e-PS, 2005, 2, 13-18
ISSN: 1581-9280
WWW.e-PreservationScience.org
www.Morana-rtd.com© by M O R A N A RTD d.o.o.

published by

F ULL PAPER

R EVIEW

T HE USE OF HALIDES FOR STABILISATION OF
IRON GALL INK CONTAINING PAPER - THE PRO -S HORT COMMUNICATIONNOUNCED EFFECT OF CATION

1*

J. M ALEŠIÈ , J. K OLAR
1 National and University
Library, Leskoškova 12, SI1000 Ljubljana, Slovenia
2 University of Ljubljana,
Faculty of Chemistry andChemical Technology,
Aškerèeva 5, SI-1000
Ljubljana, Slovenia*Corresponding author:
jasna.malesic@nuk.uni-lj.si

1,

2

M. S TRLIÈ , S. P OLANC

2

Abstract
It has long been known that transitional metal ions and acids iniron gall inks induce degradation of paper support, thus endangering numerous historical materials. Stabilization treatment
should therefore consist of deacidification and addition ofantioxidants that could be used for stabilization of all historicalmaterial, regardless to their iron gall ink composition.
This contribution discusses the effects of various quaternaryammonium and phosphonium halides on iron gall ink corrosionof cellulose in alkaline environment. It is demonstrated that thesize of the cation affects the extent of stabilization obtainedwith various quaternary ammonium bromides, with the strongest
stabilization achieved with tetrabutylammonium chloride, bromide and dodecyltrimethylammonium bromide. An addition ofeither tetrabutylammonium bromide or chloride did not adversely affect the brightness of paper during accelerated ageing.
1. Introduction
The two main processes leading to gradual degradation of cellulose
in the presence of iron gall ink have been identified as acid hydrolysis and oxidation, catalyzed by some transition metal ions present in
the ink. 1
Acid hydrolysis of cellulose is one of the key factors responsible forthe degradation of paper during aging. 2,3 A number of deacidification
techniques have been developed to reduce the acidity and thus
lower the rate of its deterioration.

received:
accepted:

01.05.2005
14.07.2005

key words: cellulose, paper
ageing, iron gall inks, antioxidants

In addition to acids, iron gall inks contain a substantial amount ofdifferent transitional metals capable of entering the so called Fenton
reaction or a related pseudo-Fenton reaction, 4,5 where they catalyze
homolytic decomposition of peroxides. During the process hydroxyl
radicals are formed, which react rapidly with a variety of compounds. 6 Their importance during autoxidation of cellulose has been
recently demonstrated, when a correlation was established between
the extent of hydroxyl radical formation and degradation of deacidified model paper during ageing. 7

13

www.e-PRESERVATIONScience.org
In addition to iron, iron gall ink contains othertransition metals, most notably copper. 1,8Compared to iron, copper exhibits a superior catalytic activity during Fenton-like reactions. The
catalytic effects of iron and copper proved to be
additive. 9

increased stability of paper at 20 C by 25-50
times, while KBr and KSCN-treated papers werestabilized to a limited extent and will last up to 3
times longer than the one which was only deacidified. 3

Considering the two cellulose degradation pathways- acid hydrolysis and oxidation, a stabilizing
treatment for iron gall ink induced decay requires
an addition of alkalis to combat acid hydrolysis of
paper and antioxidants to inhibit oxidation.

Recently, an extensive study of the production ofoxidizing species in model aqueous systems containing pseudo-Fenton reaction mixture and various halides and a pseudo-halide has been performed confirming their antioxidant properties in
the presence of both transitional metals usuallypresent in iron gall ink, copper 25 and iron. 26

A variety of antioxidants may be used to interferewith the pathway of oxidative degradation of cellulose. They can be classified into two groups
according to the mode of action: those whichinterrupt the radical chain reaction, i.e. the “chain
breaking” antioxidants; and “preventive” ones,
which inhibit or retard the formation of free radicals from their unstable precursors, particularly
from hydroperoxides. 10
Phytate (myo-inositol hexaphosphate), a complexing agent, which belongs to the “preventive
group” of antioxidants has been widely used forstabilization of iron gall ink corrosion. 11 The drawback of complexing agents such as phytate is thatthey are usually metal-specific. Therefore, webegan an extensive survey of antioxidants that
could be used for stabilization of all historicalmaterials, regardless of the iron gall ink composition. A selection of antioxidants has been madetaking into account possible discoloration of thepaper or the ink. Antioxidants should also be stable during the conditions of accelerated ageing
and reasonably harmless or non-toxic.
Halides such as iodide, bromide and thiocyanate
are well known antioxidants which act as radical
scavengers. They react with hydroxyl radicals in awide temperature and pH range. 12-14 The rate
constants for reaction of hydroxyl radicals withiodide and thiocyanate ions are nearly diffusion
controlled, 14 while the relative rate constants forreaction of hydroxyl radicals with bromide ions
are more than ten times lower. 12-15 In acidic solutions, bromide and iodide are also capable of catalyzing heterolytic decomposition of peroxides. 16-18
Potassium iodide has been long recognized as aneffective stabilization agent during oxygen
bleaching of kraft pulps 19 or oxygen pulping. 20,21
The results presented in the literature demonstrate an effective stabilization of paper in alkaline environment by the addition of iodide, 22,23
thiocyanate and bromide. 24 While iodide and thiocyanate induce discoloration of paper, this was
not observed when sodium bromide was added tothe paper. 24 Addition of potassium iodide to theWhatman paper containing calcium carbonate

14oIn the present study, evaluation of various halides
with respect to their stabilizing effect during ageing of alkaline paper and paper containing irongall ink is presented.
2. Experimental
2.1 Model paper
The model paper sample used was purified cotton
linters cellulose (Whatman filter paper no. 1;
86.0 g m -2 , degree of polymerization (DP):2630 ±0.74%.
2.2 Model inks
Two inks, denoted “Ink 0” and “Ink 70”, wereused. Both inks were composed of deionised
water, 31.4 g L -1 of gum arabic (Fluka, Buchs),
49.2 g L -1 of tannic acid (Riedel de Haen, Seelze)
and 0.15 mol L -1 of transitional metal from copper(II) sulphate pentahydrate (Riedel de Haen,
Seelze) and iron(II) sulphate heptahydrate (Riedel
de Haen, Seelze) in molar ratios Cu(II):Fe(II) ionsof 0.0 in Ink 0 or 0.7 in Ink 70.
2.3 Sample preparation
Whatman filter paper was immersed for a few
seconds into a solution of the selected ink, diluted with a solution of gum arabic (31.4 g L -1 ) indeionised water (1:1 (V/V) ratio for gum arabic
solution for Ink 0 and 4:1 ratio for gum arabic incase of Ink 70). Wet samples were then pressed
between blotting papers and air-dried.
After 1 day of pre-ageing at 70 °C and 50% RH,
the samples were immersed for 1 h into a 0.01
mol L -1 aqueous solutions of Ca(HCO ) and air
3 2
dried. Antioxidants were added by immersing thepaper samples for 20 min either in an aqueous ora dichloromethane solution of various halides inconcentrations as indicated in Table 1.
2.4 Analytical methodology
Colourimetric measurements of paper samples
were performed with a Minolta CM-3610d diffuse
reflectance spectrophotometer with the specularcomponent excluded. The reflectance was meas-

Halides for stabilisation of iron gall inks, e-PS, 2005, 2, 13-18

© by M O R A N A RTD d.o.o.

Quaternary ammonium bromides

Quaternary phosphonium bromides

Alkali bromide
Quaternary ammonium chloride

Chemical name

FormulaSolvent

Tetramethylammonium bromide
Tetraethylammonium bromide
Tetrapropylammonium bromide
Tetrabutylammonium bromide

Me4NBr
Et 4NBr
Pr 4NBrBu NBrBenzyltributylammonium bromide

BnBu 3NBr

Dodecyltrimethylammonium bromide(C 12H25)Me3NBr

Tetraoctylammonium bromide
Tetraheptylammonium bromide
Trimethyloctylammonium bromide
Tetradecylammonium bromide
Tetradodecylammonium bromide
Methyltriphenylphosphonium bromide
Benzyltriphenylphosphonium bromide
Butyltriphenylphosphonium bromide
Potassium bromide
Tetrabutylammonium chloride(C 8H17)4NBr(C7H15)4NBr
Me 3(C8H17)NBr(C 10H21)4NBr(C 12H25)4NBr
MePh 3PBr
BnPh 3PBr
BuPh 3PBr
KBr
Bu 4NCl

H O
2
H 2O
H O
2
H O
2
CH2Cl2
H 2O
CH2Cl2
H 2O
CH2Cl2CH2Cl2CH2Cl2CH2Cl2CH2Cl2CH2Cl2
H 2O
CH2Cl2CH2Cl2
H 2O
CH2Cl24c-1(mol L )
0.03
0.03
0.03
0.03
0.3
0.03
0.03
0.03
0.3
0.03
0.3
0.3
0.3
0.3
0.03
0.03
0.03
0.03
0.3

Solubility(water)
+
+
+
+
+
+
+
+
+
+

Table 1: Various halides evaluated in the study. Concentration and the solvent used are indicated, as well as their solubility in water(+ soluble and - insoluble).

ured relative to Minolta standard. Typical R.S.D.
was below ±1% (n = 3).
Viscometric determinations of the degree of polymerization were performed according to the standard procedure, 27 using 1 mol L -1 diethylenediaminecupric hydroxide solvent (Carlo Erba) prepared according to the standard procedure. 28 DP
was calculated from the intrinsic viscosity measurements using the equation 29 DP 0.85 = 1.1·[ η ].
Typical R.S.D. for duplicate analyses was below±2%.
The rates of cellulose degradation (k) were calculated from DP measurements during artificial ageing using the Ekenstam model 30 .
2.5 Accelerated ageing
Two phases of accelerated ageing were carriedout on the samples. After the ink had been
applied to the paper, the samples were aged for
1 day at 70 °C and 50% RH (pre-ageing). The
second phase of accelerated ageing took place
after the deacidification and addition of antioxidants. Some samples were aged at 90 °C with achanging relative humidity (RH), cycling between
35-80% RH every 3 h in a Vötsch VC 0020 ageing oven for up to 16 days. Other samples wereaged at 80 °C and relative humidity 65% in a
Vötsch VC 0020 ageing oven for up to 17 days.
3. Results and discussion
Two different model iron gall inks with a Cu:Fe
molar ratio 0 (Ink 0) and 0.7 (Ink 70) were applied
to a model paper and deacidified with a calcium
bicarbonate solution, or deacidified with simultaneous addition of various halides presented inTable 1. Results obtained in model aqueous systems with either iron(III) or copper(II) ions as cat-alyst and halide antioxidants demonstrated thatthe antioxidant activity depend on the size of thecation introduced with the halide. Regardless ofwhether Cu(II) or Fe(II) was used, the most pronounced antioxidant activity was exhibited by thequaternary ammonium bromide with the longest
alkyl chain. 25,26
3.1 Effect of aqueous treatments with antioxidants
First, the effect of various water-soluble quaternary ammonium bromides on iron gall ink corrosion on model papers containing Ink 0 or Ink 70
was evaluated. Results presented in Figure 1
confirm the antioxidant activity of various bromides, as compared to the sample without theadded antioxidant. As observed already in model
systems in aqueous solutions, 26 the extent of stabilization achievable with water-soluble quaternary ammonium bromides depends on the size ofthe cation - among the symmetrical quaternaryammonium bromides, tetrabutylammonium bromide was the most effective in the presence ofboth model inks (Ink 0 and Ink 70). Results
obtained on model paper are similar to the ones
obtained in aqueous solution also in the antioxidative effect of the halides is more pronounced
in the presence of copper ions. Lower degradation rate constants, obtained for samples containing the copper-rich Ink 70, are due the lower concentration of corrosive transition metals in thestarting ink.
Dodecyltrimethylammonium bromide, which has
the longest alkyl chain among the evaluatedantioxidants, proved to have the strongest antioxidant activity, when applied on the samples containing Ink 0 (Figure 2). The effect of benzyltributylammonium bromide is similar to the effectof tetrabutylammonium bromide.

Halides for stabilisation of iron gall inks, e-PS, 2005, 2, 13-18

15

www.e-PRESERVATIONScience.org

Figure 1: Effect of various water-soluble bromides on the degradation rate constant (k) of Whatman filter paper containing Ink
70 or Ink 0 during accelerated ageing (90 °C, RH cycling
between 35-80%). Sample A was treated with calcium bicarbonate, other samples were treated with calcium bicarbonate and-10.03 mol L aqueous solutions of tetramethylammonium bromide (B), tetraethylammonium bromide (C), tetrapropylammonium bromide (D), tetrabutylammonium bromide (E), benzyltributylammonium bromide (F), dodecyltrimethylammonium bromide(G), methyltriphenylphosphonium bromide (H). Error bars represent standard deviation.

Figure 2: Effect of various bromides on the degradation rate
constant (k) of Whatman filter paper containing Ink 70 during
accelerated ageing (90°C RH cycling between 35-80% RH.)
Sample A represents the sample treated only with calcium bicarbonate, other samples were treated with calcium bicarbonate-1
and subsequently with 0.03 mol L dichloromethane solutions oftetraoctylammonium bromide (b), benzyltriphenylphosphonium
bromide (c), butyltriphenylphosphonium bromide (d), tetrabutylammonium bromide (E), benzyltributylammonium bromide (F),dodecyltrimethylammonium bromide (G). Error bars represent
standard deviation.

3.2 Effect of non-aqueous treatments withantioxidants

gall ink corrosion, due to the supposed effect ofsuch ageing on migration of acids and metals
present in ink, 11 standard ageing conditions(80 o C and 65% RH) are often used in studies of
paper stability. We have therefore decided toevaluate the effects of some antioxidants also atstandard ageing conditions (Figure 3).

In addition to slowing down oxidative decay irrespectively of the transition metal, the advantage
of some of the evaluated antioxidants is also theirsolubility in non-polar solvents, which allows thedevelopment of non-aqueous conservation methods. A non-aqueous approach is important due toseveral disadvantages of the aqueous treatments:
possible migration of ink compounds, changes of
ink colour and the overall tonal balance of theobject, change of dimensions and surface characteristics of paper, necessary dismounting ofbound volumes, long drying times. A non-aqueous
treatment for ink corrosion would evade thesedisadvantages, while at the same time provide
the means to treat large quantities of endangeredarchival material.
As cellulose swelling in non-polar solvents isquite limited 31 , the resulting poorer distribution ofantioxidant in paper may result in a significantly
lower effectiveness of the antioxidants, when they
are introduced with non-polar solvents. However,as demonstrated in Figure 2, this is not the case,as comparable results to the ones presented in
Figure 1 are obtained when some of the ammonium bromides (e.g. tetrabutylammonium bromide,
benzyltributylammonium bromide and dodecyltrimethylammonium bromide), were applied tothe paper in dichloromethane.
Although ageing at conditions of dynamic humidity (90 o C and cycling relative humidity between
30-80% RH) is often used for evaluation of iron16

As with the dynamic accelerated ageing, dodecyltrimethylammonium and tetrabutylammonium
bromide were the most effective among the bromide antioxidants.
From Figure 2 and Figure 3, it can also be
observed that some quaternary ammonium bromides (e.g. tetraheptylammonium bromide,
tetradecylammonium bromide and tetradodecylammonium bromide), which are poorly soluble inwater, exhibit only a limited antioxidant effect.
This is likely explained by enhanced mobility ofwater soluble molecules in paper during ageing.
On the other hand, some quaternary phosphonium bromides with limited solubility in water (e.g.
butyltriphenylphosphonium bromide, Figure 2)
effectively stabilized paper against iron gall ink
corrosion.
In addition to bromides, the effect of tetrabutylammonium chloride on the stability of paper containing iron gall ink was evaluated in Figure 3.
Tetrabutylammonium chloride and tetrabutylammonium bromide exhibit similar antioxidant properties when tested in a model aqueous solution. 26
Surprisingly, the results presented in Figure 3
indicate superior stabilizing potential of paper
support containing iron gall ink when tetrabuty-

Halides for stabilisation of iron gall inks, e-PS, 2005, 2, 13-18

© by M O R A N A RTD d.o.o.
strongest stabilization was achieved with tetrabutylammonium chloride, bromide and dodecyltrimethylammonium bromide. Ammonium bromides, insoluble in water, do not significantly
affect the rate of paper degradation. Quaternary
phosphonium bromides also exhibit strong antioxidant properties, although the effect of cation is
not pronounced.
An addition of either tetrabutylammonium bromide
or chloride did not adversely affect the brightnessof paper during accelerated ageing.
Considering the extent of stabilization obtainedby various halides, a patent for their use for stabilization of paper had been proposed. 32
Figure 3: Effect of various bromides on the degradation rate
constant (k) of Whatman filter paper containing Ink 70 or Ink 0
during accelerated ageing at 80°C and 65 % relative humidity.
Sample a was treated with calcium bicarbonate, other samples-1
were treated with calcium bicarbonate and 0.3 mol L
dichloromethane solutions of tetraheptylammonium bromide (b),tetradecylammonium bromide (c), tetradodecylammonium bromide (d), trimethyloctylammonium bromide (e), dodecyltrimethylammonium bromide (f), tetrabutylammonium bromide (g) andtetrabutylammonium chloride (h). Error bars represent standard
deviation.

lammonium chloride is used instead of bromide.
Although the differences are small, they might becaused by the differences in electronegativity ofthe two anions.
Before certain antioxidant could be proposed fortreatment of iron gall ink corroded papers, several
other factors should be taken into account inaddition to their antioxidant efficiency in the presence of iron gall ink, such as degradation anddiscolouration of paper support during ageing.
Preliminary results demonstrate that during 17days of artificial ageing of Whatman filter paper,no significant difference in brightness of paper
was observed in samples containing tetrabutylammonium bromide or chloride in comparison to thedeacidified sample. While the degree of polymerization decreased by 15 and 16%±1% for thedeacidified control and the one with tetrabutylammonium bromide, the sample containing tetrabutylammonium chloride degraded only 8.5±0.6% during the 17-day accelerated ageing period.
4. Conclusions
The effect of various halides on the ageing ofWhatman filter paper containing iron gall ink wasevaluated. Results indicate that efficient stabilization is obtained when certain quaternary ammonium and phosphonium bromides are introduced
into the paper samples, regardless of the type oftransition metals present in the ink. The extent ofstabilization obtained with various quaternaryammonium bromides depends on the size of thecation and its solubility in water. Among watersoluble quaternary ammonium bromides, the5. Acknowledgements
The authors acknowledge the financial support
given through the EC 5 th Framework Programme
project InkCor, Contract no EVK4-CT-2001-0049
and through the Slovenian Ministry of higher education, science and technology.(Programme no.
P1-0153).
Aneta Balazic and Andrej Štolfa are gratefully
thanked for technical assistance.
6. References
1. J. Kolar, M. Strliè, M. Budnar, J. Malešiè, V.S. Šelih, J. Simèiè,
Stabilisation of corrosive iron gall inks, Acta Chim. Slov., 2003,
50, 763-770.
2. X. Zou, N. Gurnagul, T. Uesaka, J. Bouchard, Accelerated
aging of papers of pure cellulose: mechanism of cellulose degradation and paper embitterment, Polym. Deg. Stab., 1994, 43,
393-402.
3. M. Strliè, J. Kolar , Ageing and Stabilisation of Paper, National
and university library, Ljubljana, Slovenija, 2005.
4. H.J.H. Fenton, On a new reaction of tartaric acid, Chem.
News, 1876, 33, 190.
5. P. Wardman, L.P. Candeias, Fenton Chemistry: An
Introduction, Radiat. Res., 1996, 145, 523-531.
6. A.R. Barnes, J.K. Sugden, The hydroxyl radical in aqueous
media, Phar. Acta Helv. 1986, 61, 218-227.
7. J. Kolar, M. Strliè, B. Pihlar, New colourimetric method for
determination of hydroxyl radicals during ageing of cellulose,
Anal. Chim. Acta. 2001, 431, 313-319.
8. B. Wagner, E. Bulska, A. Hulanicki, M. Heck, Topochemical
investigation of ancient manuscripts, Fresenius J. Anal. Chem.,
2001, 369, 674-679.
9. M. Strliè, J. Kolar, V.S. Šelih, D. Koèar, B. Pihlar, A comparative study of several transition metals in Fenton-like reaction system at circum-neutral pH, Acta Chim. Slov., 2003, 50, 619-632.
10. G. Scott, Antioxidants in Science, Technology, Medicine andNutrition, Albion Publishing, Chichester, UK, 1997.
11. J. Neevel, Phytate: a potential conservation agent for thetreatment of ink corrosion caused by irongall inks, Restaurator,
1995, 16, 143-160.
12. I. Kraljiæ, C.N. Trumbore, p-Nitrosodimethylalanine as an OH
Radical Scavenger in Radiation Chemistry, J. Am. Chem. Soc.,
1965, 87, 2547-2550.

Halides for stabilisation of iron gall inks, e-PS, 2005, 2, 13-18

17

www.e-PRESERVATIONScience.org
13. R.W. Matthews, D.F. Sangster, Measurement by Benzoate
Radiolytic Decarboxylation of Relative rate Constants for
Hydroxyl Radical Reactions, J. Phys. Chem., 1965, 69, 19381946.
14. A.J. Elliot, A.S. Simsons, Rate constants for reaction ofhydroxyl radicals as a function of temperature, Radiat. Phys.
Chem., 1984, 24, 229-231.
15. J.K. Thomas, Rates of reaction of the hydroxyl radical, Trans.
Faraday Soc., 1965, 61, 702-707.
16. W.C. Bray, R.S. Livingston, The catalytic decomposition ofhydrogen peroxide in a bromine-bromide solution, and a study ofthe steady state, J Am. Chem. Soc., 1923, 45, 1251-1271.
17. W.C. Bray, H.A. Liebhafsky, Reactions involving hydrogen
peroxide, iodine and iodate ion. I. Introduction, J Am. Chem.
Soc., 1931, 53, 38-44.
18. J.L. Minor, N. Sayer, Oxygen/alkali oxidation of cellulose andmodel alcohols and the inhibition by iodine, J. Polymer Sci.: PartC, 1971, 36, 73-84.
19. J.L. Minor, N. Sanyer, Carbohydrate stabilization with iodidein oxygen bleaching of kraft pulps, Tappi, 1974, 57, 109-112.
20. L.L. Landucci, N. Sanyer, Influence of transitional metals inoxygen pulping, Tappi, 1975, 58, 60-63.
21. L.L. Landucci, N. Sanyer, Influence of metal and iodide ionsin oxygen pulping of loblolly pine, Tappi, 1974, 57, 97-100.
22. J.C. Williams, C.S. Fowler, M.S. Lyon, T.L. Merrill, Metal
Catalysts in the oxidative degradation of paper, J.C. Williams,(ed.), Preservation of Paper and Textiles of Historic and Artistic
Value, Advances in Chemistry Series, 1977, 164, 37-61.
23. J. Kolar, M. Strliè, G. Novak, B. Pihlar, Aging and Stabilizationof Alkaline Paper, J. Pulp Pap. Sci., 1998, 24, 89-94.
24. J. Kolar, M. Strliè, Stabilisation of Alkaline Cellulose withHalides and Pseudo-Halides, Intern. Conf. Chemical Technologyof Wood, Pulp and Paper, Bratislava, 2003, 422-423.
25. J. Malešiè, J. Kolar, M. Strliè, S. Polanc, The influence of
halide antioxidants in Fenton-like reaction systems containing
copper(II) ions, J. Mol. Catal. A-Chem., 2005, In press.
26. J. Malešiè, J. Kolar, M. Strliè, S. Polanc, The influence of
halide antioxidants in Fenton-like reaction systems, J. Mol. Catal.
A-Chem., 2005, Submitted.
27. SCAN-CM 15:88: Viscosity in cupri-ethylenediamine solution.
Scandinavian pulp, paper and board testing committee, 1988, 17.
28. S. Goldstein, D. Meyerstein, G. Czapski, The Fenton
Reagents, Free Rad. Biol. Med., 1993, 15, 435-445.
29. R. Evans, A.F.A. Wallis, Comparison of Cellulose Molecular
Weights Determined by High Performance Size Exclusion
Chromatography and Viscometry, 4th Int. Symp. Wood Chem.,
1987, 201-205.
30. A.M. Emsley, R.J. Heywood, M. Ali, C.M. Eley, On the kinetics
of degradation of cellulose, Cellulose, 1997, 4, 1-5.
31. G.I. Mantanis, R.A. Young, R.M. Rowell, Swelling of compressed cellulose fiber webs in organic liquids, Cellulose, 1995,
2, 1-22.
32. M. Anders, J. Kolar, M. Strliè, J. Malešiè, M. Šala, M. Koèevar, Antioxidationsmittel Für Organisches Material Und Verfahren
Zur Behandlung Desselben, Patent Application, Submitted.

18

Halides for stabilisation of iron gall inks, e-PS, 2005, 2, 13-18