LETTER TO THE EDITOR

1.1 To The Editor:

WE READ WITH GREAT INTEREST the article written by Frank S. Welsh concerning the characteristics of Prussian blue (JAIC 27 (1988): 55–63). Larger-than-average flakes of this pigment have also been encountered from objects examined by the Getty Conservation Institute. These flakes do indeed pose certain analytical difficulties when using polarized-light microscopy (PLM) as the sole means of identification, due the similarity in optical properties between Prussian blue flakes and ultramarine. In many cases elemental analysis by either microchemistry or energy-dispersive X-ray fluorescence was necessary in order to resolve the issue of identification.

Fortunately, a simple method can be employed to aid in the PLM identification of many blue pigments that allows mixtures of pigments to be rapidly differentiated. The Chelsea filter is commonly employed for characterization of gemstones, especially emeralds, and other minerals. Upon viewing the gemstone or mineral sample through the yellow Chelsea filter, a red color is imparted to those materials that transmit red light. The use of this filter as an aid in the identification of blue pigments has been reported by Peter Mactaggart in his course on pigment analysis taught at his home in Welwyn, U.K. (see “A Pigment Microscopist's Notebook,” P&A Mactaggart, Mac & Me: (Unpublished; Welwyn, 1988.)

To use the Chelsea filter with a polarized-light microscope for pigment identification, it is inserted into the light path of the microscope, commonly between the field and aperture diaphragms. The light source is generally used without polarizers. The filter imparts a red or pink coloration to natural and synthetic ultramarine, cobalt blue, and smalt. Other blues such as indigo, cerulean blue, and Egyptian blue are only occasionally colored red by the filter; the intensity depends on the depth of the blue coloration of the particles. Azurite, Prussian blue, and blue verditer are not affected at all by the filter and appear blue-black.

The primary advantage of using this method is that mixtures of certain blues may be visually differentiated rather quickly without the need for elemental analysis. Prussian blue flakes can be rapidly distinguished from ultramarine because the former appear blue-black, while the latter are colored red by the filter.

The Chelsea filter can be purchased for $24 from the Gemological Institute of America, 1660 Stewart Street, Santa Monica, Calif. 90404. We have found it to be a useful accessory to the polarized-light microscope and highly recommend it to anyone involved in pigment identification.–Michael R. Schilling and David A. Scott, Getty Conservation Institute, 4503 Glencoe Avenue, Marina del Rey, Calif. 90292

Author's Reply:

I AM GLAD THAT Michael Schilling and David Scott's letter calls our attention to the Chelsea filter. It has been in use by gemologists for quite a number of years. It is indeed another handy reference check for conservation microscopists who are analyzing paint pigments. I think this again shows how useful the polarized light microscope can be.—Frank Welsh, 859 Lancaster Avenue, Bryn Mawr, Pa. 19010