Light microscopy is an analytical tool which can provide unique information on a wide range of historical materials. This data can be used for a huge number of purposes, from from identifying the history of an artifact to providing insights into the preservation method used.
Historical artifacts, in general, have high heterogeneity, which means information hidden from the naked eye can be obtained by analyzing their microstructure. For instance, a paint layer detached from the surface due to pitting corrosion, or the oak species used for making furniture, can be invaluable pieces of information when studying an artifact, and can be easily found through light microscopy.
With regards to cultural heritage applications, light microscopy plays a vital role in analyzing and optimizing the degradation process of artifacts to identify a suitable conservation method. Dr Olivier Schalm, a researcher at the Department of Conservation Studies in the University of Antwerp (Belgium), has studied the process of glass and metallic degradation in cultural artifacts to identify damage in antique photographic plates, using the cutting-edge Evident DSX500 light microscope, which is capable of high resolution imaging up to 4000x.
Insights on Glass Corrosion
Glass is a homogenous and transparent substance which gradually becomes opaque and gains irregularities over the years. Analyzing glass artifacts in detail can reveal very complex properties.
Figure 1A shows the surface of a window glass fragment exhibiting lamellae formations with darkfield illumination at 555x magnification. The cross-section of the sample fragment is shown in the Figure 1B.
Figure 1A. Surface of a window glass fragment showing lamellae in unprecedented detail
Figure 1B. Cross section of the sample fragment showing the lamellae running internally.
In this experiment, a detailed analysis of large area of the sample is performed using the DSX500. The results showed the presence of lamellae formations under the brightfield and darkfield illumination.
Like most antiques, the surface of old window glass samples is imperfect. However, by obtaining the sharp images of antique samples with a z-stack created using the Extended Focal Image (EFI) function of the DSX500, detailed analysis on the formation of irregularities is possible.
Different types of glass corrosion, including the formation of manganese-rich inclusions can be visualized using various illumination techniques. Figures 2A and 2B show a glass artifact which has developed inclusions over the last 200 years, forming dendritic patterns under brightfield and darkfield illuminations, respectively.
The surface inclusions with a higher refractive index are shown in the brightfield observation, while the darkfield illumination revealed the inclusion below the surface. This study revealed an intricate three-dimensional structure, without the need for complicated tomographic techniques.
Figure 2A. Inclusions developed on glass artifact under brightfield illumination
Figure 2B. Inclusions developed on glass artifact under darkfield illumination
Daguerreotype Conservation of Photographic Plates
The daguerreotype, developed in 1839, is the first photographic process where an image is formed based on the light scattering due to silver and mercury nanoparticles on a polished silver surface. Figure 3A shows an example of daguerreotype process.
The optimization of cleaning and preserving techniques for the photographic plates has to take into account the silver corrosion process, and the extent of the damage which has occurred in the plates.
Understanding a specific feature of an image is an important aspect of analyzing the antique damages. Navigation, however, is achieved using cues of specific features for instance, identifying a scratch with the feature of the eye as a point of reference as shown in the Figure 3B.
Figure 3A. A image obtained using the daguerreotype process
Figure 3B. The daguerreotype image in under brightfield and darkfield illumination
Figure 3C. Location of scratch in the image revealed using DSX500 and BS-SEM
Reversing Corrosion Process
Modern conservation techniques not only stop corrosion, but also reverse the oxidation state. This can be achieved using atmospheric plasma afterglow treatments. Dr Schalm's research group is working towards the development of this technique as a part of the European Plasma and nano for new-age soft conservation (PANNA) project, for cleaning and protection of cultural heritage assets.
In hydrogen-based plasma, a reactive species is formed which minimizes the black tarnish of silver sulfide caused by corrosion, restoring the gloss on the surface. Pure silver can be reduced in few seconds - however, copper is resistant to current plasma treatments, and most historical silver artifacts contain a small amount of copper.
An in-depth analysis of copper corrosion can be carried out by immersing a pure copper plate in an aerated sodium sulfide solution for half an hour, followed by the examination of copper sulfide formation using the high-resolution light microscopy.
Unlike SEM techniques, which provide highly detailed images without color information, high-resolution light microscopy provides new insights to the copper corrosion. Figure 4 shows the particles of a corrosion product forming preferentially at the metallic grain border.
Figure 4. Particles of a corrosion product forming preferentially at the metallic grain border at magnifications of 150x (A), 2000x (B) and 4000x (C)
Besides carrying out gloss measurements to analyze the effect of sodium sulfide using light microscopy, the close examination of sterling silver corrosion from sodium sulfide exposure revealed unexpected information, which is shown in Figure 5.
From the series of images captured over an hour at 2000x magnification as shown in the Figure 5A, it is clear that the regions around the copper inclusions coincide with an associated drop in gloss. The graph illustrating the gloss drops as surface becomes rougher from copper regions is shown in the Figure 5B.
Figure 5A. Series of images captured over an hour at 2000x magnification
Figure 5B. Graph indicating surface topography of gloss drops
High-resolution light microscopy serves as an intuitive analytical tool for cultural heritage applications. It enables the researchers to easily characterize damage and identify minor and major differences in the corrosion process.
This approach, using the Evident DSX500, has also allowed Dr Schalm’s research group to easily obtain new information on a wide range of cultural artifacts. This technique facilitates a rapid and efficient first-line analysis which can provide additional information when combined with complementary methods.
The experimental findings revealed certain unexpected information for both metallic and glass artifacts which rise further questions on the underlying mechanisms. Although there are more material corrosion parameters to explore, it is obvious that the latest high-resolution light microscopy will continue to play a key role in the analyzing the new avenues of cultural heritage.
This information has been sourced, reviewed and adapted from materials provided by Evident Corporation - Industrial Microscopy.
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