Richard Hawkes ACR
Ed. note: a version of this article originally appeared in the September 2013 issue of Icon News.
Since the mid-1980s, especially in the field of cleaning easel paintings, the name Wolbers and gels have been closely linked. Initially, Richard Wolbers’ research focussed on the use of solvent-carrying gels based on the polymer Carbopol® and resin soaps. They have since extended to the use of new methods involving aqueous gels, culminating in the publication of his book, Cleaning Painted Surfaces: Aqueous Methods (2000).1 Richard has been teaching workshops on the use of gels in conservation throughout this period all around the world. He was in London running his New Methods of Cleaning Painted Surfaces course for International Academic Projects when he was invited by the Icon Book & Paper Group to speak on aspects of another of his workshops developed for paper conservators.
Although obviously an accomplished chemist, Richard has a background in easel painting conservation. He began his lecture by expressing his dislike of exposure to solvents, preferring water-based systems of emulsions and gels. Few, especially in paper conservation, where water is the prime solvent, would disagree with this. Most of the chemicals he went on to mention were selected due to their lower toxicity or biodegradable properties. Although his workshops cover a wider range of materials, this lecture focussed on three non-toxic polysaccharides: xanthan gum, gellan gum and agarose (fig.1). Paper conservators are used to applying cellulose ether gels as swelling agents or as a carrier for enzymes. Some other aqueous gels have been used, such as Laponite®. By comparison, the recipes for gels for cleaning painted surfaces can appear complex. As well as solvents, gels can carry enzymes, buffers, chelates, surfactants, salts and thickeners; the point being that they are tailored to act in a specific way upon the substrate. However, the gels that Richard discussed could be prepared relatively simply and, if desired, used solely for their capillary action to draw out discolouration.
Each of the three gelling agents had differing properties that could be exploited. Xanthan gum, used in the food industry as a stabiliser for ice cream, remained as a gel at a range of pH levels from 1 to 14 and regardless of certain salt concentrations. Should the substrate being treated be acidic, xanthan gum is less likely, unlike some polymers, to form precipitates. Whilst xanthan gum’s anionic nature means it is not appropriate as an enzyme carrier it can hold a wide range of salts including chelates. It is also compatible with acids and water-miscible solvents and forms stable emulsions with water-immiscible solvents up to 20% w/w without the need for a surfactant. In fact, it can act like a surfactant and pull dirt away from the substrate and into the gel. They are, though, incompatible with oxidising agents such as hydrogen peroxide. At this stage, Richard went on to mention a number of solvents, such as iso-propyl palmitate as an alternative to white spirit and dibasic ester-type solvents with similar solubility parameters to xylene and toluene. He also touched on silicone solvents classed as dimethylsiloxanes, such a cyclomethicones D4 and D5, which can be used as a masking material in aqueous treatments in a similar manner to cylcododecane and which will evaporate in a relatively short space of time.
Several articles have been presented recently on the use of rigid gellan gums and some in the audience were already familiar with its potential applications.2 Although similar to the cellulose-like, branched polymer of xanthan gum, both deriving from bio-fermentation of a sugar source, a 2% solution of gellan gum forms a rigid, water-containing sheet which can be cut and handled. It cannot be used with many other additives; chelates would remove the vital calcium ion within the molecular structure, but small amounts of ethanol or isopropanol could be added. Its potential use in paper conservation was as a swelling agent or poultice and a means of drawing out soluble discolouration. The products used, Gelzan® and Kelcogel®, were relatively cheap and gellan gum is non-toxic and biodegradable.
Finally, Richard dealt with agarose, a rigid gel-forming material derived from a seaweed source. Unlike xanthan gum it can be used as a carrier for enzymes and, unlike gellan gum, it is compatible with buffers and chelates. One of Richard’s former students has described its use as a carrier for trypsin3, a protease enzyme but it can also be used with other families of enzymes. The gel can be formed in petri dishes, cut and lifted into place for local enzyme treatments. Potentially it is also an effective carrier for EDTA, with or without the addition of a reducing agent like sodium dithionite, as a chelator for iron(III) oxide. Richard described another useful application. Small agarose gel discs placed in temporary contact with paper can be removed and tested with a pH probe to give an indication of surface pH of the paper, without risking the tidelines sometimes formed when probes are placed in direct contact with paper. He suggested that when agarose gel was used for cleaning paper or textiles, the capillary forces involved can be greater than those from a vacuum suction disc. For selecting an agarose product, Richard advised us to use one that was pH neutral, had a melting temperature of 85-95ºC and a gel point between 32-45ºC. A low electroendosmosis (EEO) value, a measure of the movement of liquid through the gel, can also be a guide to higher purity.
I have personal experience of using agarose gels with enzymes successfully, although one issue with them is that water is rapidly drawn from the gel block into the paper, potentially leaving tidelines. Richard had explained that with 1-3% w/v gels this can be a problem but with gels of 5-6% the water is released much more slowly and the capillary forces tend to draw material from the substrate into the gel. Unfortunately, for enzymes to move from the gel to the substrate successfully a low percentage of 1.5%, with the added risk of tide-lining, was necessary.
Another question on chelates in agarose gel led Richard to suggest the potential of HBED chelates and Tiron® as alternatives to EDTA / dithionite that could be explored. The use of barrier layers was also raised and, although residues of all three gels should be removable with water rinsing, Bondina and barrier papers could be used to aid handling and removal.
Richard’s years of teaching on this subject make him an eloquent and authoritative advocate for seeking out more efficient, less toxic and greener alternatives to orthodox conservation treatments and we were very grateful to him for taking the time, after a full day’s teaching, to come and talk to us. The Getty Conservation Institute carried out the Gel Cleaning Project (1998-2003)4 to investigate further the properties and actions of the solvent gels proposed in the 1980s. Although much data is already available from the food industry, something similar would be welcomed to investigate further the use of aqueous gels in a conservation context and explore the potential of Richard Wolbers’ innovative approach to problem solving. Thanks go to Amelia Rampton and those involved with the Co-operative Training Register (CTR) who organised this event. The lecture and question session was filmed and is available for viewing on The Gathering’s YouTube channel.
- Cleaning Painted Surfaces: Aqueous Methods, Archetype, London 2000
- S Iannuccelli and S Sotgiu Wet Treatments of Works of Art on Paper with Rigid Gellan Gels, AIC Book & Paper Group Annual Vol 29 2010, pp. 25-40
- Y Van Dyke Practical applications of enzymes in paper conservation, Art, Biology and Conservation: Biodeterioration of Cultural Property, The Metropolitan Museum of Art, New York, 2004, pp94-109
- Solvent Gels for Cleaning Works of Art: The Residue Question, edited by V Dorge Research in Conservation, The Getty Conservation Institute, 2004
fig. 1 Table showing performance comparison and compatibility of additives with gels, reproduced courtesy of Richard Wolbers