For this issue of Hand Papermaking devoted to paper sizing, we offer a review and extension of pertinent results obtained in our investigations of the gum-dichromate photographic process, commonly known as the gum-bichromate process.1 We have published three articles to date on our findings; this article is derived mainly from "Understanding the Gum Dichromate Processes in Pictorialist Photographs: A Literature Review and Technical Study," which appeared in Studies in Conservation in 2013.2 The process of gum-dichromate printing consists of coating paper with a mixture of gum arabic, pigment, and a potassium dichromate solution. The dichromate and colloid mixture—in this case the colloid is a gum—forms the light-sensitive layer within which the pigment is embedded. A negative is placed on top of the dried, coated paper and is exposed to light. Light hardens the gum in the exposed areas making it insoluble. The pigment trapped in the hardened gum structure forms the dark tones of the print. After exposure, the paper is placed in a water bath which dissolves the gum in the unexposed areas, revealing the paper substrate beneath, thereby forming the light areas of the image. On a macro scale, the gumdichromate image is three-dimensional, with the thickest pigment layer in the dark areas, thinner in the midtones, and only the paper substrate in the lightest areas. The triumph of obtaining a successful image is not due solely to the retention of the pigmented colloid in the dark areas, it also is based on the creation of white highlights. This step is largely dependent on the paper quality and the sizing used. Identification of gum prints can sometimes be done visually and with the aid of magnification. These methods highlight the greater amount of pigment covering in the darks versus the lights, and the isolated pigment particles in the midtones and light areas in particular. Gum prints made on top of images made by other processes can sometimes hinder absolute identification. In these cases scientific analysis is invaluable. In theory, one will find chromium from the dichromate along with elements representing the pigments with non-invasive X-ray fluorescence analysis (XRF). Analysis done on a group of Pictorialist prints from The Metropolitan Museum of Art—by Raman spectroscopy and XRF—showed that, even when pigments were observed, chromium could be either absent or present and, when present, the amounts could be constant throughout the print or they could vary. In our research we compiled data on the materials most likely to have been used by Pictorialist artists, including historic papers, sizings, pigments, colloids, and sensitizers; as well as the range of methods used, comprising exposure, development, clearing, and coating in the creation of gum-dichromate prints. With this base of knowledge, we prepared samples and conducted scientific analyses to better understand the dichromate process. In our tests, we noted that, when a gum process had been used, chromium and pigments were found in the areas of maximum image density and little or no chromium was observed in the highlights. Results from this research also underline the impact that the paper support and the type and application of sizing materials have on the visual appearance and identification of the gum-dichromate technique. The forty manuals, journals, and historical notes reviewed from the Pictorialist period describe a diversity of paper qualities as being appropriate for use in gum-dichromate printing. These recommendations include watercolor and drawing papers, ordinary writing paper, as well as papers by an assortment of manufacturers like Canson, Ingres, Michallet, Allonge, Lalanne, Julio, Montgolfier, Schleicher & Schull, Steinbach, Johannot, Whatman, Cartridge, Joynson, Helios, Van Gelder, Zander, Angora, and Rives, and with a variety of surface qualities. However not all the papers and sizings have the same effect on the print's final look and not all the authors agree on which are the best. Paper sizing is recommended in order to obtain the desired image in the dark areas as well as clear whites in the minimum density areas. The sizing deters pigment particles from getting caught between the paper fibers and muddying up the image clarity. In the literature reviewed, different opinions were collected regarding the use of paper sizing to obtain a successful gumdichromate print. Tennant experimented with different formulas and his final statement was that "sizing is a waste of time"3 whereas other authors such as Carlin mentioned that the support should be any sized paper and sometimes even white shellac could be applied to the back. Carlin stated that the sizing of the paper could be done by dampening the paper with a gelatin solution or by brushing with a starch solution.4 These two materials were already remarked as good sizing materials by Maskell and Demachy in 1897 in The Photo-Aquatint or The Gum-Bichromate Process.5 In an issue of Camera Notes from 1900, Stevens cited the use of a gelatin solution to size the paper.6 Gelatin sizing is also described in an 1897 issue of Wilson's Photographic Magazine and in The Dictionary of Photography for Amateur and Professional Photographers published in 1902.7 Other authors, such as Anderson, outlined that, with certain papers, sizing is not necessary, however if sizing is required, they recommended the use of starch.8 Hunt9 and Wenzel10 also agreed on the use of starch, as is the case of articles in different issues of the British Journal of Photography11 and similar publications that indicate in addition that arrowroot is the most popular starch. There are also some formulas consisting of a mixture of materials to improve sizing properties overall.12 Gelatin and chrome alum are the favorite of authors such as Wall, Jordan, and Carroll;13 Wallon;14 and Tennant.15 Other mixtures described in the literature are gelatin and starch,16 gelatin and alcohol,17 or gelatin and formaldehyde.18 The latter still is described as a good sizing in modern manuals.19 Insofar as the recommended proportions, the most common solutions are those of pure gelatin or starch in water, with concentrations between 2 and 5% (w/v, or weight per volume), with 3% (w/v) being the favorite, and 2% (w/v) for chrome alum in water. When a mixture of gelatin and chrome alum is used, the recommended volume ratio is 1:1. Prior to sizing, the paper should be dampened in hot water to avoid any shrinking of the support while printing and developing. Sizing should be applied to the paper while hot. The process can be done by immersing the paper sheet in a tray or by brushing the solution onto the paper surface. The brush must not pass twice over the same spot if a homogeneous surface is desired; however if the paper is porous, a double coating, in two directions, should be applied, and finally the sized paper always should be dried flat.20 For our experimental samples, we used Arches 88 and Arches MBM from Arjowiggins Graphic (Boulogne-Billancourt Cedex, France) as the two paper supports. For solutions, we used: 3% (w/v) gelatin in deionized water, 3% (w/v) arrowroot starch in deionized water, and a mixture of 3% (w/v) gelatin and 2% (w/v) chrome alum. We applied the gelatin and starch solutions to the paper following two different procedures: by brushing the paper and by immersing it in the solution. To apply the mixture of gelatin and chrome alum, we used two different methods. The first method consisted of brushing the paper with the 3% solution of gelatin in deionized water, letting it dry and, on top of this first layer, brushing the 2% solution of chrome alum in deionized water solution. The second method consisted of brush-applying the 1:1 (v/v, or volume per volume) mixture of gelatin and chrome alum solutions directly to the paper surface. We performed XRF analysis on the raw paper and on the sized paper samples. In the raw Arches 88 paper and the samples of Arches 88 paper sized with starch, XRF showed the presence of a relatively high amount of calcium (Ca) and a low amount of silicon (Si). When Arches 88 paper was sized with gelatin, a relatively lower amount of sulfur (S), together with Ca and Si, were observed in the XRF spectra. In all the Arches 88 paper samples sized with chrome alum, we detected chromium (Cr) along with relatively low amounts of sulfur (S) and potassium (K), and Ca and Si. The XRF analysis of the Arches MBM paper samples gave us similar results: the raw samples, the samples sized with starch, and the samples sized with gelatin, all showed the presence of relatively high amounts of Ca and relatively low amounts of S, while the samples sized with the chrome alum solution also show the presence of Cr, relatively low amounts of K, Ca, and S. The source of potassium and sulfur may be linked to the chrome alum (KCr(SO4)2.12H2O), however potassium could also be related to the potassium dichromate (K2Cr2O7) used as sensitizer. Arches 88 paper is an unsized, waterleaf, 100-percent cotton paper,21 where calcium and silicon has been detected by XRF; however Arches MBM is a mixture of 75-percent cotton fibers and 25-percent sulfate pulp, where XRF showed the presence of calcium and sulfur. These test results, along with the literature review of the materials used during the Pictorialist period, demonstrate that when the amount of chromium is relatively higher in the maximum density areas in a photographic image, a dichromated colloid technique has been used. On the other hand, when chromium is detected in similar amounts in both dark and light areas, it is not possible to establish via XRF if the chromium comes from a dichromated colloid process or whether chrome alum has been used to size the support. Although many types of sizing are cited in the literature, chrome alum is one that can be detected with non-invasive XRF analysis. The prevalence of its use by Pictorialist artists can only be determined with more exhaustive analysis of Pictorialist collections internationally. It is clear, however, that the paper sizing material and application method were used very consciously to obtain superior results with the dichromated colloid photographic process. The authors wish to thank the Department of Photograph Conservation, the Department of Photographs, and the Department of Scientific Research at The Metropolitan Museum of Art (MMA) where the research was carried out; and Amy Hughes, then a graduate student at the Conservation Center, Institute of Fine Arts, New York University for her assistance. Anna Vila thanks Annette de la Renta and the MMA Education Department for the Senior Research fellowships that allowed her to complete this research project. ___________ notes 1. "Gum dichromate" is the chemically correct term that has replaced the more commonly used "gum bichromate" in recent years. 2. See three previously published articles: Anna Vila et al., "Understanding the Gum Dichromate Processes in Pictorialist Photographs: A Literature Review and Technical Study," Studies in Conservation vol. 58, no. 3 (2013): 176–188; Anna Vila and Silvia A. Centeno, "FTIR, Raman and XRF Identification of the Image Materials in Turn of the 20th Century Pigment-Based Photographs," Microchemical Journal vol. 106 ( January 2013): 255–262; and Anna Vila, Silvia A. Centeno, and Nora W. Kennedy, "A Technical Study of Three Pictorialist Photographs by Demachy and Le Begue," Museum Studies in Art, Science, and Technology vol. 2 (2011, in press). 3. John A. Tennant, "Gum-Bichromate Printing," The Photo-Miniature vol. 2, no. 22 (January 1901): 399–419. 4. W. E. Carlin, "The Gum-Bichromate Process," Camera Notes vol. 3 no. 2 (October 1899): 66–72. 5. Alfred Maskell and Robert Demachy, Photo-Aquatint or The Gum-Bichromate Process, first ed. (London: Hazell, Watson & Viney Ld., 1897). 6. Charles W. Stevens, "Improved Gum-Bichromate Process," Camera Notes vol. 4, no. 2 (October 1900): 102. 7. "The Bichromate Gum Process," Wilson's Photographic Magazine vol. 34 (1897): 174–176. Also, Edward J. Wall, "Gum-Bichromate Process," The Dictionary of Photography for Amateur and Professional Photographers (London: Hazell, Watson & Viney Ld., 1902), 366–374. 8. Paul L. Anderson, Pictorial Photography: Its Principals and Practice, first ed. (Philadelphia and London: J. B. Lippincott, 1917). 9. Robert Hunt, "Mr. Ponton's Process (Bichromate of Potash)," chap. 6 in A Manual of Photography (London: John Joseph Griffin & Co., 1853), 72–74. 10. Henry Wenzel, Jr., "The Gum-Bichromate Process," in The American Annual of Photography and Photographic Times Almanac for 1901 (New York: Scovill & Adams Co., 1901), 73–92. 11. A. D. Pretzl, "The Bichromated Gum Process," The British Journal of Photography vol. 44 (March 19, 1897): 183–184. Also, James Packham, "The Gum- Bichromate Process," The British Journal of Photography vol. 44 (December 10, 1897): 789–791. And, "The Gum-Bichromate Process," The British Journal of Photography vol. 44 (December 3, 1897): 776–777. 12. Henry G. Abbott, Modern Printing Processes: Gum Bichromate and Platinotype Papers (Chicago: Geo. K. Hazlitt & Co., 1900). 13. Edward J. Wall, F. I. Jordan, and J. S. Carroll, Photographic Facts and Formulas (Boston: American Photographic Publishing Co., 1924). 14. E. Wallon, "Color Printing in Gum Bichromate," The Process Engraver's Monthly vol. 6 (1899): 170–172. 15. Tennant, "Gum-Bichromate Printing." 16. Abbott, Modern Printing Processes. 17. Johannes Gaedicke, "The Gum Bichromate Process," Wilson's Photographic Magazine vol. 35 (1898): 406–407. 18. Wall, Jordan, and Carroll, Photographic Facts and Formulas; Wall, "Gum- Bichromate Process," The Dictionary of Photography for Amateur and Professional Photographers; and Abbott, Modern Printing Processes. 19. David Scopick, The Gum Bichromate Book (Rochester, New York: Light Impressions, 1978). 20. Wall, "Gum-Bichromate Process," The Dictionary of Photography for Amateur and Professional Photographers; Abbott, Modern Printing Processes; Wall, Jordan, and Carroll, Photographic Facts and Formulas; Wallon, "Color Printing in Gum Bichromate;" and H. M. Levoy, "Printing in Bi-Gum," American Photography vol. 1, no. 4 (1907): 192–193. 21. Silvie Turner, The Book of Fine Paper (New York: Thames and Hudson, 1998).