Protocol for Sampling Individual Lacquer Layers

An important component of the research was the development of a procedure for sampling individual lacquer layers for analysis. Isolating sample material from a single layer made it possible to definitively identify the materials present in each layer, which is of utmost importance considering the complex layering structure of lacquers. If the layering structure had been ignored during sampling, the analytical results would reflect the bulk composition, thereby missing the subtle compositional differences between layers.

Physical sampling of the lacquer for analysis is preceded by careful examination to ensure that the sample site is well preserved and representative of the object as a whole. Areas beneath gilded mounts afford excellent sampling locations on furniture because the sample locations will be covered once the mounts are reattached. Additionally, the mounts protect the lacquer from abrasion as well as light damage.

Promising sample sites are identified during examination under a stereomicroscope. Next, these areas are examined under ultraviolet (UV) light and imaged by X-radiography to reveal modifications and restorations that are not otherwise detectable. After identifying well-preserved and representative sample sites, samples are taken for visible and UV cross-section microscopy. These yield a clear understanding of the layer structure at the sample site, including the number, appearance, and thickness of original and restoration layers present. Ideally, sampling for organic analysis is done in situ, directly adjacent to the cross section sample site.

Sampling for analysis by TMAH-Py-GC/MS is done by scraping with a scalpel or a custom-made micro chisel. In general, an area approximately 2 mm x 2 mm is selected for sampling and excavated layer by-layer, collecting the scrapings from each stratum separately. This area yields sufficient sample material for TMAH-Py-GC/MS analysis on individual layers as thin as 20 μm; the area can be reduced proportionately for thicker layers.

The work of excavation is conducted at moderate to high magnification under a stereomicroscope using both visible and UV illumination. The use of a high-intensity UV spotlight, such as the Labino TrAc Light UV, is extremely useful in differentiating the layers as they are sampled. The previously prepared cross-section photomicrographs are regularly consulted as sampling progresses to aid in the identification of each layer.

Before sampling any given layer, all overlying material is removed with the excavating tool. Then, scrapings of the target layer are carefully extracted and placed in the well of a single-depression microscope slide. Collection of sample material is halted when the next, underlying layer begins to be exposed, because it poses a risk of interlayer contamination. All remnants of the sampled layer are then scraped away and discarded, so that sampling of the next layer can begin.

Layers greater than 20 μm in thickness can usually be sampled discretely with little or no contamination from adjacent layers. For sample locations beneath gilded mounts, microcracking of the lacquer and subsequent contamination by restoration coatings is generally not encountered because the areas sampled are well protected from light.

Finally, for layers less than 20 μm in thickness, adjacent layers may prove to be indistinguishable even under UV light. In these cases, multiple layers are sometimes knowingly sampled together.