The conservation of two contemporary Chinese woodblock prints using Gellan gum (以結冷膠修復兩幅近代中國木刻版畫) By Vivian Yip

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Print before treatment. Copyright V. Yip

Printed by artist Chen Qi just under twenty-five years ago, the two contemporary Chinese woodblock prints ‘Qin No. 2’ and ‘Bridge’ showed heavy foxing-like discolouration ranging from light yellow-ochre to dark red-brown. A hybrid treatment combining traditional Chinese washing technique with the use of gellan gum was developed as a possible alternative for dealing with the particular challenges presented by water-soluble ink on Xuan paper.
Foxing is a descriptive term based on a range of visual phenomena rather than on chemical information. Current hypotheses point to multiple causes but one common mechanism – localised cellulose oxidation.
Oxidation is a degradation reaction that occurs slowly in pure cellulose but the rate and severity of the chemical process can increase significantly in the presence of a catalyst, such as impurities in paper and contaminants in the environment, as well as stress in the paper caused by local moisture condensation. Oxidation chemistry in cellulose also sets off other oxidation reaction cycles and catalyses acid hydrolysis.

Chemical analysis
X-ray fluorescence (XRF) analysis indicated low levels of iron in both foxed and non-foxed areas of Qin No. 2. Organic origin of foxing could not be confirmed from Fourier Transform Infrared (FT-IR) spectra. Foxed areas tested to be slightly more acidic than non-foxed areas for both woodblock prints.
Treatment approach
To stabilise the prints, washing would help remove catalysts and reactants of oxidation whereas an alkaline wash would reduce both the acidity of the paper and the intensity of the foxing simultaneously. A reducing agent would have to be considered if washing alone did not give satisfactory results.
Aqueous treatment should be sympathetic to the low wet strength and fragile state of the Xuan paper, which was showing creases and splits at different stages of development. Water-soluble Chinese ink was mostly stable with the exception of a few isolated areas. Tests on life-size aged samples indicated that the prints should not be washed for a prolonged period of time.
Humidification
A challenge encountered during testing on life-sized samples was humidification. Xuan paper is thin and absorbent and has a tendency to adhere to the support as soon as it begins to absorb moisture. This prevents the lateral movement of the paper as its cellulosic fibres expand and as a result, creates and retains creases. (See Fig. 2). Xuan paper has low wet strength, which made it difficult to handle and manipulate after humidification, especially for practitioners untrained in conservation of Chinese art on paper.
Instead of humidification, the traditional Chinese washing method was adapted to wet the print briefly but thoroughly before further aqueous treatment.
Chinese paintings are often washed recto side up while stilled lined. This helps to swell the adhesive layer and release the lining. The water reservoir gathered on top of the painting is removed by rolling a clean white towel over the surface. Washing is usually done within minutes as Chinese papers are absorbent and the water-soluble ink cannot be wetted for an extensive period of time. The process may be repeated several times depending on the work. To prepare for washing, the woodblock prints were placed onto Bondina resting on a Perspex sheet then wetted quickly with a broad goats hair brush called “paibi”, dipped into a bowl of water then sprinkled and dripped onto the recto.
This process was repeated until a pool of water had gathered on top. Xuan paper was able to relax and existing creases were released. The paper could be manipulated at this point using a rolled up white towel or a soft brush but contact was kept to minimum. The perspex was then tilted slightly to drain. The prints were now ready to be transferred onto gellan gel for washing.
Gellan gum
The use of rigid gels in conservation had been introduced by Richard Wolbers in the early 2000s. Rigid gels such as gellan can deliver moisture and draw out water-soluble degradation products from the paper substrate without leaving any significant amount of residue after treatment, according to FTIR analysis on the use of gellan gum (Mazzuca, C. et al. 2013).
Research on the use of gellan gum on paper artworks has been published by Iannuccelli et al. (2010), Berzioli et al. (2011) and Mazzuca et al. (2013) and the results have been encouraging.
Gellan gum is a high molecular-weight polysaccharide produced by the bacteria Sphingomonas elodea. In its deacylated form and in the presence of cations, gellan forms a hard and brittle aqueous gel, a network of long
polymeric chains dispersed in water. Effectively, gellan gel holds water and releases and absorbs it in a uniform way. The level of water transferred to the paper is modulated by the concentration of the gel.
The advantage of using gellan gum for the treatment of the prints was that it provided a stable support to the large fragile Xuan paper during washing. Its efficiency is comparable to immersion (Mazzuca et al. 2013) yet the paper absorbs less water overall and less swelling occurs within the paper structure and the media. The rate of washing is even with a visibly observable process that can be halted easily. Gellan gel is also a stable carrier for other additives, such as certain enzymes, deacidification agents, reducing agents and water miscible solvents.
The disadvantage of washing on gellan gum during this project lay mainly in the logistics of making the large sheet of gel due to limited equipment in the university studio. However, once made, the gel was reused after being washed thoroughly by soaking in several changes of clean water.

Making the gels
Qin No. 2 was washed on 1% (w/v) alkaline gellan gel. To make the gel, calcium hydroxide was added to 5L of distilled water to raise pH to 8.5. Calcium acetate was then added at 0.04% (w/v) to provide the cations needed for a more stable gel. Due to the large quantity of gel needed, concentrated gellan gum dispersion was microwaved in two 1L beakers before being diluted with hot alkaline water to give 5L of gel solution, which was poured quickly but steadily into a large tray for gelation.
Reducing gellan gel was obtained the same way as the alkaline gellan gel, with the exception that a 0.7% (w/v) borane tert-butylamine complex was added after gellan gum powder had fully hydrated, indicated by the clear gel after heating. The mixture was stirred in a fume cabinet until the reducing agent had dissolved. Reducing gel should only be used in a well-ventilated area and a fume extraction hood should be used. Borane tert-butylamine was chosen in place of the more commonly used sodium borohydride for its slower and milder action, which did not disrupt gelling of the gellan gel.
Washing and reducing
After wetting, Qin No. 2 was moved onto a layer of alkaline gellan gel that was left to set in a large shallow tray. The use of Bondina as a support for the print also acted as a barrier and slowed the washing rate to that similar to a 2% (w/v) gellan gel. The print was left to wash on gel, covered with Perspex, for 25 minutes. A small amount of water was then added to the gel, just enough to release adhesion of the print and Bondina and any small creases that might have formed. The print was lifted to drain on Perspex before being placed onto a reducing gel for 20 minutes while covered with Perspex. Again water was added to the gel to release print before being drained on Perspex. Qin No. 2 was put back onto a cleaned alkaline gellan gel for 15 minutes, while ink fugitivity was monitored throughout the treatment. The print was then released from the gel and drained on Perspex before being pressed between Bondina and felt overnight.
The larger print, Bridge was also washed on 1% (w/v) alkaline gellan gel but was reduced with 0.5% (w/v) sodium borohydride in industrial methylated spirit (IMS), delivered locally with a small hand spray, then blotted. Concentration was later raised to 0.7% (w/v). The reducing agent was cleared with IMS then 50% (v/v) IMS in water. The print was then wetted on Perspex and drained, the procedure repeated then pressed between felt and Bondina overnight.

Results
Both treatments produced good results. While some discolouration is still visible, it is much less distracting and the prints show a natural brightness. Localised reducing treatment with sodium borohydride was effective but took longer than reduction on gellan gel with borane tert-butylamine.