About the evidence within paintings showing they were produced with the aid of projected images, historian of science Wolfang Lefèvre writes “Deducing a production technique solely on the basis of the finished product is clearly a questionable position to adopt.” Lefèvre’s statement reflects what art education professor Graeme Sullivan recently wrote, that “…practice-based understanding is not normally part of a curatorial skill set.” The fact is, practice-based disciplines (optics, archeology, materials science, etc.) are well versed at deducing production techniques based solely on the finished products, without any of the documentation that historians rely on. Two examples to illustrate this, from the disciplines of archeology and materials science are:
In “A History of Flint-Knapping Experimentation, 1838-1976,”anthropologist L. Lewis Johnson reviews hundreds of experimental studies that determined how a wide variety of prehistoric flint tools were produced. Johnson writes “The first scientist to use his own knapping experience to help explain prehistory was Sven Nilsson, who stated (1868:6):
When, more than forty years ago, I first began to collect, I found here and there stones which had evidently been fashioned by the hand of man for some special purpose, and which showed distinct traces of strokes or knocks against some other equally hard, but more brittle stone. Having from my earliest youth made a practice of chipping flint-stones, and giving them any shape which I desired, I was able to recognize in these stone hammers the instruments by means of which the flint weapons had in ancient times been made.
“[Nilson’s] application of his understanding of chipping to ancient tools marks the beginning of the scientific study of knapping and adds to his reputation as one of the founders of prehistoric archaeology.” As this example shows, an entire academic discipline is based on being able to deduce production techniques based solely on the finished product, without any written documentation whatever.
A second example, from the discipline of materials science, is “Wootz Damascus steel of ancient orient,” by Juha Perttula. Pertulla writes that “Verhoeven et al. claim that they have revealed the lost art of making genuine Damascus steel. According to them, the formation of the original damascene pattern required the ore from which the wootz was prepared contained critical impurity elements, particularly vanadium, which segregated between dendrites during the solidification of the ingots. The impurity element induced the growth of interdendritic carbides during the forging process and consequently the formation of parallel carbide-rich layers.” I quoted this passage in full because it nicely illustrates that quite complex information about production techniques can be deduced by a scientist solely on the basis of the finished product, again without any written documentation whatever.
As the above two examples illustrate, scholars in practice-based disciplines routinely determine with certainty what production techniques had been used based only on the evidence contained within the finished product. In our case, Hockney and I were able to analyze the non-documentary evidence within the images because of our training in the practice-based disciplines of art and optics. However, the principle academic audience for our discoveries are historians who, like Wolfgang Lefèvre quoted at the top of this page, rely almost exclusively on written documentation. Because of this, I suspect that most historians are likely to remain no more than neutral unless contemporary written accounts are located of the actual use of projected images by artists.
 Wolfgang Lefèvre, “The Optical Camera Obscura I, A Short Exposition,” in Inside the Camera Obscura – Optics and Art under the Spell of the Projected Image, Wolfgang Lefèvre, ed., (Max-Planck-Institut für Wissenschaftsgeschichte, 2007)
 Graeme Sullivan, “Painting as Research: Create and Critique” in Handbook of the Arts in Qualitative Research, J. Gary Knowles and Ardra L. Cole, eds. (Sage Publications, in press).
 L. Lewis Johnson, “A History of Flint-Knapping Experimentation, 1838-1976 (and Comments and Reply)”. Current Anthropology, Vol. 19, No. 2. (June 1978), pp. 337-372. Critical comments on this manuscript by 19 archeologists and anthropologists are at the end of the published article. All of them implicitly accept the validity of deducing the production technique from the prehistoric finished products alone.
 Juha Perttula, “Wootz Damascus steel of ancient orient.” Scandinavian Journal of Metallurgy, Vol. 33, 92-97 (2004).