Scientists and historians have devoted great time and effort into understanding how historical and archaeological artifacts degrade and what can be done to better preserve them. Whether viewing a fifteenth century oil painting, a handwritten document on parchment from ancient Rome, or clothing and tools from prehistoric times, researchers have investigated the preservation process in order to ensure these items are not lost forever.
![Artefacts such as these at Zuiderzee Museum in the Netherlands show signs of ageing - researchers are working to better understand how materials age to protect heritage items. [pixabay]](https://images.squarespace-cdn.com/content/v1/556d800ae4b0e8f91507450c/1606220661645-O6VYDPA3VW5WPZB1ZG2X/zuiderzee-museum-315416_1280.jpg)
Artefacts such as these at Zuiderzee Museum in the Netherlands show signs of ageing - researchers are working to better understand how materials age to protect heritage items. [pixabay]
Researchers at the Advanced Research for Cultural Heritage (ARCH) Lab at the National Research and Development Institute for Textiles and Leather, at the University of Craiova, Romania, used Linkam’s LTS120 Peltier system to study the photo-oxidative deterioration of parchment provoked by sunlight irradiation. Meanwhile, researchers at the Department of Analytical Chemistry, at the University of the Basque Country in Spain, employed a Linkam THMS600 stage to study the phase transition of plattnerite [ß-PbO₂ lead (IV) oxide]-based historical painting samples when analysed by Raman spectroscopy. These are just two of the many examples that showcase the role of temperature-controlled microscopy in wide-ranging scientific fields; from materials and metallurgy, to life science, oil and gas, space exploration and food research.
The team at the ARCH Lab monitored and evaluated the physical–chemical changes of parchment induced by accelerated ageing by applying non-invasive Fourier transform infrared spectroscopy in attenuated total reflectance acquisition mode (FTIR-ATR) suited for biological and natural samples, Raman spectroscopy and colorimetric measurements, together with the Micro-Hot-Table (MHT) method. MHT is a micro-invasive technique which is widely used for measuring the shrinkage temperature (Tₛ) of collagen-based historical and archaeological materials (mostly leather and parchment). The Tₛ value depends on the structural stability of collagen, and therefore on its deterioration degree.
Analyses were performed using MHT equipment consisting of a Linkam LTS120 temperature control Peltier stage equipped with an automatic heating rate adjustment system and a stereomicroscope. The shrinking motion was recorded by a digital camera mounted onto the microscope [see Figure 1]. This simple method delivers sophisticated analysis and enables in situ evaluation and diagnosis of collagen-based artefacts.
MHT equipment for in situ analysis of shrinkage activity. (a) micro heating plate (Linkam LTS120); (b) temperature controller; (c) water circulator; (d) digital microscope; (e) computer with image MHT software.
Dr. Elena Badea, Project Director at the ARCH Lab, commented: “The experiment was carried out to study the damaging effects of mixed light-thermal ageing on collagen in the solid phase, as found in parchment. If we can better understand how collagen deteriorates in a particular artefact, we can get helpful insight into its manufacture, how damaged it is, as well as how best to formulate conservation treatments and apply appropriate storage and handling conditions to preserve it.
“We have been using the MHT method since 2006 and are now strongly advocating a more analytical use of this method when analysing both new and artificially aged samples, and ancient materials, by including the analysis of other shrinkage parameters to limit the risk of over or underestimating the conservation (damage) condition of parchment/leather.
“The FTIR-ATR and Raman techniques coupled with MHT method provided qualitative results that allowed us to characterise the ageing pattern of collagen in parchment exposed to light irradiation at 52 °C and 30% relative humidity over time.”
The LTS120 Peltier stage is an easy-to-use temperature control stage for use with optical microscopes. It uses a 40mm x 40mm Peltier element to control the temperature of the sample from -25°C to 120°C with accuracy of 0.1°C and rates of up to 30°C per minute, with options for gas purging, humidity control and electronic connections. The sample can be mounted on a regular microscope slide and can be moved 15mm in both X and Y directions.
Plattnerite is a by-product found upon the degradation of lead-based pigments, which have been widely used for centuries in artwork. The formation of plattnerite and other lead compounds is thought to be responsible for an undesirable blackening in aged artworks. Better understanding of how plattnerite forms may assist in the analysis and restoration or laser cleaning of aged artwork. The research team at the University of the Basque Country coupled a Raman Spectrometer with a THMS600 temperature control stage to heat commercial plattnerite samples and increase the cell temperature to verify the temperature range at which the phase transitions of lead dioxide occurred and to show the gradual degradation of plattnerite and the formation of the secondary products during the Raman analyses. The team found that pigments from historical painting samples behaved differently to those from commercial samples under the effect of the laser.
One of the researchers, Dr Juan Manuel, commented: “The THMS600 freezing–heating stage can be operated over a temperature range of -195°C to +600°C and it enabled us to verify that the first phase transformation of plattnerite takes place at a temperature between 365°C and 370°C. The stage allowed us to obtain a gradual transformation of lead-based compounds from red lead into litharge and massicot (types of lead oxide minerals), generated by the thermal plattnerite degradation [see Figure 3].
“However, we also found that the degradation process was reversible, since only the use of the laser beam, even if used at maximum power, was not enough to obtain a stable form of massicot. The intermediate products of the decomposition of plattnerite were evident when the sample was analysed again at low laser power, after cooling.
“The THMS600 allowed us to see the Raman spectra belonging to the different phase transitions, which had not been possible via varying the laser power alone. Our findings indicate that, in the presence of lead compounds, it is important to use a low laser power during Raman measurements, on mural paintings’ surfaces.”
Figure 11 3D representation of Raman spectra shows the phase transition from red lead to litharge and then massicot, with increasing temperature (from 500 °C to 600 °C) using temperature-controlled stage.
Through the analysis of manuscripts, documents and artefacts which have been seldom read, opened or exhibited, important new information has emerged. For example, the origin of the materials, the spread of production techniques and interaction with the environments where they were used are all pieces of knowledge that must be understood in order to grasp the full picture of a historical artefact.
References
Studies on the effects of mixed light‑thermal ageing on parchment by vibrational spectroscopy and micro hot table method, Cappa et al. Herit Sci (2020) 8:15 https://heritagesciencejournal.springeropen.com/articles/10.1186/s40494-020-0353-z#rightslink
Use of Temperature Controlled Stage Confocal Raman Microscopy to Study Phase Transition of Lead Dioxide (Plattnerite), Costantini et al. Minerals (2020) https://www.mdpi.com/2075-163X/10/5/468
Images in this article are taken with permission from the above references under CC BY 4.0.
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