|Speaker:||Barbara H. Berrie, Senior Conservation Scientist, National Gallery of Art|
|Topic:||“Art and Science: Two Ways of Seeing”|
President Collins called the 2141st meeting to order at 8:19 p.m. on February 8, 2002. The Recording Secretary read the minutes of the 2140th meeting, and after discussion, they were approved. The Recording Secretary announced that most of the minutes since 1994, except for Retiring Presidents' and Joseph Henry lectures, were now available on the Society web site.
The speaker for the evening was Barbara H. Berrie, Senior Conservation Scientist, National Gallery of Art. The title of her presentation was “Art and Science: Two Ways of Seeing”.
Art conservation is a field where chemists, conservators, art historians and connoisseurs work together to gain knowledge about art and how it can be preserved. A chemist working in art conservation investigates the properties of materials used in making art with the objectives of determining which techniques would be most effective in preserving the art while being appropriate to use in conservation and restoration. Any material added by a conservator should be unobtrusive to a casual observer, detectable by an expert, and be reversible with no permanent effect. Chemistry aids in the practice of conservation by helping to determine the best method for preserving art while it is on exhibition. Paints are tested to evaluate their light stability, and whether they would be appropriate to use in restoring lost portions of paintings.
Physical examination techniques such as X-ray absorption spectroscopy and X-radiography can show, for example, where a white pigment that contains lead was used, or that a picture was painted over as the artist decided to reuse a canvas or rethought the work, as was found for A Young Girl Reading by Jean-Honoré Fragonard. Another technique, air-path X-ray fluorescence spectroscopy, provides information about the presence of elements heavier than argon without having to remove a sample from the work. For example, the presence of zinc and mercury in an orange paint indicates that the artist probably used white zinc oxide and vermilion mercuric sulfide (also known as cinnabar).
In collaborating with conservators, analytical examination of an artwork sometimes provides insight into its original appearance. The painting Saint Sebastian by Tanzio da Varallo in the National Gallery was to be cleaned and have the varnish removed. A microscopic chip of paint was taken at the edge of a damaged portion of the painting that was to be restored. The chip was mounted, cut at right angles, and examined under polarized light to determine the undercoat layers. This revealed that the Saint's green mantle was actually painted on top of the first varnish. This green mantle covered a mantle that was originally yellow. This had possibly been done because the color yellow was associated with evil in iconography.
Another objective of chemical analysis is pigment identification. Looking at lead based yellow pigments used since the 1330's, one pigment called massicot had been described in a 1941 paper as an oxide of lead and tin. Our analysis has now shown that it was probably one of two types of lead tin oxide. Lorenzo Lotto (1480-1556/1557) kept a notebook of his materials in which he described another pigment called “pot maker's yellow”. The Lute Player painted by Orazio Gentileschi around 1612-1620 when examined by X-ray fluorescence showed that a yellow pigment contained tin and antimony in a constant ratio with varying proportions of lead that was probably related to the pigment described by Lotto. X-ray powder diffraction and scanning electron microscopy were used to characterize the pigment, which is similar to a colored glass. It was shown to be a ternary oxide of lead, tin and antimony. Analytical chemistry applied to the analysis of materials used in works of art does not in any way detract from the beauty of the art. The knowledge gained from chemical analysis can not only aid in its preservation, but also provide insights into the artist's original intention, the art's original appearance, and how the artist created it.
Ms. Berrie kindly answered questions from the floor. President Collins thanked Ms. Berrie for the Society, and welcomed her to its membership. The President made the announcements about the next meeting, parking, and refreshments, and adjourned the 2141st meeting to the social hour at 9:40 p.m.
|John S. Garavelli|
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