AIC 53rd Annual Meeting

The Cooper Hewitt, Smithsonian Design Museum has a collection of approximately 300 sketches by Jan van der Straet (called Stradanus) a 16th century court artist to the Medici in Italy. A group of these drawings may have important information on the verso written with black chalk, but unfortunately, have been lined with paper. The presence of inscriptions/drawings on the verso has been detected using transmitted visible light when the media is ink but not for chalk. To minimize carrying out interventive conservation treatment to remove the linings from the fragile drawings, transmitted IR imaging was investigated to determine whether the technique could be used to detect chalk drawings on the verso without removing the linings from the drawings.

This initial imaging investigation involved four Stradanus drawings. Reflected and transmitted visible light and IR images were acquired of the recto and verso of the drawings. The investigation started with the three drawings that had the paper linings removed and had known verso chalk drawings. The first step involved testing whether transmitted IR imaging of the recto could resolve the verso chalk drawing which could be verified with reflectance images of the verso. Preliminary processing, during the image acquisition, indicated that transmitted IR and image processing could detect the verso drawing, so the next step involved imaging one of the unlined drawings with a paper support placed behind the object to mimic the lining. The final step involved the imaging of a fourth drawing that had not had the paper lining removed and it was unknown whether there was a verso chalk drawing. 

Transmitted IR images on their own did not provide much information, but image processing, both false color and image subtraction, was essential for further analysis. The false color image processing involved combinations of reflected and transmitted visible light and IR images including newer techniques that have only been introduced and used on paintings. The image subtraction processing was the difference between the reflected and transmitted IR images. The most promising methods were the image subtraction and the false color processing using transmitted visible and IR images. The image subtraction was able to fully resolve the verso chalk drawing for one of the drawings (both with and without a tertiary support), but the same processing was less conclusive for a verso chalk drawing that did not have identifiable features. When the difference mode was less conclusive, some of the false color processing seemed to be able to reveal some features that do not correspond with the recto ink drawing and could suggest that there might be verso chalk drawings.



The imaging of four Stradanus sketches suggests that transmitted IR imaging and additional processing is promising for detecting verso chalk drawings without removing the paper lining, but the results were not always definitive. Additional testing with a larger subset of drawings is needed to further investigate the potential of transmitted IR imaging and image processing.