Person in a lab uses a tool on a used paint palette

This research seeks to understand mechanical properties of naturally aged artists’ paints to inform active conservation treatments and passive preventive conservation measures.

Artists’ paints differ considerably in their material properties depending on a variety of intrinsic factors such as binder composition, the nature of pigments, extenders, driers and other additives. They also change over time, initially being in a liquid-like form that evolves into a solid-like paint film. The timescale over which drying and/or curing processes occur varies from a few hours for acrylics to months and decades for alkyds and oil-based paints respectively.

Existing scientific research indicates that properties such as stiffness, strength, and fracture toughness of paints very strongly depend on their composition, manufacturing as well as aging. An exact knowledge of the mechanical properties of paints being a part of an artwork and the way those properties change with variations in moisture content and temperature can inform active conservation treatments and passive preventive conservation measures.

Measuring the mechanical properties of original paints that are part of a work of art is a serious practical challenge. In the heritage conservation field, it is important to limit sampling to the absolute minimum. This disqualifies conventional tensile and compression tests that require the destruction of macroscopic sample sizes.

An alternative approach, applied in our laboratory, is the use of nanoindentation technique suitable for the direct examination of submillimeter samples. A standard test consists of continuously measuring the load and displacement of an indenter as it is pressed into and withdrawn from the material. The load-displacement data contain an abundance of information about the tested material. The standard test provides the Young’s modulus and hardness of the sample, but when more complex measuring schema are employed, its complex modulus, yield stress, creep and fracture toughness can also be characterized.

A major advantage of nanoindentation is that the material properties of submillimeter samples taken from artwork can be analyzed in a virtually nondestructive way—no chemical alteration and minimal physical impact—allowing the sample to be used for subsequent testing.

Nanoindentation can be especially effective in tracing subtle changes of material properties and when combined with more conventional techniques, such as quasistatic (UTM) and dynamic mechanical analysis (DMA) applied on reference materials, can provide unique insight in properties of paints.

Current Work

Developing methods for analysis cross-sectional samples of historic and artistic paints

The complexity of analyzing the mechanical properties of cross-sectional paint samples stems from two separate issues.

First, the intrinsic properties of the pigment-binder composite material may vary due to its heterogeneity, time-dependent behavior, and sensitivity to temperature and relative humidity. Testing such materials requires a statistical approach to address a heterogeneous composition and a consistent method to establish the contact depth and, consequently, the probe-material contact area.

Second, the analysis of mechanical properties may be impacted by the specific geometry of paint samples and, in the case of historic paints, their small volumes. High surface roughness combined with the substrate effect requires a careful selection of indentation depths. Examination of cross-sectional samples must also consider edge effects.

In our research, we propose specific measuring protocols and original methods of data analysis to address these issues and obtain meaningful mechanical characterization of cross-sectional paint samples. We also focus on optimizing sample preparation protocols.

Correlation between micro and macro properties of paints

The study examines the relationship between the time-dependent mechanical properties of artistic paints measured by nanoindentations and dynamic mechanical analysis (DMA). The difference is not only in the scale of the exerted deformation but also in the deformation method: compression-shear for the nanoindenter and tension for DMA.

Experiments are performed on various acrylic and oil mockup paint samples. The same material is used in free film form for DMA testing and in cross-section form for nanoindentation. Systematic experiments will result in collecting a comprehensive, multi-scale set of mechanical data for various artists’ acrylics and oil paints.

The work is particularly relevant for studying historic oil paints. They age for a very long time, and the aging process cannot be artificially simulated. This means properties of historic oil paints can be established only based on small-scale measurements such as nanoindentation. Defining a multiscale calibration curve would allow for translation of micro mechanical properties to macro mechanical ones. The latter can be directly used to inform models predicting risk of climate induced damage.

Analysis of soft and dripping paints

The aim of this study is to analyse the correlation between chemical, visual, and mechanical properties of soft and dripping paints used by a number of significant artists in the mid-twentieth century.

Understanding the mechanical properties of these paints at various humidity conditions can help to improve strategies for their exhibition and storage.

We have developed a novel flat-punch nanoindentation methodology which can be applied for minute, unembedded samples of very soft materials. It was applied for testing bone black oil paints from paintings by Karel Appel and Asger Jorn, across a range of relative humidifies.

The experiments reveled that analyzed paints possess softening points below the typical humidity range of modern museums. It convincingly explains why these paints exhibit conservation problems even in a standard climate-controlled museum environment.

Effects of treatment–cleaning studies

Very few analytical techniques are available that have the sensitivity to study the effect of surface cleaning on the mechanical properties of the uppermost microns of paint surfaces. The combination of nanoindentation and dynamic mechanical analysis provides a unique possibility to compare mechanical changes of the very surface and bulk of paint resulting from any conservation treatment, including exposure to solvents.

Our recent study analyzed naturally aged yellow ochre oil paint films after immersion and sponge-cleaning using selected solvents common in conservation practice. The modern solvent choices and the gentler application method of sponge cleaning resulted in a limited mechanical effect on the surface.

The study emphasizes the importance of focusing on appropriate choice of solvents and application systems during cleaning procedure.

Development of mock-up paint compatible with original historic material

Conventionally, macro-sized mock-ups are used to approximate the mechanical properties of historic artists’ materials. These properties cannot be obtained directly from the microsamples without understanding the relation between results of measurements performed in micro and macro scale. While this relation may be difficult to establish for any given material it is possible to test mock-up samples using nanoindentation and directly compere results with results of test on historic samples.

The main focus of our work is to understand how parameters obtained from measurements in the microscale can inform preparation of mock-ups for different type of paints. The methodology was previously tested for distemper paint from Eidsborg Stave Church in Norway.

For this type of paint, the effectiveness of the developed methodology was limited by the heterogeneity of the ested material. Obtained results suggest however, that mechanical compatibility of mock-ups of more homogeneous samples can be successfully validated using nanoindentation measurements.

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