Ageing of Laboratory Irongall Inks Studied by Reflectance Spectrometry
| Main Authors: | , , |
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| Format: | Article |
| Language: | English Slovak |
| ISSN: | ISSN 0034-5806 |
| Online Access: | http://www.viks.sk/chk/res_3_4_99_151_166.doc |
| Abstract: | SUMMARIES. This study tackles changes in colour of irongall inks over time. Different chemical processes, in which ink composition has a major influence, cause colour change. Two series of inks were prepared, one using ferrous sulfate partially oxidized, and another using pure ferrous sulfate hepta-hydrated; pure gallic acid and gum arabic. The inks were applied on Whatman filter paper, and then aged in a climatic chamber. The colour of ink spots was measured with a spectrophotometer. Changes in lightness, chroma and hue were related with ink composition. All inks become darker and browner after ageing, including those not containing gallic acid. Inks prepared with an iron/gallic ratio nearer to that needed for the complex formation are the darkest ones initially and show the slightest change in colour. Gum arabic containing inks darken more than inks without it. Strong corrosion was observed in the aged dark spots. CONCLUSION. ˙ Ferrous and ferric ions yield different initial colour to the ink, but final colours are quite similar, showing that new chromophoric groups appear in cellulose by their catalytic influence. ˙ Blue colour of ink (iron-gallic complex) changes to brown and black after ageing. Browning and darkening can be caused by oxidation of ink components, giving quinonoid structures, and also by degradation products of cellulose and/or gum arabic. ˙ Since arabinose units oxidize more readily than glucose, inks containing gum arabic always become darker than those prepared without it. ˙ Spots of inks containing gum arabic are more uniform due to the better ink stabilization. This fact can also increase their darker appearance. ˙ Changes in colour can be accurately studied by reflectance spectrometry, so that quantitative information can be correlated with chemical hypothesis. ˙ The main hypothesis in order to explain colour, its changes, and cellulose degradation are:˙ Autoxidation of cellulose with atmospheric oxygen, catalyzed with Fe + and Fe2+ ions, with formation of reactive intermediates, such as peroxides, hydro-peroxides and radicals. ˙ Autoxidation of gallotannates with atmospheric oxygen and other oxidants present gives quinones susceptible to photochemical activation. ˙ Fenton's reaction, between Fe2+ and hydrogen peroxide formed by autoxi-dation, with formation of very active hydroxyl radicals. ˙ Oxidation of cellulose by hydroxyl radicals, with formation of carbonyl and carboxyl groups. ˙ Fenton-Hamilton mechanism explaining the role of iron-gallic complex as the hydroxyl radical conveyor in cellulose oxidation. ˙ Acid hydrolysis of cellulose and acid condensation of hydrolytic fragments. ˙ Some of the hypothesis proposed need to be confirmed experimentally in further studies, by means of analytical analysis of ink degraded paper. Other hypotheses are well sustained by numerous bibliographic data. |
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| ISSN: | ISSN 0034-5806 |
