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Purpose:

 
The goal of this project is to use cross-section microscopy analysis techniques to identify the finish histories on four areas of primary wood and three areas of secondary wood.
This analysis will help to identify the original finish, as well as identify later coatings applied to re-saturate or alter the aged finishes.
 
   

Procedures:

 
Seven wood samples with darkened finishes were provided to Susan Buck from seven different areas of the table. All the samples included wood fibers to ensure that the complete finish stratigraphies remained intact. The samples were first examined at 30X magnification and small portions of each were removed with a scalpel and cast into polyester resin cubes for permanent mounting. The cubes were ground and polished for cross-section microscopy analysis and photography. The sample preparation methods and analytical procedures are described in the reference section of this report.  The cast samples were analyzed and photographed using a Nikon Eclipse 80i epifluorescence microscope equipped with an EXFO X-Cite 120 Fluorescence Illumination System fiberoptic halogen light source and a polarizing light base using SPOT Advanced software (v. 5.1) for digital image capture and Adobe Photoshop CS for digital image management. Digital images of the best representative cross-sections are included in this report. Please note that the colors in the digital images are affected by the variability of color capture and color printing.
 
   

Cross-section Microscopy Results:

 
The comparative finish evidence in all the samples can be best interpreted using thereflected ultraviolet (UV) light images. In these UV photomicrographs it is possible to discriminate shellac coatings which typically autofluoresce light to bright orange, from natural plant resin varnish coatings which typically autofluoresce white to pale yellow, depending on age and oil content. The coatings which contain pigments can be observed as resinous translucent mediums with suspended red, brown and yellow particles.
 
   
An aged, oxidized shellac coating may have a paler orange autofluorescence at its surface, and may exhibit a recognizable, regular, cracking pattern (sometimes described as “bricking”). This type of “bricking” pattern was found to be characteristic of a group of elaborately carved Chippendale-style Philadelphia furniture identified as belonging to the General John Cadwalader.1 Aged shellac coatings may also have darkened surfaces with quenched autofluorescence because of degraded oil coatings or embedded grime.  Degraded plant resin varnishes with oil components can have a dull autofluorescence and will react positively for the presence of oils with biological fluorochrome stains. Wax layers do not fluoresce, and synthetic resin coatings generally have a smooth, sinuous appearance and recognizable pale lavender or blue autofluoresence colors.
 
   
Samples 1 through 4 were identified as being from the primary wood, which is mahogany. Samples 5 through 7 were identified as being from the secondary wood, which is possibly birch. Cross-section microscopy analysis reveals that the first coating layer in samples 5 through 7 is a pigmented, reddish, oil-based stain, which was likely applied to darken the lighter secondary wood to match the mahogany.
 
   
It is more difficult to confidently identify the first coating layer in samples 1 through 4 as the finish history on the mahogany elements is more disrupted and uneven, likely due to cleaning and refinishing. Sample 1 retains the most complete finish history, with three generations of coatings, beginning with a degraded, uneven shellac layer. Generations 2 and 3 are pigmented plant resin varnishes with suspended red, brown and orange pigments. The most recent coating is an unpigmented plant resin varnish (identified based on its characteristic white autofluorescence).
 
   
The evidence in each cross-section is illustrated and discussed in this section of the report, and the finish histories found in the seven samples are compared in the conclusion.