Raman Spectroscopy Analyzes Illumination in Gutenberg Bibles
Anne L. Fischer
Johannes Gutenberg is considered the inventor of European printing because he was the first to use movable metal type, back in the mid-15th century. His workshop in Mainz, Germany, produced approximately 180 copies of the Bible in 1455, and considerable remains of 48 of these books exist today.
Their production took three years and was no small undertaking. First, they were printed. Illumination -- the illustrations and decorations of the manuscripts -- was left to the purchaser. The variations among them are interesting because pigments were used depending on the location, tastes and financial resources of the owner.
Raman spectroscopy has revealed the compounds present in the illuminations on seven Gutenberg Bibles. The work offers a better understanding of the production of the texts and enables their preservation and restoration.
A previous study of a Gutenberg Bible used particle-induced x-ray emission to determine the elements present in the ink of the printed type. Now a group from University College London reports in the June 1 issue of Analytical Chemistry that it used Raman spectroscopy to examine seven Gutenberg Bibles and reveal all but one of the compounds present in the illuminations. Researchers are interested in the palettes of the illuminated copies to learn about, and from, the pigments that were involved.
Gregory D. Smith, a former member of the group and now assistant professor of conservation science at Buffalo State College in New York, said that this examination of the Bibles revealed the complete chemical palettes of these precious documents. A specific analysis is important, he said, because these richly decorated texts can leave the observer with the impression that the pigments must have been the most valuable available, which can be misleading. For instance, in the King George III copy at the British Library in London, there is no lapis. The blue color is azurite, a much cheaper copper carbonate.
Another reason for the study was to determine how such works of art should be treated. Smith is confident that the Gutenberg Bibles are well- preserved and -maintained, but by understanding the chemical makeup of the pigments, curators can be ready with a conservation plan. Treating the texts properly requires an understanding of their chemical makeup.
The researchers analyzed the pigments using a Renishaw plc System 1000 spectrometer equipped with a 632.8-nm HeNe, a 514.5-nm argon-ion and a 783-nm diode laser. They used two setups. At the British Library, they analyzed the King George III copy, supported either on a book cradle or on an extended microscope stage. Because of their fragile nature, it is important not to open the books all the way to prevent cracking the binding, so the book holder was helpful in preventing damage.
They operated the lasers at their lowest convenient powers for the job, explained Robin J.H. Clark of University College, the project director. This ensured that nothing would be damaged or altered in the spectroscopic study.
When the Bible was held by a book cradle, it was illuminated with light through a 103 objective on the remote probe head of either the HeNe or the diode laser, mounted over the cradle. The Raman-scattered light from the pigments was transmitted to the spectrometer by a fiber optic cable and onto a thermoelectrically cooled CCD detector. With the microscope system, 1003 objectives brought individual grains into focus.
The second evaluation method analyzed the pigment debris that collected in the gutters between the pages over centuries of use. The researchers gently swept the debris out of the binding and placed it on a glass slide for examination ex situ. They examined the grains under the Raman microscope using the same experimental conditions used to examine the pages and compared the Raman spectra with those in libraries of spectra obtained by Clark's group from reference samples.
Both the in situ and ex situ methods were used to examine the King George III Bible, and the results were compared to determine the validity of ex situ. The latter method is preferred when access to the Bible is limited or if spectroscopic equipment is not available to be moved to the site housing the Bible. The study concluded that the latter method does not necessarily expose the complete palette, a matter that is of some concern if restorative curatorial work is ever needed.
Overall, the researchers achieved their goal of understanding the chemical composition of the palette, with one exception. They could not identify a reddish-purple pigment, which Clark said may be a fluorescent organic chemical extracted from a plant. Many organic materials fluoresce, a process that may obscure their Raman spectra, diminishing the chances of identification.
The information gained helps enlighten curators, art historians and conservators as to the pigments used in the exquisite illuminations of these increasingly valuable texts. The Bible owned by the Huntington Library in San Marino, Calif., was purchased for $50,000 in 1911. Clark surmised that it might be worth at least $25 million today.
Although the illuminations on these works of art are not currently in need of restoration, the goal of curators is to preserve them indefinitely. The knowledge gained through spectroscopic examination will help them understand what degradation has already taken place and what steps may be taken to prevent further deterioration.
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