Non-Destructive Compositional Analyses of Gutenberg's Inks and Papers by Proton Milliprobe
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Submitted to Archaeometry December 1982 NON-DESTRUCTIVE COMPOSITIONAL ANALYSES OF GUTENBERG'S INKS AND PAPERS BY PROTON MILLIPROBE Thomas Ao Cahill, Bruce H. Kusko, Robert A. Eldred Department of Physics and Crocker Nuclear Laboratory, and Richard N. Schwab D~partment of History University of California, Davis, CA 95616 USA Introduction The development of printing with moveable type began a revolution that continues to this day. During the fifty years that followed the invention of printing the new technique spread over Europe, resulting in the publication of hundreds of thousands of books, numbers that were completely inconceivable in the era of manuscript book production. Many aspects of the incunabula period (pre-1500) are shrouded in uncertainty, partly because records of what we wish to know have not survived or were never written down, and doubt- less partly because of the tendency of craftsmen then, as always, to guard their secrets. The extraordinary level of craftsmanship of the first great printed book, the 42-line Bible (the Gutenberg Bible) stands out in sharp contrast to the inferior quality of many later works during the incunabula period and afterward. The blackness and permanence of its ink, the technical perfection of its printing, and the excellence of its papers have rarely been surpassed. Little is known with certainty about the invention of its typographical ink, which in the new printing pro~ess was second in importance only to the development of moveable type itself. Even the exact role of Gutenberg in the introduction of printing into the western world and the publication of the 42-line Bible is open to debate. The paths by which the new technique spread and varied from town to town and country to country are in many cases obscure. We hope that our studies of the composition of inks and papers in the fifteenth century will shed light upon some of the early printing mysteries of this important period. The purpose of this paper is to explain how the technical problems that we encountered in the analysis of the 42-line Bible and other early printed works were handled and to present evidences about the composition of inks and papers that bear upon Gutenberg's own work and its impact upon his contemporaries and successors among the printers of the incunabula period. -1- The Problems of Non-Destructive Analysis of Delicat~ Objects: The paramount problem associated with obtaining compositional data on pages of the 42-line Bible is that because of their uniqueness and value any method of analysis must be non-destructive in the fullest sense of the term. By this we mean that the techniques must put no stress whatsoever on the object beyond that normal to its use or display, and that after the investi- gation one should not be able to tell, even with instruments far more sensitive than human senses, that any analysis has been performed. Chemical methods can only approach this ideal state by the use of minute samples, or aliquots, taken from the object. Since chemical methods involve changes of chemical state they are inherently destructive. If the aliquot cannot be seen by eye, the ideal is approached. Unfortunately, this procedure then faces the logical hurdle of associating a minute aliquot to the bulk sample, and spatial inhomogenities can make the results misleading. Many nuclear and atomi c methods are basically non-destructive, but by accident of design only certain types of samples can be analyzed non- destructively. A good example is neutron activation analysis (NAA), (DeSoote, 1972; Arniel!1~811which can perform quantitative trace analysis of numerous elements in grams of sample. Yet, most systems involve insertion of the sample into a nuclear reactor core, where stressful and even destruc- tive radiation exists,and one sometimes has the problem of lingering radio- activity of the sample. Methods of x-ray analysis, such as x-ray fluorescence (XRF), electron microprobe and scanning electron microscopes (SEM), and particle induced x-ray emission (PIXE) come closest to the non-destructive ideal. Electron beam methods, however, require very small samples~ vacuum irradiations, and often -2- sample coatings; they also entail some beam heating. These are usually unacceptable for the analyses of most rare or valuable ojects. X-Ray fluores- cence is widely and successfully used for analyses of fragile objects, and it is truly non-destructive (Banks, 1963; Hall, 1973; Hansen, 1981; Carriveau, 1982) However, in the form used by most museums and laboratories, it finds a limited range of elements, has spatial resolution on the scale of centimeters and has limited sensitivity. Particle induced x-ray emission (PIXE) has millimeter spatial resolution and can find a wider range of elements than XRF, including some of the biologically important elements from sodium through calcium so important in papers and many biologically derived inks (Johansson, et al 1970; Folkmann et al 197~; Cahill 1980). Such systems have previously been successfully used in the study of archaeological objects made from ceramics and metal objects '(Gordon and Kraner 1972; Ahlberg 1976; Baijot-Stroobants and Bodart 1977; Boulle and Peisach 1979; Mommsen et al 1980; Chen, et al,1980). PTXE systems, however, are generally operated in vacuum, limiting sample size, and many have potential ,for sample damage by beam heating. The first problem is solved and the second mitigated by bringing the beam out of vacuum into an air or helium atmosphere, for an external beam PIXE system, a proton milliprobe (Seaman and Shane 1975; Katsanos et al 1976; Lodhi and Sioshansi 1977; Deconninck 1977; Huda 1979; Chen et al 1980). In the Davis system the beam heating problem was reduced to insignificance by placing the x-ray detector so close to the sample that beam currents could be reduced to less than 10-9 amperes, and by using on-demand beam pulsing to turn the cyclotron off whenever an x-ray was being analyzed. The specific problems of analysis of inks and rubrics is complicated by the presence of paper or parchment substrates. One must make a quantita- tive subtraction of the chemically complicated paper from the inks, requiring -3- the ability to analyze a known amount of ink-plus-paper, and then subtracting the paper content. This can not be done if the excitation covers an unknown ratio of ink to paper, on the front and back sides of the sheet. Thus, one requires excitation narrower than the inked letters and rubrics. The problem is compounded by examples in which the inks on each side of a leaf have different compositions. One must then be able to select a letter on each side without interference from the letters on the other side. This requires not only a small proton beam but ability to set its location to sub-millimeter precision on both sides of partially opaque objects. Thus, in the program undertaken at Davis, the attempt was made to develop a proton milliprobe of such inherent sensitivity that one could analyze even the most fragile and sensitive objects, with the capability of quantitively analyzing millimeter portions of large objects to parts per million sensitivity for any and all elements from sodium to the end of the periodic table. -4- The Davis Proton Milliprobe: A beam of 4.5 MeV protons is used as the source of excitation in the Davis external beam proton milliprobe. The 76" isochronous cyclotron (Gendreau 1981) circulates a beam of 9 MeV H~ ions at relatively high currents corresponding to a good fraction of the maximum possible circulating internal beam for this ion in the Davis machine. The extracted beam passes through highly confining external slits, the electrostatic deflection plates of the on-demand beam pulsing system, (Thibeau et al 1973) a bending magnet, and another set of slits before entering the experimental area. Through these precautions, the nominal 0.5 nA delivered to the sample could not suddenly increase more than a factor of 2 due to changes in internal cyclotron parameters, as the accelera- tor is operating close to maximum conditions. Additional collimation is provided by a set of continuously adjustable collimators upstream of the final micrometer slits (Figure 1), designed to minimize current on these micrometer slits. The beam exits into a helium.atmosphere slightly above ambient pressure through a 2 mil Kapton* window. This window is routinely replaced after every 10 hours of use, although a darkening of its normal golden color to jet black provides visual confirmation of radiation damage to the window well before failure. The chamber is lined with polyethelyne (CH2) foils to prevent secondary radiation from the aluminum walls. The beam then passes through the sample to a moveable Faraday cup and is inte- grated. Location and size of the ion beam is set by the micrometer settings, and confirmed by the use of easily bleachable blue dye in a paper which is run for about 5 minutes at 10 times the normal beam current. A low power He-Ne laser mounted below the beam line provides a spot of about 1 mm diameter at *Dupont trademark -5- the exact location of~ and collinear with~ the ion beam~ through use of a mirror that can intersect the ion beam. This laser spot~ completely harmless to the object being tested~ can be seen through most papers and parchments~ as well as on the front (analyzed) surface of opague objects by another mirror. A white light of variable intensity shines on the front surface of papers and parchments, allowing one to detect letters simultaneously on each side and compare it to the laser spot. Although the energy delivered by the proton beam to a sheet of paper is roughly equi~alent to that provided by a 100 watt light bulb at 50 cm~ and much less than sunlight~ extensive tests were performed to verify that the proton beam was harmless to papers and parchments.