**1. Introduction**

*Metamorphosis Insectorum Surinamensium*, first published by Maria Sibylla Merian (1647–1717) in Amsterdam in 1705, is among the most famous examples of early modern insect imagery. The volume consists of 60 copperplate engravings, designed by Merian, engraved by professional engravers, Pieter Sluyter, Joseph Mulder, and Daniel Stopendael, and printed in Merian's Amsterdam workshop. The original volume was published in Dutch and Latin editions and was sold either containing unadorned engravings or, for an added fee, hand-colored by Merian or her daughters, Dorothea Maria and Johanna Helena [1]. The product of two years of research conducted on site in Suriname, the images in *Metamorphosis* constitute a landmark in the history of scientific imagery. They contain detailed portrayals of the different stages of metamorphosis of insects native to Suriname and they set these depictions of transformation in the insects' botanical habitats. Although *Metamorphosis* contains inaccuracies, embellishments, and mistranslations, the images in the volume as a whole are unique for the depth of information that they provide about the flora and fauna of Suriname, for their vivid yet concise compositions, and, notably for this study, for the fact that they number among few early modern natural history prints to survive with original coloring.

Whether color could be used as reliable and reproducible descriptive feature of natural history specimens was hotly contested amongst early modern botanists, naturalists, and artists [2]. Reproducing the color of natural phenomena, especially in the context of expeditions like Merian's, challenged naturalists to develop means to systematize their

**Citation:** Dill, O.; Vermeulen, M.; McGeachy, A.; Walton, M. Multi-Modal, Non-Invasive Investigation of Modern Colorants on Three Early Modern Prints by Maria Sibylla Merian. *Heritage* **2021**, *4*, 1590–1604. https://doi.org/10.3390/ heritage4030088

Academic Editor: Francesco Soldovieri

Received: 26 June 2021 Accepted: 30 July 2021 Published: 4 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

terms and observations [3] and demanded expertise in art, craft and science, making the valence and materiality of early modern color a rich and active area of research for historians of art and science [4]. Merian herself chose to include color as a significant feature of insects' descriptions. In the texts accompanying her images she was careful to include lively textual descriptions of insects' color as identifying features [5]. In the images themselves she relied on a system of modelli, painted in watercolor on paper and vellum, in order to standardize the colors used to represent insects in colored editions of the prints [1]. *Metamorphosis* constitutes an important case study in the history of early modern colored prints.

The study of color in early modern European woodcuts and engravings poses challenges to print scholars, notably that it is often difficult to assess when and by whom coloring is applied [6]. It was common practice among early modern European print owners or book readers to interact with printed works [7] and to add their own annotations or coloring using ink, watercolor, or gouache [8]. It was also common for dealers and collectors especially in the nineteenth century to add color to prints either as enthusiasts or as forgers [9]. In the case of *Metamorphosis*, these challenges associated with authenticating colored prints more generally are compounded by the fact that Merian's original copper plates survived after her death. They were embellished and reprinted in several posthumous editions in the The Netherlands and France in 1726, 1730 and 1771, thus often making it challenging to assess the proximity of loose-leaf prints to Merian's workshop [10].

Analytical methodologies have been employed in the study of color in early modern works on paper; in particular, X-ray fluorescence (XRF) and multispectral photography have been shown to be effective methods of characterizing colorants used to color prints [9,11,12] and illuminated manuscripts [13]. Some have used art historical and technical analyses (XRF and Raman spectroscopy) to study the relationship between printing and color workshops and practices in the sixteenth and seventeenth centuries [6]. These studies concern themselves with works whose coloring can be verified to be early modern. Some have applied XRF to identify modern coloring on early modern prints [8] but there remains further work to be done in specifying and characterizing the practice and materials of modern coloring of earlier prints. Merian's work in particular has been the subject of some technical analyses, which used microscopic investigation to distinguish between prints and counterproofs but not in order to classify or propose dates for the pigments used [14].

Three painted prints from *Metamorphosis* (Call no. Folio 581.9 S357.m; Figure 1) reside in Northwestern University's Charles Deering McCormick Library of Special Collections. These prints are referenced in Merian's text as plate 9, plate 54 and plate 55 and in the following we refer to individual prints by these in-text plate numbers. The coloring on these prints is carefully done and thickly applied. With the exception of two peppers in plate 55, the color scheme of the Northwestern Special Collection prints follows that of counterproof and watercolor versions of the same images that are securely attributed to the Merian workshop, such as those in the British Museum [15–17]. The care of coloring, as well as the aforementioned factors surrounding the history of colored prints by Merian and others, made it initially unclear when, why, and by whom the Northwestern University Special Collections prints and their coloring were produced. Still, given the significance of *Metamoprhosis* in the histories of botany and entomology and with respect to the eighteenth century use of color in illustrated natural history, these prints represent an invaluable archival resource.

**Figure 1.** Prints from Maria Sibylla Merian's *Metamorphosis* in Northwestern University Chalres Deering McCormick Special Collections, call no: Folio 581.9 S357.m: plate 55 (**left**), plate 9 (**middle**), plate 54 (**right**).

In order to assess how, why, and by whom the prints were made, and in particular whether they were made by the same maker, the prints in Northwestern Special Collections underwent technical analysis with macro X-ray fluorescence (MA-XRF), hyperspectral imaging, transmitted light photography, and photometric stereo imaging. These data were analyzed in dialogue with archival art historical research to characterize and date three separate components of the prints: pigments, printed lines, and paper support. MA-XRF and, in two cases, hyperspectral data were used to investigate the prints' pigments, photometric stereo imaging was used to investigate how the printed marks were made, and transmitted light photography was used to visualize paper structure. Exhaustive characterization of pigments, including modern organic pigments, may require the use of molecular techniques such as Raman spectroscopy [18–21] or FTIR [22]. While the use of these two techniques would have been ideal in this study, no sampling was possible on these prints, and we did not have access to the necessary portable instrumentation for in situ analyses. Therefore, the investigation of the coloring materials could only be performed using MA-XRF and hyperspectral imaging, the combination of which may give good insights into inorganic colorants but limited insights into organic ones. The analysis of the prints presented here can better contextualize these specific prints, and shed light on whether and how the coloring on these prints may contribute to the study of the eighteenth century use of color in natural history. More broadly, this analysis provides valuable information for characterizing modern coloring of early modern prints and demonstrates the use of a multimodal workflow that combines XRF with other modes of analysis.
