Abstract
Research into paper artworks and the identification of drawing techniques dating from the 15th to the 19th century from the collection of Slovak National Gallery revealed that the most frequently used means of writing was a set of brown-gray inks. This set includes bistre, sepia, and iron-gall inks (IGI). The light fastness of these organic compounds is not considered very high. Moreover, iron gall inks attack paper support by the processes commonly known as iron gall ink corrosion. In the experimental part of the research, the stability of three different brown-gray inks was studied. Model inks, i.e. bistre, sepia and IGI with two molar ratios Fe : gallic acid were prepared according to historical recipes. Whatman paper was used as a support material. Samples were submitted to accelerated aging in closed vessels at 100°C with and without the presence of NO2 in concentration of 100 ppm as well as to the light aging in a light chamber. Changes of mechanical, optical and chemical properties of the samples as well as spectra in the IR region were studied. The observed changes allowed an evaluation of the effect of particular inks on paper support and effect of various external factors of aging (temperature, light, pollutants) on the model systems. Simultaneous thermal analysis (DTA/TG) was carried out in order to study the thermo-oxidative stability of the materials. Sepia and bistre inks have been assessed as generally more stable than iron-gall inks.
Highlights
Bistre is a brown dye obtained by extraction of soot in boiling water followed by evaporation and drying [1]
Materials Three types of inks were studied: bistre ink 12100 (Kremer Pigmente GmbH & Co KG) – B, sepia ink 12401 (Kremer Pigmente GmbH & Co KG) – S, iron gall inks prepared from monohydrate of gallic acid (Sigma Aldrich) and ferrous sulfate heptahydrate (Lachema, Czech Republic)
Color stability assessment of the observed materials can be performed with respect to a set of blue wool standards [20] (BWS)
Summary
Bistre is a brown dye obtained by extraction of soot in boiling water followed by evaporation and drying [1]. Its aqueous suspensions may have a golden-brown, grayish or brown color, depending upon the thickness of the coating. Color is affected by the type of the wood used. The main part of a wood tar composition, which is similar to that of asphalt, is dehydroabietic acid with a lower portion of pyrolytic products [2]. According to other sources [3], the main color components in the bistre ink are formed by substituted, water-soluble phenols. Historical sources refer to the bistre changes of its hue when exposed to light. After being exposed to a strong light source, the tar part oxidizes and the dye gains cooler hue and possibly bleaches [4]
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