THE DEVELOPMENT AND INITIAL APPLICATION OF A GAS CHROMATOGRAPHIC METHOD FOR THE CHARACTERIZATION OF GUM MEDIA
SARAH L. VALLANCE, B.W. SINGER, S. M. HITCHEN, & J. H. TOWNSEND
It is important that conservators are provided with detailed information regarding the exact nature of the materials used by artists to assist in devising safe treatment plans that accommodate the artists' intention for the appearance of their works. Specific knowledge of the nature of the media used in particular works may shed light on why some paintings are in better condition than others of similar age. This article reports the initial investigations into the development of a simple procedure for the analysis of gum-based binding media.
Gums are a group of non-crystalline, polysaccharide materials that can be found in vegetable matter and are often exuded when a plant is “wounded.” They are water-soluble or water-dispersible compounds with complex composition, usually consisting of a number of surgars (e.g., galactose and mannose) plus their uronic acid derivatives (e.g., galacturonic acid) (Gettens and Stout 1966).
The plant gums arabic (Church 1901; Merrifield 1966), tragacanth (Gettens and Stout 1966), and cherry (Doerner 1934; Gettens and Stout 1966; Mills and White 1994) have been used for many centuries as the principal media for watercolors, minitures, and manuscript illumination and, on occasion, as sizing materials; indeed there is documentary evidence to suggest that gum was used as a medium prior to the use of drying oils (Laurie 1911; Mills and White 1994).
Honey, cane sugar solution, and glycerine have long been used as additives in aqueous media (Gettens and Stout 1966), preventing the extreme drying that results in the brittleness of these particular media. Today, dextrin is more usually used for this purpose.
Karaya gum, the dried exudate of the native Indian tree Sterculia urens, has been used as a cheaper substitute for gum tragacanth; alternative names include Indian tragacanth, Indian gum, and Sterticula gum (Furia 1981). Other plant gums, including carob or locust bean, tamarind, cholla, plus plant resins like myrrh and olibanum, have found uses in an artistic context either for painting or papermaking. It is thought that the Egyptians used locust bean gum for binding mummy wrappings, and tamarind seed gum was the principal paint medium for Indian miniatures and murals (Mills and White 1994).