Scientific Research and Analysis Laboratory (SRAL)
The scientific analysis of objects is carried out to answer questions about an object's condition, technology of manufacture, history and authenticity. Answers to these questions can be critical for formulating an appropriate conservation treatment. Winterthur's well equipped laboratory includes:
|•||a Fourier transform infrared spectrometer (FTIR)|
|•||a laboratory-based and handheld energy dispersive x-ray fluorescence spectrometers (XRF)|
|•||a scanning electron microscope with energy dispersive x-ray spectroscopy (SEM-EDS)|
|•||a gas chromatograph-mass spectrometer (GC-MS)|
|•||reflective and transmitted ultra violet and visible light spectrometers|
|•||a near infrared reflectance spectrometer|
|•||a liquid chromatograph-mass spectrometer (LC-MS)|
Many types of analysis can be done non-destructively without taking a sample. In other cases, technology has advanced to the point that only a very tiny sample is necessary.
A Paint Question
A Windsor settee (1959.1662) from Winterthur’s collection was analyzed to understand its finish history, and to compare the technology of the finishes to the period literature. This settee was chosen for scientific study because of its rare form and for its extensive finish history. The settee is a rare sack-back settee with an s-curve upper rail and scrolled handholds from Norwich, Connecticut (New London County) ca. 1795–1803. It was made by Ebenezer Tracy Sr. (1744–1803), and is stamped with his brand under the seat. Windsor furniture was commonly painted green in the late eighteenth century so that the pieces would be pleasingly camouflaged in a garden landscape. In fact, a French visitor to Philadelphia in 1790s observed that the fashion was “wooden chairs painted green like garden furniture in France.”
Winterthur’s Scientific Research and Analysis Laboratory uses a number of nondestructive and minimally invasive instrumental methods of analysis to identify the materials used to make and conserve objects of art. To understand the finish history of the settee shown here, and to study this history on an atomic and molecular level, tiny samples of its painted finish were removed with a scalpel blade. These samples are approximately one milligram, the size of the period at the end of this sentence. Once removed, the samples are embedded in a clear polyester resin mounting material and then the mounted sample is cut, polished and viewed under a microscope in cross-section (end-on) to reveal the finish layers. A popular green paint recipe from this period, from Paints and Receipts for Wooden Work, published about 1801, involved mixing white lead, a drying oil such as linseed oil, and verdigris (basic copper acetate).
To determine whether this recipe was followed for the Tracy settee, a cross-section was placed in the scanning electron microscope, a high powered microscope that allows us not only to get a very highly magnified image of the cross-section, but also to determine the elemental composition of the paint layers.
This cross-section shows the full finish history, a rare survival on a piece of this date. We can use Raman spectroscopy, a molecular analysis technique based on laser light scattering, to identify the different blue, yellow, and green pigments used in each of these layers. Raman spectroscopy allows us to identify pigment molecules based upon their characteristic patterns of molecular vibrations. Analysis of the second generation paint finish revealed the presence of chrome yellow and Prussian blue pigments which are the components of a compound green pigment known as chrome green that was introduced after 1810.
Using the electron microscope and the Raman spectrometer for elemental and molecular analysis respectively, we can identify the pigments used in each finish generation on the Windsor settee, allowing us to understand the evolution of painted finish technologies from the eighteenth century up to the twentieth century. Such studies are critical to our understanding of the object’s authenticity and also to the history of technology.