturer Golden Artist’s Colors these were: Mineral Spirits Acrylics (poly(n-butyl methacrylate)), Fluid Acrylics and Heavy Body Acrylics (both poly(n-butyl acrylate/methyl methacrylate)butyl acrylate). Paraloid B48-N (methyl methacrylate/butyl acrylate) with pigment-pastes was also included. For all paints, a variety of additives were tested, separately and in combination: adhesion promoters, UV-stabilizers and matting agents. A number of paints and additives combinations were custom formulated by Golden for the project. To evaluate the durability of the paints and the influence of the additives, samples with and without additives were prepared and exposed to artificial and natural aging. The natural ageing was performed in collaboration with Dow Chemicals at several of their exposure sites. For both natural and artificial aging, visual changes were recorded and FTIR-, color-and gloss measurements were carried out at regular intervals.
To assess the workability of the materials, local treatments were simulated on mock-ups. The mock-ups were exposed outdoors, to observe the long-term durability of the repairs. The data on natural ageing is still being gathered, but the artificial ageing indicated that all manipulated paints would show high outdoor durability; the modified Mineral Spirits Acrylics showed especially promising results. From a practical perspective, Fluid Acrylics were the best performers: they are easy to handle, have high hiding-power and can be applied with an airbrush, allowing for almost invisible touch-ups. The research provides technical and practical information of a broad range of materials for local treatment. If natural ageing confirms the positive outcomes of artificial aging than a possible application of the study could be to make the paint formulas commercially available to the conservation field.