Materials and Scientific Analysis

Victor F. Hanson, a retired DuPont physicist, established Winterthur’s Scientific Research and Analysis Laboratory in 1969 with the development of the first x-ray fluorescence (XRF) spectrometer designed specifically for the nondestructive analysis of museum objects. XRF is now one of the most valuable techniques used by museum scientists around the world since it does not require a sample; no material is removed and there is no damage to the artwork.
With x-ray fluorescence analysis, a beam of x-rays is aimed at an area of an object such as, for example, the red colorant on a print.  The high energy of the x-ray beam excites the elements in the red colorant, causing them to emit secondary, or fluorescent, x-rays.  The energies of these fluorescent x-rays coming from the red colorant are characteristic of the elements present in the pigment, and so can be used to identify them.  As a result, x-ray fluorescence data can be used to distinguish between a mercury-based red pigment and a cadmium-based red pigment, for example.  The ability to distinguish between mercury- and cadmium-based reds can be critically important to determine the 20th-century application of colorants to 19th century prints since cadmium-based red pigments were not commercially introduced until 1907.

Using x-ray fluorescence (XRF) spectroscopy, Winterthur scientists identified the inorganic pigments below, such as chrome yellow, zinc white and emerald green, all recent inventions in the opening decades of the 19th century. Organic pigments such as gamboge and indigo were identified with microscopy. Fanny Palmer’s Quail Shooting includes viridian—an expensive, vibrant green synthetic pigment that was widely available in its more transparent, hydrated form only after 1850. (A Winsor & Newton catalogue from 1850 listed watercolor cakes of viridian—“chromium green oxide”— in its third pricing tier: three shillings versus one or two for materials in the lower tiers.) An alternative to viridian,  the common, inexpensive mixture of Prussian blue and the organic yellow pigment gamboge, was also found in a number of Palmer’s landscapes, as were the problematic pigments verdigris (which is unstable) and emerald green (which is toxic). It is impossible to know why viridian was used in Palmer’s print, but apart from its high cost, it was ideal: singularly transparent, it could add color without obscuring Palmer’s draftsmanship and design.

Reference:

Winsor and Newton.
“Illustrated List of Colours and Materials for Drawing and Watercolour Painting” in Thomas Rowbotham and Thomas L. Rowbothan, The Art of Landscape Painting in Watercolours (London: Winsor and Newton, 1850), 5.