Lacquer of Interest
Using GC-MS to investigate degradation of Asian Lacquer.
Jonas Veenhoven |
I work within an interdisciplinary project called Profound study of Hydrous and Solvent Interactions in Cleaning Asian Lacquer (PHySICAL). We use state-of-the art analytical techniques to develop safe and effective cleaning methods for cultural artifacts coated with Asian lacquer. Asian lacquer, made from the sap of trees in the sumac or cashew family (Anacardiaceae), is a highly durable coating, used on all sorts of objects, weapons, armor and even architecture. When exposed to light, degradation products start to appear in the lacquer, making the surface highly water-sensitive. Cleaning the degraded lacquer with solvents can cause swelling when the cleaning agent is absorbed into the lacquer layer during cleaning. Leaching of organic compounds from the polymer matrix can also occur – a complex problem with unpredictable long-term effects.
I’m responsible for the chemical side of the research at the Belgian Royal Institute of Cultural Heritage (KIK-IRPA), the Separation Science Group at Ghent University and Conservation and Restoration of Cultural Heritage Program at University of Amsterdam. The other partner in the project is the Royal Museums of Art and History (RMAH) in Brussels. Asian lacquer and polychrome sculpture conservator Delphine Mesmaeker is in charge of the RMAH research – studying their lacquer collection, improving preventive conservation conditions and investigating the visible changes to the lacquer surfaces through cleaning.
Let’s get PHySICAL
My group applies chromatographic techniques coupled to MS to:
• Analyze objects
• Carry out quality control of materials
• Analyze our polymerized mock-up samples
• Monitor the effects of artificial aging on mock-up samples
• Investigate aqueous and solvent extractions using immersion and dedicated surface extraction methods.
For the analysis of objects incorporating insoluble polymers, such as the Asian lacquers, we use pyrolysis coupled to GC-MS (Py-GC-MS). This allows us to identify material specific markers after deconvolution of the compounds. The small sample size, reduced sample preparation time and dedicated mass spectral libraries developed for this technique have proved to be valuable for the analysis of cultural heritage materials. Nevertheless, quantification is difficult, although peak area ratios can be used to semi-quantify material contributions. In addition, separation and identification using Py-GC-MS is complex because the numerous components that are formed through pyrolysis result in many coeluting peaks, which are not necessarily material-specific.
Right now, we are carrying out lacquer immersion experiments, assessing leaching of aged lacquer following exposure to solvents using conventional GC-MS. For this work we omit pyrolysis, to avoid overcomplicating interpretation and make sure the pyrolyzed fragments don’t fall below the limits of detection. By working in splitless mode and using dedicated derivatization protocols in combination with MS we can achieve the required sensitivity and selectivity. Analyzing the chemistry of leaching helps us to understand ongoing/accelerated degradation phenomena caused by solvents and ultimately predict the effect of cleaning on ancient lacquer.
Back on display
A thorough knowledge of possible solvent interactions with the lacquer surface will have a major impact on conservation of these objects. Because knowledge of the cleaning process is currently very limited, cleaning is often postponed, and objects put in storage rather than displayed in the museum. Not only does this remove them from public view but long-term storage could also lead to unwanted effects such as dust deposits. With a better understanding of solvent–lacquer interactions, we can propose cleaning methods that are not harmful to the object, so that they can be displayed once more.
Plans for the project include the continuation of the immersion extractions after artificial aging of the mock-up samples and development of surface analysis techniques in combination with stir-bar sorbtive extraction. These techniques are being developed at Ghent University and, in combination with thermal desorption GC-MS, allow us to reach ppt and even ppq level sensitivities, lowering the limits of detection of GC-MS by up to three orders of magnitude. The use of direct extraction of solutes from leachable surfaces has thus far been limited to some life sciences applications (such as skin sampling) and this will be the first time it is applied for the analysis of historical objects in general and for Asian lacquer in particular. LC-MS and other separation techniques will also be evaluated and, if needed, we will make use of high-resolution MS using LC-Time of Flight MS (LCToFMS) or Orbitrap to elucidate structures and form hypotheses about reaction mechanisms. Both GC-MS and LC-MS analysis techniques will be optimized for the analysis of Asian lacquers and compounds extracted from the damaged surface.