| Abstract: | SUMMARIES. This article presents a historical and scientific investigation into commercially prepared British watercolour cakes from London-based artists' colourmen Reeves, Rowney, Ackermann and Roberson, dating from before 1180 through to the early twentieth century. Analysis focused on the identification of the plant-gum binding medium via the use of gas chromatography-mass spectrometry (GC-MS) and pigment identification via energy dispersive X-ray analysis (SEM-EDX) and polarized light microscopy. The historical context is provided by an investigation into the historical uses of plant gums in European art and the history of importation of plant gums into the United Kingdom. The analysis of watercolour cakes used by the British artist J.M. W. Turner offered further insight into the use of specific plant gums and gum mixtures by eighteenth- and nineteenth-century British painters.
CONCLUSION. The investigation of historical watercolour manuals and treatises provided valuable insight into the use of plant gums in European art from the seventeenth to the early twentieth century and proved to be useful in the interpretation of analytical results. The gums listed in earlier manuals include arabic, tragacanth. ammoniac, some Prunus species such as almond and cherry, as well as lesser-known gums such as hedera, guaiacum and gum lake. The historical investigation confirmed that the primary gum employed in watercolour painting has been, and continues to be, gum arabic and that the other gums tended to be used for specific purposes, such as gilding. Documentary sources examined in this paper mention that several other substances, such as isinglass and animal glue, were commonly added to plant gum-based paints, as well as other gums, sugar or honey and preservatives, in order to modify the properties of the resulting paint and/or dry paint films. These additives have yet to be confirmed by analysis, except in the case of honey or sugar and gum admixtures. The GC-MS analysis of historical watercolour cakes consistently supported the notion that the dominant gum present is gum arabic, which concurs with the historical accounts and archival recipes. However, the analysis of the Reeves and Roberson watercolour cakes, as well as Turner's watercolour palettes, also revealed that small amounts of xylose, fucose and mannose were present in some of the cakes as either xylose-fucose pairs, xylose-mannose pairs, or xylose alone. These additional monosaccharides suggest the possible presence of small amounts of cherry, tragacanth or sarcocolla gums and, in one case, a bottled Roberson medium was confirmed as containing a mixture of gum arabic with a minor amount of added gum tragacanth. Aside from the addition of other plant gums, the presence of trace amounts of xylose may result from other factors including the importation of several subspecies under the larger 'gum arabic' umbrella, or an aging process that requires further investigation. Interestingly, many of the cakes containing xylose were red in colour, and some were confirmed as containing the pigment vermilion. Hence, it is possible that a more viscous gum such as tragacanth was added to the vermilion cakes to bind this relatively heavy pigment. The 'gamboge' cakes in most cases contained varying amounts of gum arabic and what is probably genuine gamboge resin, as indicated by the presence of elevated arabinose levels and, in one case, confirmed by Raman spectroscopy. Other variations in monosaccharide content arose from the presence of other organic colourants such as the Rhammus berries used for sap green, which produced elevated rhamnose levels. To summarize the trends seen in the use of plant gums over the eighteenth, nineteenth and into the twentieth century, it appears that early watercolour cake manufacturers such as Ackermann produced cakes using gum arabic alone, as well as experimenting with 'honey' colours that contained very little or no gum binder. Other early watercolour cake manufacturers such as Reeves and Roberson occasionally added other gums such as tragacanth, cherry or sarcocolla to their paints while retaining gum arabic as the primary binder. By the early twentieth century, gum arabic was again being used as the sole binder, with no sign of the presence of any other gums, although the number of samples representing this period was too small to be definitive. The amounts of honey or sugar added to the paints may be either pigment-dependent, with the brown and black cakes tending to have the highest sugar or honey content, or quality-dependent, with a significant and consistent increase in sugar/honey content identified in what is probably a student-grade Reeves watercolour box from the early twentieth century. |
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