Lacquered surfaces can be affected by light, water, oxygen and coatings such as waxes and varnishes. Sarah Houlton explores how a piece in the UK's Victoria and Albert museum's collection is being conserved.
Lacquered surfaces can be affected by light, water, oxygen and coatings such as waxes and varnishes. Sarah Houlton explores how a piece in the UK’s Victoria and Albert museum’s collection is being conserved.
Museum pieces are all too often extremely delicate. Even furniture designed to be sat on or have things put in can, after centuries of use and abuse, become badly damaged. If the piece is to survive the rigours of life as a museum exhibit, careful conservation is essential, particularly for sensitive surfaces like lacquer.
Lacquering is a common decorative technique in Far Eastern furniture. In Japan, lacquering is known as urushi, with the base lacquer, which is often black, being combined with metal powder and a host of layering and inlaying techniques to create works of art. Good quality lacquer is extremely durable, and initially it is very resistant to both water and organic solvents. But as the water which is an essential part of the lacquer’s structure is lost in low relative humidities, it becomes more brittle, less strong, and susceptible to attack by water and oxygen.
In the Far East, conservation and restoration of lacquered objects has centred on using the same materials that were used to make the lacquer in the first place. However, pieces that found their way to the West - whether because they were initially made for the export market, or have since been sold on - were often treated with substances such as waxes, oils and natural resins. While these non-traditional treatments might have looked good when they were first applied, as the treatments age, they tend to discolour and dull the lacquer’s surface. Nowadays, Western conservators are much more likely to use synthetic (or otherwise non-traditional) materials chosen because of their photostable properties, and because the treatments ought to be reversible.
One of the biggest dangers to lacquered objects is exposure to light. Light can cause substantial damage to urushi lacquer, and if the photodegradation is severe then even simple cleaning can cause irreparable damage to its surface. Conservators in Japan commonly use water to clean the lacquer, with experience giving an intuitive understanding of the materials. In the West, conservators prefer to avoid aqueous solutions, turning instead to hydrocarbon solvents that are less likely to damage the surfaces. But while mild mineral spirits are good at removing oily and non-polar dirt, they are far from ideal for removing the soiling typically found on museum objects after prolonged storage or open display, as this is much more likely to be of a polar nature. So the strongly polar water is often a more effective solvent for cleaning the surface.
Yet when the lacquer is severely light-damaged, tap water can cause blanching of the surface, and at the very least a diminution in the lacquer’s characteristic glossiness. Even deionised water may have the same effect, as the lacquer can be sensitive to slightly elevated pH. The lacquer has a pH of around 4.5 when it is fresh, and tends to become more acidic as it ages and gets affected by light. The photodegraded lacquer is easily damaged, with even fingertips able to etch the lacquer, resulting in permanent fingerprints that ruin the surface.
The way the lacquer behaves as it is cleaned will likely be different depending on the amount of exposure it has had to light, or treatments that have been used on it in the past. Even on the same object the lacquer can behave differently - for example, the outside of a cabinet might have suffered extreme light damage, while its interior, protected from light by the closed doors, could be in near pristine condition.
The damage that an object has suffered has a great bearing on the type of dirt that will have adhered to it, too, and hence the form of cleaning it will need. The surface of photodamaged lacquer becomes more polar the more UV exposure it has suffered, and hence the dirt will be more polar, too. So, increasingly polar solvents are needed to remove the dirt from the hydrophilic surface.
Further damage has frequently been caused by well-meaning people trying to protect or improve the lacquer. A layer of varnish was often added to the surface to restore its shine, but this can have a detrimental effect on the decorative finish, and often yellows with age. However, removing the varnish can cause further problems, as it can damage the lacquer underneath. If removal is essential, then the least polar solvent possible should be used to minimise the damage caused by bonding of the solvent to the lacquer’s surface.
Cracks in the lacquer cause huge problems, as they may go all the way through the layers, trapping the solvents being used to clean it. This may discolour the lacquer further, or even cause it to delaminate. In these cases, minimal quantities of solvent should be used to reduce the possibility of inadvertently causing further damage.
The Victoria and Albert (V&A) museum in London, UK, has one of the world’s largest collections of decorative art. ’There is not the physical space to display it all on permanent exhibition, so there are rotating displays to increase access to the collection,’ explains Shayne Rivers, senior conservator and lacquer conservation expert at the museum. ’This means that much of the collection remains in storage, and that needs looking after.’
Rivers is one of a team of 55 conservators at the museum, who are divided into specialisms such as paper, textiles, furniture, metals, ceramics or sculpture. ’While the curators are responsible for interpreting the collection and tracking the objects, our job is to make sure the pieces stay in good condition,’ she says. ’For example, the curators will send us a list of objects that are about to go on loan, and we will assess them and decide whether they are safe to travel, if they are too fragile to move, or whether they will be OK if some work is done on them. We also look after preventive measures, such as monitoring and recording light levels, humidity levels, and pest management.’
Chemistry is key in practical conservation - it helps in the understanding of why a piece is deteriorating, how it’s going to deteriorate in the long term, and how this can be prevented.
Perhaps the best example of Japanese lacquer in the V&A’s collection is the Mazarin chest. It was made in Japan for the export market, coming to Europe over 350 years ago. It has been associated with numerous illustrious families, including Louis XIV of France’s principal minister Cardinal Mazarin (hence the name), and one of the Dukes of Hamilton. ’It is the most stunningly beautiful object, and one of the most important of its type in the world,’ Rivers claims. ’If it were in Japan, it would certainly be considered a national treasure.’
The detail on the chest is extraordinary. It uses a wide range of different decorative techniques, such as metal fittings, foil inlays, mother-of-pearl inserts, and glass for the eyes of the dragons. Much of the decoration is done with sprinkled metal powder, a typically Japanese technique. And the inside of the chest - protected from dirt and light by the lid - positively glows. ’The chest is the equivalent of an Old Master in terms of quality,’ Rivers adds, ’so it’s important that we handle it extremely carefully’.
The museum wanted to include the chest in the Encounters: the meeting of Asia and Europe 1500-1800 exhibition, due to open this month. ’The chest is perfect for this exhibition: it’s all about the interaction between East and West, and the chest was made in a Western style but with Eastern decoration,’ Rivers says. ’But the decoration is too fragile at present. The chest needs careful cleaning and we must make sure that the solvents we use to remove the accumulated grime will not damage the chest further.’
Rivers spoke to Andy Holton at ExxonMobil Chemical’s Fareham, UK, site, where he is product stewardship and regulatory affairs manager. Holton says that choosing the best solvents to use was not simple. ’This was uncharted territory for the company,’ he explains. ’Fortunately, Shayne already had an idea of the characteristics she wanted, and we sent samples of various solvents we thought might work to the museum.’
Several solvents were picked out, and they were first used in cleaning tests before being applied to the chest. ’We found Nappar 10 to be particularly useful, either on its own or in combination with an aromatic hydrocarbon,’ Rivers explains. ’This is because the chest has in the past been treated with wax and, on another occasion, a non-drying oil. This is an example of matching the solvent to the material you want to remove, but needing to avoid interactions with the original substrate; hydrocarbons are usually non-damaging to lacquer and are effective for removing waxes and oils.’
The problem becomes much more complex when the lacquer has been coated with a traditional varnish, for example one based on natural resins or drying oils, because many of the solvents that might remove these materials are almost certain to damage the lacquer underneath.
Rivers adds that the chest appears to have several different coatings, in different places and with different stratigraphies. ’Our first step has been to remove the wax and oil coatings where we can. We are considering our options for other, more problematic areas,’ she says.
Conservation is not just a case of cleaning. In Japan, a traditional consolidation treatment is often applied to old lacquer pieces, where urushi is diluted in a hydrocarbon solvent and applied to the surface in order to impregnate the micro-cracks caused by light damage. The aim of this is to strengthen the surface. ’We plan to undertake research on the long term effects of such an impregnation treatment, before we consider whether it is appropriate for the chest,’ she says.
Rivers also asked ExxonMobil if it could help pinpoint a solvent or solvent mixture that had similar properties to the Japanese solvent, which is not available in Europe. ’Andy suggested that a mixture of their solvents DSP 80/110 and Nappar 6 might be a good place to start. We will be testing this solvent blend next year, if our application to the Getty Foundation [in the US] to fund a combined research and conservation treatment programme is successful,’ she says.
’Conservation is a non-starter for the commercial production of solvents because we use such small quantities,’ Rivers adds. ’Andy had to send us samples because we couldn’t find a supplier to supply in quantities of around five litres at a time. It has been great to be able to discuss our needs and come up with some possible solutions, and to be able to try out solvents that are new to us.’
Sarah Houlton is a freelance science writer
Urushi, as Japanese lacquering is known, is itself the sap of the tree Rhus verniciflua, of the Sumac family, known as Urushi-no-ki in Japanese. It grows exclusively in Japan and China, with other members of the genus growing in Vietnam, Taiwan, Thailand and Myanmar; this sap is softer and takes longer to set than that from R. verniciflua. A tree yields around 250g of sap a year.
In China, where the lacquer is also heavily used as an industrial plastic, for instance for oil pipelines, there were estimated to be over 400m R verniciflua trees in 1978, producing around 2000 tonnes of lacquer a year. Its use is exclusively decorative in Japan, and Japanese production that year was closer to five tonnes.
The sap is collected through cuts in the bark, in much the same way that rubber is tapped. The fresh sap is a water-in-oil-in-water double emulsion containing 27-50 per cent water, which matures into raw lacquer, a water-in-oil emulsion. The main ingredient of the raw lacquer is the chemical urushiol, which constitutes 60-65 per cent of the oil phase, plus 2-5 per cent glycoprotein. The water phase contains 5-7 per cent polysaccharide, and 1-2 per cent of the enzyme laccase.
Urushiol is a catechol, which has one of a range of aliphatic chains containing 15 or 17 carbon atoms attached to the benzene ring, the majority in the cis position. It is also the active ingredient in poison ivy, so it can cause skin irritation. Laccase is a phenol oxidase enzyme, which plays a pivotal role in the lacquer’s hardening process.
The raw lacquer is also a strong adhesive, especially when mixed with wheat flour or animal glue. If it is mixed with rice paste and sawdust, then it can be used as a filler. However, to create a stable external coating layer, it must be refined. This is done by taking the matured sap and stirring it, initially at room temperature and then at temperatures up to 45?C, in an open vessel until the viscosity and colour reach the right levels.
Much of the water evaporates during this process, with the water content reducing from 20-25 per cent in the raw lacquer to just 2-4 per cent. Several other chemical changes take place, too. The polysaccharides precipitate and then break up into fine particles which disperse in the urushiol. The urushiol partially polymerises, and complexes with the glycoproteins. Some of these complexes then combine with the polysaccharides.
Creating lacquered objects is a slow, painstaking process - it’s not just a case of painting the lacquer onto a wooden or metal piece. Rather, it is applied to the object in several very thin layers, with each layer being left to dry and harden before the surface is sanded down and the next coat applied. It can be applied as the bare lacquer, but typically inorganic pigments are added - often mercuric sulfide (cinnabar) or iron oxide to give red or black lacquer.
While other natural resins like shellac harden as the solvent in which they are dissolved evaporates, urushi lacquer hardens either by heating, or by an enzyme-catalysed polymerisation reaction with oxygen. In the former, the lacquer is heated to 110-180?C, and atmospheric oxygen reacts with the urushiol’s sidechains to give peroxides. These attack the benzene ring, forming free radicals which undergo further reactions, or they cross-link with other sidechains. While this method works well for the base layers of a metal object, the enzyme-catalysed process is normally used on wooden objects, and for the decorative upper layers on metals.
Enzyme-catalysed curing takes advantage of the laccase that is present in the lacquer. This works best at a relative humidity of 75-85 per cent and a temperature of 20-30?C, so is usually carried out in a humidity chamber. The laccase catalyses the oxidation of urushiol to free radicals, which then react to give the cross-linked polymer. The curing process can take anything from hours to days, and the water content is ultimately reduced to 1-3 per cent. Some of this water is an integral part of the urushi’s structure, and if it is lost, the coating will be damaged.
High humidity is essential for the cross-linking reaction to take place, and the humidity level has a big effect on the quality of the final product. If the humidity or temperature is too high, then the finish will be dull, wrinkly or weakened, rather than hard with a characteristic glossy shine.
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