Iron-gall ink corrosion
From the first International Conference for Preservation and Conservation Access of Antique Manuscripts (i.e. Internationale Konferenz zur Erhaltung uns Ausbesserung alter Handschriften) in 1898 at St. Gallen, until the Ink Corrosion Conference—IIC in 2019 at Krems, and the development of several projects in recent decades (e.g., InkCor, 2002–2005), a great deal of effort was made and continues to be made in terms of the study of the efficacy of various conservation treatments to solve the cultural heritage institutions’ problem caused by the so-called “iron-gall ink burn” [28].
An early treatment approach was mainly directed toward recovering the strength of the paper support. From the late nineteenth century to the early twentieth century several consolidation materials and methods were applied, from traditional lining and lamination (sandwich-like method) procedures with adhesive and thin papers or chiffon-silk to new synthetic materials, such as the commercial product Zapon, a cellulose nitrate [29,30,31]. The latter was first used for waterproofing of geographic maps by the German army, but due to its flammability, in 1909, the Royal Materials Testing Office/Berlin (e.g., Königliches Materialprüfungsamt Berlin) recommended the use of a safer product, Cellit, a cellulose acetate (possibly with a degree of substitution (DS) of 2.2–2.6, considering it is soluble in acetone) [30, 31].
In the mid-twentieth century, cellulose acetate and poly(vinyl chloride) films were used for the consolidation of deteriorated documents. At the time, the bookbinder/conservator William Barrow, collaborator of the well-known Library of Congress, recognized acid hydrolysis as one of the main causes of paper deterioration and corrosion increase of iron-gall ink, when present. Therefore, he developed a two-step method (immersion in a saturated calcium hydroxide bath, followed by a calcium bicarbonate bath) plus his lamination method that involves the application of cellulose acetate and a tissue paper, on both sides of the document, to avoid a plasticised appearance [29, 32]. The result was a rather heavier, stiff, and uncharacteristic flat paper sheet document, but quite alkaline.
Already in the nineteenth century, the use of the ‘ammonia collodion process’ was recommended, which involved the application of ammonia vapours, followed by mechanical stabilization with collodion [31]. Collodion is a cellulose nitrate solution in ethanol and ethyl ether [33]. The invention is attributed to Schönbein, who mixed the two solvents in a 50:50 ratio. Reilly proposed a DS of 2 for cellulose nitrate in collodion [33]. Again, applying the highly flammable cellulose nitrate with shrinkage and low penetration problems was intended [31]; adding that long-term stabilizing was not achieved with ammonia neutralization [34]. In the mid-twentieth century, Barrow established an alkaline treatment prior to lamination as a regular procedure, namely, to treat ink-corroded documents. Due to the high pH of treated documents and ink colour changes observed in the mid-1960s, Barrow suggested the use of a single bath of saturated magnesium bicarbonate, known as the “Barrow One-Step” [29, 35].
After Barrow’s achievements, several authors followed the idea of iron-gall ink document stabilization through deacidification.
Minogue was one of the first to mention washing with distilled water as a possible treatment [36]. Nevertheless, Peter Waters established washing with water as a regular step for iron-gall ink corroded documents. Waters became a main figure in the field after his role in the 1966 flood of Arno in Florence and was invited in the 1970s to coordinate the conservation services at the Library of Congress [32]. At the Library of Congress, he set up as current praxis an immersion bath in warm water for acid removal, followed by an immersion bath in diluted calcium or magnesium bicarbonate for paper buffering [37]. The type of water used was not described, but currently, it can be deionized or “purified” tap water obtained using an activated carbon filter. The diluted calcium or magnesium bicarbonate solutions aimed to avoid “gripping” (deposition of a thin whitish layer causing a rough surface, mostly visible in dark areas), a phenomenon already described by Brannahl as the main drawback for inked documents [38].
Waters also recommended the use of newly available materials (e.g., “heat-set mending tissue”) to be applied as much as possible locally, only on the damaged affected areas of the documents; and the substitution of the complete lamination by polyester film encapsulation, providing physical support for the weaker documents [37].
The ink discolouration problem, namely after alkaline treatments, plus the risk of iron spreading during aqueous treatments, also promoted different studies.
Nonaqueous methods for treating manuscripts were also investigated early on, namely that of barium hydroxide dissolved in methanol. According to Baynes-Cope (1969), folding endurance tests indicated the method’s safety, and pH measurements before and after treatment showed that this method was effective; however, he also recognized that when insufficient buffer was deposited, the acidity would return [39]. In the mid-1970s the use of methylmagnesium carbonate, patented by George Kelly, was also seen as a possibility for water-soluble iron-gall inks. A study on its efficacy proves that both methods, spraying and immersion, leave the considered adequate alkaline reserve (approximately 1%, which can be measured as described in [40]). Nevertheless, the solvent’s fast evaporation rate left an uneven deposit [41].
In the 1980s, Hey assumed that the main cause of degradation was the presence of sulfuric acid and ferric oxides in the ink and considered that, whenever possible, washing should be a mandatory first step. In her research, she compared four different solutions for deacidification: 4% sodium borate; 1/2 saturated calcium hydroxide; magnesium bicarbonate and methylmagnesium carbonate dissolved in methanol and Freon. She concluded that sodium borate was unsuitable and that the best performance was of calcium and magnesium baths. She also suggested that the higher the ratio of calcium or magnesium carbonate to iron, the greater the protection conferred to cellulose [36].
“Simmering” or “boiling” water treatments for iron-gall ink-containing manuscripts were also seen as a possible solution and have been used for over 40 years. Carried out in the Conservation-Restoration Laboratory of the Vatican Library in the 1970s, the treatment was used in a few other European laboratories (e.g., Poland, Austria) and was later adopted by American [42] and Canadian scientists and conservators [43, 44]. It was confirmed that high levels of the destructive iron (II) ions (Fe2+) could be removed from the paper into the simmering wash water and that the concentration of possibly redeposited Fe2+ in other areas of the support was negligible (below the limit of detection by the analytical methodology used) [42]. However, on the other hand, the long-term effects of filler and size loss are a concern, plus the fact that this method is not yet completely proven to be safe on rather weak and fragile papers [44].
It is also worth mentioning that some authors were especially concerned with the regeneration of texts by applying chemical compounds that can later damage both the ink and support, adding to the complexity of the deterioration process of iron-gall ink documents [45, 46].
Searching for proper treatment was still ongoing in the 1990s, namely, in the field of deacidification/alkalinization. A work comparing the effect of fully aqueous and ethanol-diluted solutions of magnesium bicarbonate on six iron-gall ink documents dating from the eighteenth and nineteenth centuries was developed. Test results suggested that the addition of ethanol preserves the visual appearance of aged iron-gall inks, while both fully aqueous treatments (of 100% and 25% saturated magnesium bicarbonate) both caused loss of intensity and colour change in the ink of four of the six documents [47].
Since 1997, a nonaqueous deacidification method composed of submicron-sized particles of magnesium oxide dispersed in perfluoroalkane has been applied to a selection of iron-gall ink manuscripts in the Library of Congress [29]. When sprayed, the particles become lodged in paper and it is supposed that afterwards, they react with ambient moisture to form magnesium hydroxide. Further studies on this nonaqueous system revealed uniform spraying and an adequate alkaline reserve on the tested papers [48].
The phytate treatments
Han Neevel, a conservation scientist at the Netherlands Institute for Cultural Heritage, proposed in 1995 an innovative aqueous iron-chelating treatment based on the premise that excess Fe2+ was mainly responsible for ink corrosion on paper [16]: the application of myo-inositol hexakisphosphate salts (phytates), which are naturally occurring antioxidants that would inactivate the iron ions responsible for cellulose oxidation [49]. Phytic acid (myo-inositol hexakisphosphate) forms complexes with a variety of divalent and trivalent cations, Fig. 3. The antioxidant action of phytic acid is based on its ability to coordinate all sites of Fe2+ and Fe3+ [50], Fig. 3. Phytate also offers protection against oxidation by diminishing the concentration of free Fe2+ as it lowers the redox potential of the Fe3+/Fe2+ couple [51].
Myo-inositol hexakisphosphate forms high-affinity complexes, 1:1 stoichiometry, with Fe2+ and Fe3+ (as with many other transition metal ions), and the stability constants are pH-dependent [51]. Bearing in mind that during the conservation procedure of the iron-gall ink, the pH is kept in the range of 5–5.8, the species in solution will possibly be: for Fe2+, [Fe(H6L)]4− and [Fe(H5L)]5− with logK = 5.95 and 7.7 respectively; for Fe3+, the only complex should be [FeH3L]6−, logK = 18.20. On the other hand, in solution, gallate-Fe3+ constants are logK = 14 [52].
These stability constants are measured in solution and refer to soluble species. However, the brown pigments found in aged iron-gall inks can be insoluble, particularly those based on Fe3+. Thus, a first complexation with iron ions not complexed with gallotannins is expected, but considering that phytate salts can complex both Fe3+ and Fe2+, it will always be important to carry out preliminary tests to assess the safety of this type of treatment.
Calcium phytate (CaPhy) treatment is usually composed of these primary steps: wetting and washing of the paper; calcium phytate immersion, deacidification (neutralization and deposition of alkaline reserve) with calcium bicarbonate, application of gelatin sizing, mending any cracks and losses, and drying [53, 54], Fig. 4. For the aqueous washing step, instead of deionized water, the use of tap water of good quality or recalcified water is recommended to prevent removing original substances that are known to contribute to the chemical stability of paper, such as finely distributed calcium carbonate deposits [53, 54]. Gelatin is generally used as a resizing agent for iron gall inked documents rather than the other adhesives commonly used in paper conservation, due to its demonstrated ink corrosion protection effect [55].
Several studies have attested to the effectiveness of CaPhy treatment in preventing paper deterioration caused by iron-gall ink by comparing different properties of treated and untreated samples after artificial ageing, such as bursting strength; folding endurance; tensile strength; degree of cellulose polymerization; colour or whiteness; pH; alkaline reserve; or fluorescence labelling of carbonyl and carboxyl groups in combination with GPC-MALLS [56,57,58,59]. This treatment has been, at least partially, adopted by the international paper conservation community [60].
Variants of the CaPhy treatment have been proposed, such as the use of magnesium phytate (MgPhy) instead of calcium [61], Fig. 3. MgPhy prevented paper deterioration similarly to CaPhy, while having the advantage over CaPh of not requiring the use of toxic ammonia to adjust the pH of the phytate solution [61]. Other myo-inositol derivatives, such as myo-inositol 1,2,3-tris(dihydrogen phosphate) and myo-inositol 1,2,3,5-tetrakis(dihydrogen phosphate) were investigated as they could be derivatized to give less polar compounds and constitute a nonaqueous alternative to CaPhy or MgPhy [62].
Dilution of CaPhy in ethanol could also be an alternative for documents with water-soluble inks, but the higher the dilution is, the lower the treatment efficacy, manifested by a decreased mechanical resistance in the treated paper [63]. Völkel and colleagues [64] tested the addition of fibrillated nanocellulose into the different steps of the CaPhy treatment and proved its potential as a mechanical stabilizer of iron-gall ink-damaged paper. This addition would eliminate the need for subsequent local mending.
CaPhy treatment, however, introduces a new chemical into the paper (calcium phytate precipitate), which can be visible on the surface of the paper as a white powder. Although this superficial deposit can be removed by brushing, this operation is not advisable on paper severely deteriorated by iron-gall ink. One of the major limitations of this treatment is the poor solubility of phytate in nonaqueous media, hampering its application in water-sensitive items. As an aqueous treatment, it has the additional shortcoming that only unbound volumes are eligible for it. Additionally, the multiple immersion steps required [53] imply significant mechanical stress of such damaged papers [65], in addition to ink colour alteration [56], and a significant modification of the paper/ink composition [66].
Pos-phytate treatments
To overcome the drawbacks of CaPhy treatment, several alternatives have been proposed. Jana Kolar and colleagues, proposed for the first time the use of halides as antioxidants to stabilize iron gall inked paper [57]. An aqueous solution of tetrabutylammonium bromide, a peroxide decomposer, was shown to prevent cellulose depolymerization to a higher extent than CaPhy [57]. Malesic et al. continued testing this class of compounds using a nonaqueous solvent: dichloromethane [67]. Tetrabutylammonium chloride, bromide and dodecyltrimethylammonium bromide exhibited the strongest stabilization effect and were the first nonaqueous alternatives to CaPhy [67]. Later, Kolar and her research team tested alkylimidazolium bromides in a less toxic organic solvent: ethanol [68]. 1-Ethyl-3-methylimidazolium bromide and 1-butyl-2,3-dimethyl-imidazolium bromide, in combination with alkali magnesium ethoxide in ethanolic solution, had a higher stabilization effect on iron gall inked paper when compared with the previously tested tetraalkylammonium bromides, CaPhy or MgPhy, while causing no significant colour alteration on the treated ink [68]. Data on the toxicity and environmental impact of these imidazolium-based ionic liquids are quite limited, though [69].
Rouchon et al. [70] also tested the use of halides to treat iron gall ink-damaged papers, but in this case, using them as salts (NaCl, NaBr, CaBr2) and compressing the iron gall inked documents between two interleaves charged with the active compound. However, for the migration of the active compounds from the interleaves to the documents to occur, high relative humidity conditions (above 80%) for several days are required, and these conditions may additionally induce the migration of iron and acidic compounds out of the ink line and across the paper.
Kolar and colleagues demonstrated that the transition metal content of historical iron-gall inks varies greatly, and due to its superior catalytic activity, it is copper, not iron, the main oxidation catalyser on paper containing copper-rich iron-gall inks [57].
To address this problem, Zaccaron et al. compared CaPhy treatment with a new method using glucose as a reducing sugar, which based on the Fehling reaction, would selectively remove free copper ions by precipitating them as an insoluble cuprous oxide in the treatment bath [71]. However, this glucose treatment caused severe hydrolytic and oxidative degradation with remarkable yellowing on the paper and is not a viable conservation option. Moreover, the authors concluded that CaPhy treatment was still very effective and safe even for iron-gall inks with a high percentage of Cu ions.
Piero Baglioni’s group, which specializes in nanotechnology, has also studied stabilization treatments for iron gall inks, including copper-containing ink. They compared the effect of two nonaqueous deacidification solutions: magnesium hydroxide nanoparticles dispersed in isopropanol and a commercial Bookkeeper solution [72]. The pH of paper deacidified with the nanoparticles was maintained at approximately pH 7 to reduce the rate of cellulose oxidation, since the catalytic activity of iron and copper ions is minimal when the pH is approximately neutral [73]. Both magnesium hydroxide and Bookkeeper treatments partially prevented cellulose depolymerization caused by iron gall ink with artificial ageing. The nanoparticles performed slightly better while having the advantage of not using fluorinated solvents [72]. How the final pH of treated paper was controlled to be near 7 is not clear, and this is a crucial step due to the influence of pH on the efficacy and safety of the treatment. Sequeira et al. showed in a previous study that when using calcium hydroxide nanoparticles, the final pH of treated papers will depend not only on the concentration of applied nanoparticles but also on the initial pH of the paper itself [74].
Later, this same research group developed a combined deacidification and strengthening treatment consisting of hydroalcoholic gelatine solutions (ethanol or isopropanol) mixed with Ca(OH)2 nanoparticles called GeolNan, which could increase the resistance of cellulose to hydrolysis and oxidation induced by iron gall ink [75]. According to the authors, this achievement is mainly due to the nanoparticles, even if gelatin itself partially hampers the depolymerization of cellulose, probably slowing down the oxidation reaction by reducing ion mobility or complexing metal ions. A previous study on the effects of nonaqueous deacidification with Ca(OH)2 nanoparticles on iron gall inked paper also revealed that the nanoparticles alone may diminish the depolymerization of cellulose under artificial aging, although to a lesser extent than aqueous Ca(OH)2 saturated solutions [74].
Due to the high alkalinity of calcium hydroxide nanoparticles in the presence of moisture [76], special caution should be taken to control the pH when treating heavily oxidized cellulose, such as iron gall ink corroded paper, owing to the higher risk of alkaline degradation.