One of these unique moments, March 6-7 2018, location Royal Library Brussels (KBR): five drawings bearing a signature by Bruegel brought together to image them all with the same equipment, procedures and settings. Four drawings form part of the KBR collection of the Print Room and one is kept at the Royal Museums of Fine Arts of Belgium, likewise in Brussels.
In a previous blog post the toolbox of the applied imaging techniques within the FINGERPRINT project was already summed up. Since then, the applied band-pass filter set for the multispectral recordings was improved and expanded. A set of new filters (MidOpt), starting from 365nm in the UV and ranging over the visual spectrum up till 940nm in the IR, was selected. All together they assure a full coverage of the wide spectral reflective responses of the paper and applied inks of the drawings. The set consists of the following filters:
BP550 (VIS); BP324 (Near-UV & Near IR); Bi405 (Violet edge); Bi440 (Violet); BP505 (Cyan); BP590 (Orange); BP635 (Light red); BP660 (Dark red); BP695 (Red & Near IR); Bi725 (Red edge); Bi780 (Near IR); Bi830 (Near IR); Bi880 (Near IR); BN940 (Near IR).
For every drawing the recto and, when possible, the verso were recorded with all filters, i.e. each filter mounted separately on a full spectrum Nikon D610 while the drawings were exposed to the incident light of two studio flashlights (Elinchrom style RX600) or with the BP324 filter exposed to the light of a UV 365nm tube; 14 recordings per side in total. One extra capture was produced with UV radiation and the VIS filter mounted to obtain a UV florescence image. As intended with MS photography, the use of various spectral bands is able to reveal and accentuate other characteristics compared to standard visual / white light photography; as illustration the example below.
The Bruegel-signature in light brown ink on the Strait of Messina drawing (i.e. a drawing after Pieter Bruegel the Elder) is difficult to read and to distinguish from the paper background. When a number of the multispectral results are presented next to each other, the letters become better legible when only spectral bands with shorter wavelengths are used. Further processing of these images with techniques such as PCA could reveal other information.
A second example (just below) with the results of the MS photography can be observed on the Bruegel drawing from the Royal Museums of Fine Arts of Belgium: Prudentia. In certain cases spectral imaging allows you to reveal information on the drawing’s conservation history. Details and features otherwise difficult to distinguish can be monitored and visualized. Standard high resolution photographic documentation (i.e. VIS) barely visualises small, perfectly executed restorations of the paper. However, as the composition of the modern paper differs from the original historical paper, the UV Florescence of both similar but nevertheless different materials react with the incident light and emit this energy in their own easily to distinguish manner.
In addition to standard high resolution photography (Phase One IQ1) and the MS Filter photography (Nikon D610), all drawings were also imaged with the KU Leuven Portable Light Dome (PLD) system; both with the White Light (WL) and Multispectral (MS) modules. These reflectance images, based on the principles of photometric stereo, provide, pixel per pixel, interactive in-depth visualisations and the extraction of metric data on the material characteristics, i.e. their spectral behaviour and on the topography of the artefact.
Bruegel’s design on paper was transferred onto copper plates by the engraver through tracing the ink lines with a stylus. When the drawings are observed with the naked eye, or inspected by means of the standard high resolution Metamorfoze photographs (see below), these indentation lines can barely be distinguished on the surface. These lines are very shallow and remain largely obscured by the ink design. When raking light is introduced these indentations become more visible; therefore all drawings were photographed twice in a standardized manner, with light coming from the left side of the drawing and light coming from the bottom/below. Although the presence of the original design still makes it challenging to identify all the indentations made by the engraver on Bruegel’s drawing, some of them are clearly visible.
The interactive PLD renderings provide even better insight in the presence of these indentation lines. In the MS PLD dataset of the original drawing, the PLDviewer software can isolate and visualize all the tracing lines by combining both the virtual relighting tool and the shaded filter. The first provides the necessary raking light, which drops shadows, illuminates edges and so makes the indentations contrast with the main flat surface. The latter omits the color of the surface, so Bruegel’s ink design. (In the example below the shaded MS PLD image has a underlying normal map based on the dataset with the Blue spectral responses)
In the upcoming months these various types of images of the Bruegel drawings will provide art historians and conservators a solid set of data to empower there study, work strategies and aimed insights. The imaging of the these five drawings was made possible thanks to the joined efforts by the entire FINGERPRINT-team.