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Diagnosis and decision making for current oxidative and reductive bleaching treatments used on paper

Sigourney Smuts

Abstracts

Bleaching of paper artefacts has recently become a highly contentious issue with scientific research indicating risks to both media and support. However, there remain occasions where bleaching enhances legibility or improves aesthetic appreciation. A thorough review of the literature is presented to offer practical advice and a better understanding of the chemistry behind the processes, thus minimising risks.

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Introduction

1Bleaching is defined as the practice of removing colour or stains using chemical agents or light, thereby rendering the object whiter or lighter (Smith, A. 2012: 223). This is achieved by chemically altering the molecular structure of the cellulose and breaking the functional chromophore groups that cause colour, consequently rendering them colourless.

2Bleaching is potentially one of the most damaging treatment options in conservation. Termed a ‘cosmetic’ procedure, its main objective is to improve the aesthetic qualities of a work (Henniges and Potthast 2009: 294). Although there are minimal benefits to the paper artefact with regard to longevity and chemical stability, the treatment is motivated by the perception that discolouration and staining of artworks are undesirable qualities, giving the impression of neglect, lack of appreciation or value (Caple 2000: 90 – 91). Art objects are meant to be enjoyed aesthetically and staining and discolouration negatively affects this appreciation. In this respect, bleaching is one of the most effective methods of improving the aesthetics of an object, especially in cases of extreme discolouration. It may, however, also negatively change the work and distort the artist’s intent.

3The risks of any proposed bleaching treatment must therefore be balanced in the context of the above considerations against the benefits and assessed in the context of the age value of the work (Caple 2000: 98) . This gives consideration to the history of the work, which is evident in the discolouration and staining that has occurred over time. In the case of archival artefacts these qualities need to be preserved. However, in artworks for exhibition where the object has an aesthetic function, visual aspect of the work is more important and bleaching may be appropriate (Saito 1985: 141). Historically valuable works are sometimes best left untreated as the risk to the work is too great. Future conservators may also develop better methods of dealing with discolouration and staining or a different outlook on what is required for works to be appreciated (Caple 2000: 98). It may therefore be beneficial to leave a work untreated rather than create a situation where future conservators cannot repair the damage previously done.

4Bleaching of works of art on paper has in recent times become a highly controversial issue, resulting in many conservators refusing to practice the process. This controversy stems from technological and scientific advances that have provided a greater understanding of the bleaching process and its associated severe degradative properties (Smith, T. 2012: 260). Up until the 1970s, bleaching using chlorine-based chemicals as well as hydrogen peroxide and potassium permanganate, was widely practiced and endorsed by conservators (Brückle 2012: 274 – 276; Smith, T. 2012: 249 – 258; Lienardy and van Damme 1988: 178 – 195; Donnithorne 1979: 20; Meynell 1979: 30). This practice was even considered to be possibly beneficial to the object (Donnithorne 1988: 17 – 19). However, a more cautious approach has since been adopted after previously treated objects were re-examined and the results reported in the literature (Smith, T. 2012: 260). Behind this controversy are the two opposing viewpoints: improved aesthetics versus degradation. An uneasy balance has been struck where most published research discusses the specifics of bleaching methodologies, but with the caveat that the authors do not advocate the practice or that this approach should only be used as a last resort and in extreme cases (Burgess 1988: 12; Hey 1977: 10). All other alternatives to reduce discolouration should be considered and employed before a decision to bleach is undertaken. However, there are situations where bleaching may be beneficial to the object by removing metal or oil stains, which may accelerate the degradation of the paper support (Burgess, et al. 1989: 1).

5This paper will discuss the various factors that need to be considered when deciding whether or not to bleach, as well the practical considerations involved in bleaching.

Diagnosis and Decision-Making

6Before bleaching can be considered, it is important that the conservator determine the function of the object and whether bleaching is necessary. Since bleaching is an irreversible procedure, it should not be undertaken lightly. The relative importance of artistic and historical value needs to be considered in order to determine whether the work should be preserved in its current form or aesthetically modified through bleaching. It is generally accepted that archival artefacts or those of high cultural value should not be bleached, as the risk of irreversible damage and loss of historical integrity is too great. However, where the primary function of an artwork is visual appreciation, bleaching may be considered as it improves the overall appearance of the work (Brückle 2009: 326). As most types of degradation are part of the history of the work, the point at issue is whether these need to be preserved to fulfil the object’s function.

7The extent of bleaching is another important factor, with the ideal target colour between the object’s original colour and the current state (Brückle 2009: 326). There has been a recent shift away from requiring objects to have an unspoiled appearance to allowing the age and history of the object to be seen. This is a result of a greater understanding of the scientific implications behind bleaching and other ‘cleaning’ treatments and the damage that they may cause. The current view is that stains and discolouration should be reduced, but not removed. Staining should be reduced to a point where they are no longer disfiguring or distracting, thus allowing for better appreciation of the work. Discolouration should be diminished until the paper has a ‘warm’ tone allowing some residual discolouration, rather than a stark white colour, which can make the paper look grey and over bleached (Brückle 2009: 327). This simultaneously maintains the historical integrity.

8One consideration that needs to be taken into account is whether the work was originally painted/drawn by the artist on aged paper. If so, this aged appearance should be maintained, consequently making bleaching an inappropriate option.

9First and foremost however, all other options of reducing discolouration and staining should be exhausted. Bleaching is a last resort methodology as it is potentially damaging both to the paper support and media, causing changes to the molecular structure of the various components (Brückle 2009: 322). As paper tends to retain chemicals that it comes in contact with, it is best practice to minimise the use of chemicals whose residues may in the future damage the object (Keyes 1987: 168). Depending on the type of stains and their causes, washing and alkalisation may remove a considerable amount of soluble discolouration.

10Water is a strong polar solvent and washing alone is capable of removing soluble discolouration present in the paper. The rate of washing can be improved by increasing the temperature and changing the water frequently as it discolours (Banik and Brückle 2011: 297). Alkalisation may further increase the removal of discolouration by swelling the fibres more than water does alone and neutralising the non-soluble acids in the stains. This converts acids to more soluble products that can then be removed by the same mechanisms as washing (Banik and Brückle 2011: 309). The effectiveness of washing may be further improved by the addition of a detergent,surfactant or sequestering agent, allowing polar molecules to be removed (Banik and Brückle 2011: 293). Using these types of treatments the discolouration may be reduced to a point where bleaching is not necessary (Burgess 1988: 12).

11There are alternative methods for removing various types of stains. Ammonium hydroxide is a weak alkali that in low concentrations has a brightening action on some types of stains, such as matt burn or foxing marks. Alcohols and other organic solvents may also be used for reducing certain types of staining (Keyes 1987: 166 – 168). Alcohol may enhance the washing action by improving the wetting up of paper or allowing for less water to be used on moisture-sensitive objects. Pressure sensitive tape or grease residue stains, such as wax, fat or oil, may be removed with organic solvents such as xylene, toluene or methylethylketone (Bos 1979: 54). Enzymes are also an option for difficult and imbedded stains that cannot be removed with water, such as oil stains or wheat starch paste. Although enzymes are an extreme measure and may in themselves be difficult to remove and have adverse effects on support or media, certain stain compounds may be almost entirely removed using a specific enzyme (Keyes 1987: 168).

12The components of the paper and associated media need to be identified before a bleaching plan can be decided upon, as these will directly determine the type of bleach that may be safely and effectively used (Burgess 1988: 13). Some bleaches have adverse effects when certain elements are present. Lignin is the most problematic, reacting negatively with many bleaches (Schaeffer 1997: 1; Hey 1977: 11). The media and its sensitivities also need to be considered (Brückle 2009: 322). Some media have alkali or acid sensitivities, which influence the bleaching conditions used. Bleaches may negatively affect organic media, causing them to fade or change colour. The colour of many organic pigments is due to the conjugated bonds, examples being the blue pigment indigo. As most bleaches work by breaking this conjugation, pigments may change colour or fade caused by a reduction in the number of these bonds (Feller 1971: 41 - 42). Alkaline sensitive media also need to be diagnosed and taken into consideration as some bleaches have a high working pH, such as sodium borohydride and hypochlorites (Hey 1977: 21).

13Some bleaches, for instance hydrogen peroxide, when used in the presence of metal-based media, such as iron-gall ink or verdigris, may cause degradation of the paper and staining (Hey 1977: 20). Metal ions cause hydrogen peroxide to degrade at an accelerated rate producing oxygen gas. The oxygen gas may cause blistering of the paper or oxidise the metal which may result in cellulose degradation and the formation of dark stains as the metal ‘rusts’ (Niehus et al. 2012: 356).

14As demonstrated above, the chemistry behind bleaching needs to be understood so as to predict and also avoid these adverse effects. Taken together, the media and components of the paper may exclude an object from any bleaching treatment, as the risks could outweigh the benefits.

15The condition of the object, degree of degradation and oxidation, as well as the cause of staining needs to be recognized and considered before choosing the type of bleach. A degraded object is more likely to have significant discolouration due to high acidity, which may also encourage mould or biological damage (Burgess 1980: 171). Embrittlement of paper is another degradation indicator as a result of chain scission of the cellulose molecule. Where there is severe oxidation the object will be more sensitive to bleaching procedures (Burgess 1988: 12). This is the result of an increase in functional carboxyl and carbonyl groups, primarily aldehydes and ketones, converted from primary hydroxyl groups. These functional groups result in the cellulose becoming vulnerable to hydrolysis over a greater pH range (Bicchieri and Brusa 1997: 2). Sodium borohydride is known to damage highly oxidised papers due to its acidity. This bleach decomposes in reaction with acids resulting in physical disruption of the paper and the formation of blisters. Highly oxidised papers are also more at risk of degradation due to alkaline hydrolysis at the high working pH of sodium borohydride (Burgess 1988: 23).

16The cause of the staining also needs to be considered. Staining or foxing from metal particulates excludes the use of some bleaches, for example hydrogen peroxide (Hey 1977: 20). This type of stain may respond better to a non-bleaching method (Keyes 1987: 166 – 168). Chelation is an effective method of sequestering and removing metal particulates (Burgess 1991: 36). The use of a mild acid, such as oxalic or citric acid, may also prove to be a more appropriate treatment as acids dissolve iron, thus removing the stain (Bos 1979: 54).

17The choice of bleach and the method of application are based on the paper support, its condition and components, the media as well as the ­­discolouration or staining. In order to make an informed decision it is of great importance to keep up to date with current research in the field. Over the past few decades there have been significant advances in the available knowledge of bleaches and their properties as well as when they are deemed acceptable to use as a conservation tool. For example, chlorine-based bleaches are no longer considered acceptable and new bleaches, such as reductive and light have come to the fore. Methods of application have also advanced, with the low pressure table being a relatively new option in addition to the traditional overall bleaching with immersion, floating and local application methods with swabs, brushes and poultices (Burgess 1988: 16).

18Once the type of bleach has been chosen, the pH and concentration need to be taken into account. Concentration affects the rate of bleaching and therefore the rate of degradation, but not the total amount of degradation. For example, the same amount of degradation will occur at 3% as at 1%, but the latter will take longer to achieve a similar result. This applies to oxidative bleaches only (Hey 1977: 11 – 12). However, low concentrations are often better, as the slower reaction makes it easier to judge when sufficient bleaching has been achieved, thereby reducing the risk of over bleaching (Brückle 2009: 322). pH affects the safe working conditions of the bleach and the correct pH minimises the risk of degradation. However, should the pH be unsuitable for a particular bleach, the rate of degradation is exponentially exacerbated in relation to the increase in concentration of the bleach (Hey 1977: 11). Bleaching sensitises objects, thus the type of bleach and concentration required to achieve the desired effect needs to be carefully considered so that the process does not have to be repeated, which would cause more damage (Burgess and Hanlan 1979: 20).

19The pH of the bleaching solution affects the reduction-oxidation reaction and is dependent on the specific type of bleach. For safe bleaching conditions to be carried out, the pH needs to favour the formation of carboxyl groups, which are the oxidised products of carbonyl groups and removal thereof. The acids may later be removed through alkalisation. However, should carbonyl groups be favoured, more of the stable hydroxyl groups are then converted to carbonyl groups, degrading the cellulose, resulting in the production of more carboxylic acids (Bicchieri and Brusa 1997: 1 – 2). For most of the oxidative bleaches, such as hydrogen peroxide and hypochlorites, alkaline conditions favour the production of carboxyl groups, whilst at slightly alkaline or acidic pHs carbonyl groups are produced (Margutti, et al. 2001: 70). Chlorine dioxide is the only oxidative bleach that is preferably used under slightly acidic conditions (Smith, A. 2012: 239). The pH of the bleach is also important as it may improve the bleaching efficacy. The active bleaching agent in hydrogen peroxide is the perhydroxyl anion (OOH-) which is favoured in alkaline conditions, therefore to raise its concentration the pH must be increased. However, the anion degrades at pH 11, thus the pH of the solution cannot be too high as no bleaching action will take place (Carter 1996: 1070). The pH of chlorine dioxide needs to be kept low to promote the formation of chlorine dioxide from the sodium chlorite and formaldehyde solution (Carter 1996: 1071).

20The pre-treatment of an object needs to be carefully considered and correctly conducted. This is based on the type of bleach to be used. As previously discussed, prior washing and alkalisation needs to be performed in order to ascertain whether bleaching is required. Removing as much discolouration as possible will also minimise the amount of bleaching required. As bleaching is degradative, it is also good practice to ensure the paper is in as good a condition as possible before any treatment is executed, thus reducing damage (Brückle 2009: 323; Burgess 1988: 15 – 16). It may be necessary to carry out additional treatments, such as chelating with oxalic acid to remove metal particulates, particularly if hydrogen peroxide bleaching is undertaken (Burgess 1991: 36).

21The post treatment of a bleached object is extremely important as it removes a significant portion of residues, degradation products and stabilises the paper (Smith, A. 2012: 232; Brückle 2009: 323). The exact post treatment protocol will be dictated by the type of bleach used. To halt the bleaching process and aid removal of residues, a chemical may be needed. For example the anti-chlors sodium thiosulphate or sodium borohydride are used when chlorine-based bleaches, like chlorine dioxide, are employed. However, for all bleaching processes it is important to thoroughly wash the object, followed by an alkalisation step. Washing will remove soluble decolourised degradation products, minimising the future risk of colour reversion (Smith, A. 2012: 229). Alkalisation will neutralise and aid in the removal of any acids formed during bleaching as well as stabilise the paper. Thus an object should not be bleached if it cannot be thoroughly washed both before and after treatment (Burgess 1988: 12). Taken together, the risks associated with bleaching may be greatly reduced by methodical deliberation of pre- and post-treatment options.

Conclusion

22The fundamental underlying tenet of conservation is that of preservation and stabilisation. Procedures that undermine these principles need to be carefully considered by the conservator. Bleaching is one such treatment that has been shown to significantly damage and cause deterioration to paper artefacts. Yet, bleaching may also substantially improve the visual appreciation of an artwork or document. Thus the controversy amongst practitioners is whether the potential risks outweigh the aesthetic benefits.

23Bleaching should not be discounted based solely on the possible risks. If these risks are carefully considered and evaluated, through the thorough analysis of the object and its components and the type and cause of stain or discolouration, the potential damage to the artefact can be minimised.  This is achieved by selecting appropriate bleach, method of application and working conditions. Pre- and post-treatments will also minimise the risks to the object by reducing soluble discolouration and minimising treatment time, removing residues and degradation products, as well as neutralising acidity. Part of this evaluation process is to consider the function of the object, its future use and the environmental conditions of storage and display. If these issues are addressed, bleaching may greatly improve the aesthetic appearance and legibility of an object.

24Bleaching, as a method of reducing disfiguring and distracting discolouration and staining, improves the aesthetics of an artwork or document, and in aiding in the appreciation and legibility it continues to have value in paper conservation.

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Bibliography

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References

Electronic reference

Sigourney Smuts, « Diagnosis and decision making for current oxidative and reductive bleaching treatments used on paper  », CeROArt [Online], EGG 4 | 2014, Online since 05 March 2014, connection on 18 August 2017. URL : http://ceroart.revues.org/4068

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About the author

Sigourney Smuts

Sigourney Smuts is a South African Paper Conservator. She has an undergraduate Bachelor of Fine Arts degree (Honours, 2010) from the University of Cape Town, South Africa, and recently completed a Masters in Conservation of Fine Arts (specialism Works of Art on Paper) from Northumbria University, United Kingdom. She has completed internships and placements, including at the Parliament of the Republic of South Africa Conservation Centre, Kew Royal Botanical Gardens and the Iziko South African National Gallery. She currently works as a private conservator and at the University of Cape Town Special Collections Library.This paper is based on the final year research project leading to the award of Masters in Conservation

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CeROArt – Conservation, exposition, restauration d'objets d'arts est mis à disposition selon les termes de la licence Creative Commons Attribution - Pas d'Utilisation Commerciale - Pas de Modification 4.0 International.

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