- Research article
- Open Access
Winsor & Newton original handbooks: a surface-enhanced Raman scattering (SERS) and Raman spectral database of dyes from modern watercolor pigments
© Pozzi et al.; licensee Chemistry Central Ltd. 2013
Received: 23 May 2013
Accepted: 25 July 2013
Published: 29 July 2013
Winsor & Newton Ltd. has been one of the main fine art products providers since its establishment in 1832, being responsible for the manufacture of a wide assortment of materials ranging from oils and pigments to brushes and papers. All the items produced over the years have been indexed in a comprehensive historical archive. Original Winsor & Newton handbooks are a powerful resource which can offer insight into the world of artists’ materials, and knowledge of artists’ choices through the identification of substances employed to obtain particular colors. Scientific analyses of various kinds have been carried out on Winsor & Newton art materials over the years; however, a detailed study of the organic dyes contained in the watercolors manufactured by the company has never been performed thus far to our knowledge.
In the present study, we examined a number of color washes on drawing paper from two historical Winsor & Newton catalogues dating to the 19th and 20th century. An appropriate database was thus built, including surface-enhanced Raman scattering (SERS) and Raman spectra of organic colorants from a wide variety of shades. While the selection of colors offered by the company in the 19th century mostly included lakes prepared from plant and insect dyes, i.e. madder and cochineal, some tints based on synthetic dyes were also found in the 20th century handbook.
The present article sheds new light on the chemical composition of a number of original Winsor & Newton color washes in terms of organic colorants contained in each shade. A special attention was dedicated to the analysis of those colors for which the formulation was ambiguous or not specified by the manufacturers, such as dragons’ blood and most of the alizarin-based pigments. In addition, we were able to correct erroneous indications provided by Winsor & Newton on the composition of some tints, as in the case of violet carmine, and study how the formulation of certain pigments has been modified over the centuries.
Founded in 1832, Winsor & Newton Ltd. has always been one of the main art materials suppliers in the world, manufacturing a wide assortment of fine art products such as oils, alkyds, watercolors, acrylics, pastels, brushes, canvases and papers. Since the company was established, an extensive archive has been created, including bound records of processes and shopfloor accounts, as well as handwritten books of recipes and notes for making artists’ pigments, oil colors, watercolors and a great variety of other art materials. The 19th century Winsor & Newton archive, containing 87 manuscripts for an overall amount of 17,000 pages, is considered to be the most comprehensive historical collection of this kind dating to the 1800s .
Winsor & Newton catalogues, with their collections of swatches showing the results obtainable with the firm’s colors, are an important historical and scientific resource, which affords scholars a precious insight into the world of artists’ materials. Information provided by chemical examination of colors from historical handbooks can be of utmost importance for interpreting analytical data from actual paintings in technical studies or authentication efforts. Understanding whether an artist decided to overlap a given set of colors to obtain a particular shade or if such mixture had been already created by the paint supplier is essential to expand our comprehension of the artist’s choices and to shed new light on the techniques employed. Furthermore, chemical analysis applied to the study of original art products may also contribute to set up suitable conservation and restoration approaches, as paint defects as well as the deterioration degree of pigments in works of art can be deeply characterized and properly treated by examining actual recipes for a certain color.
Among several analytical techniques currently employed in the study of art materials, surface-enhanced Raman spectroscopy (SERS) has proven to be one of the most suitable for characterizing organic dyes , which are typically very difficult to identify by normal Raman spectroscopy because of their high fluorescence emission. In SERS, dyes are adsorbed onto nanosized metal substrates such as silver colloids, and this provides greatly enhanced Raman signals and fluorescence quencing, thus allowing for the ultrasensitive detection of trace amounts of analytes .
A few studies recently published in the literature describe the analysis of Winsor & Newton acrylic, alkyd paints and watercolor pigment cakes by gas chromatography - mass spectrometry (GC-MS), attenuated total reflection - Fourier transform infrared spectroscopy (ATR-FTIR), matrix-assisted laser desorption/ionization - mass spectrometry (MALDI-MS), electrospray ionization - mass spectrometry (ESI-MS), X-ray diffraction (XRD), and normal Raman spectroscopy [4–7]. In recent times, a SERS characterization of four color washes from a Winsor & Newton catalogue has been also performed in order to provide reference spectra for the identification of specific colorants in a watercolor by the American print-maker and painter Homer . However, to the best of our knowledge, a comprehensive Raman and SERS study of dyes from historical Winsor & Newton handbooks has to date never been carried out.
Silver nitrate, sodium citrate, sulfuric acid, glucose and hydrofluoric acid were purchased from Fisher Scientific; madder lake (lake of alizarin/purpurin), carmine naccarat (alumina lake of carminic acid) and indigo from Kremer Pigments; alizarin, carminic acid, ethanol and potassium nitrate were obtained from Sigma-Aldrich. All the aqueous solutions used for the silver nanoparticle synthesis were prepared using 18 MΩ ultrapure water (Millipore Simplicity 185 water purification system).
SERS methods: HF hydrolysis, Ag colloid synthesis and sample preparation
Since most lake pigments are manufactured by precipitating a dye with a metal salt, SERS analysis of Winsor & Newton colors was carried out following hydrolysis with HF. This step is designed to hydrolyze the dye-metal complexes and maximize dye adsorption onto the colloid surface, in order to obtain the greatest signal enhancement. For this purpose, microscopic samples of Winsor & Newton lakes on paper, typically smaller than 0.3 x 0.3 mm in size, were taken using a tungsten needle. HF hydrolysis was then performed by exposing the samples to HF vapor in a closed microchamber for 5 minutes, according to an optimized procedure previously reported in the literature  and explored in detail elsewhere .
Silver nanoparticles prepared by microwave-supported glucose reduction of silver sulfate in the presence of sodium citrate as a capping agent were used as a SERS substrate. The synthesis was carried out according to a previously published procedure .
Reference solutions of alizarin and carminic acid were prepared in ethanol at a concentration of 10-4 M, and aliquots of 6 M HNO3 were employed to adjust the pH. For SERS analyses, 0.2 μL of the dye solution were added to 0.8 μL of the Ag colloid, followed by the addition of 0.1 μL of a 0.5 M KNO3 aqueous solution to induce aggregation of the nanoparticles. On the other hand, reference madder and carmine lakes as well as all the pigments from the two Winsor & Newton catalogues under investigation were analyzed upon deposition of 0.8 μL of the Ag colloid and 0.1 μL of a 0.5 M KNO3 aqueous solution after HF hydrolysis.
Normal Raman experiments were performed using a Bruker Senterra Raman spectrometer equipped with an Olympus 100x long working distance microscope objective and a charge-coupled device (CCD) detector. A Spectra Physics Cyan solid state laser and a continuous wave diode laser, emitting light at 488 nm and 785 nm respectively, were used as the excitation sources, and two holographic gratings provided a spectral resolution of 3–5 cm-1 (1800 rulings/mm for the 488 nm laser, 1200 rulings/mm for the 785 nm laser). Output laser powers of 0.25 or 2.5 mW for 488 nm excitation and 10 or 25 mW for 785 nm excitation were employed for the analysis, according to the Raman response of the different pigments.
SERS spectra were recorded with the same Bruker Senterra Raman instrument, using a 20x long working distance microscope objective and excitation at 488 nm, with a single integration of 30 s. SERS analyses were executed by focusing onto the microaggregates which were formed inside the dye-colloid droplet a few seconds after the deposition of the Ag nanoparticles and KNO3.
Results and discussion
Detailed results obtained from the SERS and Raman analyses of pink, red, violet, brown and gray shades from the two Winsor & Newton handbooks under investigation are presented in this section. Spectra collected from a number of colors (solid lines) were compared with those of reference pure dyes and lakes (dashed lines) for identification purposes, and the pigments examined could be thus divided into different classes according to the outcomes of spectroscopic analysis. It is worth pointing out that the chemical composition is declared by Winsor & Newton only for some of the color washes included in the catalogues; the trade names given to each shade by the manufacturers are typically indicative of the color rather than the chemical constituents.
Spectra of lakes belonging to the same class were found to be often characterized by slight shifts in wavenumber and changes in relative intensities, which can be attributed to complexation of the dyes with different metal ions in the lake manufacture .
Particularly interesting is the case of the dragons’ blood color wash, only present in the 20th century edition studied. The name “dragons’ blood” has been used since antiquity with reference to a red natural resin native of East Indies which was traditionally extracted from plants belonging to the Dracaena and Daemonorops genera. However, as also pointed out by Burgio , such dye was replaced in the 19th century by a more lightfast one due to its lack of permanence upon exposure to light. Accordingly, Winsor & Newton dragons’ blood is described as an imitative colorant which is a semi-permanent substitute of the original homonymous pigment. However, more detailed information concerning its chemical composition was not provided. Spectroscopic analyses allowed us to number Winsor & Newton’s dragons’ blood among cochineal-based colors, as both its SERS and Raman spectra are in agreement with the results obtained for the other shades of this category (Figures 2 and 3). In particular, the Raman spectrum of this shade shows a remarkable resemblance with that acquired by Burgio from a dragons’ blood pigment cake belonging to a 19th century Winsor & Newton watercolor box , suggesting an analogous composition for these two materials.
Madder lake-based pigments
As clearly suggested by SERS, madder lake was used for warm sepia as well. This dark brown pigment is described by Winsor & Newton as a mixture of a secretion of the cuttlefish, Sepia officinalis, with browns of a red hue, the composition of which was not specified by the manufacturers, but could be here ascertained by means of spectroscopic analyses (Figure 6).
In addition, as anticipated in a previous paragraph, scarlet lake, neutral tint, Payne’s grey and violet carmine were found to have a different chemical composition in the two Winsor & Newton catalogues studied. Indeed, while in the 19th century edition these color washes were mainly based on cochineal, in the 20th century edition they were found to be made of alizarin (Figures 7 and 8).
Various synthetic pigments
A few violet and red colors based on synthetic dyes different from alizarin were also found in the 20th century Winsor & Newton handbook examined in this article.
In the present study, we examined pink, red, violet, brown and gray lakes on drawing paper from two historical Winsor & Newton handbooks of watercolor pigments belonging to the 19th and 20th century collections, aiming to identify the organic colorants contained in each of them. An appropriate database was built, including both normal Raman and SERS spectra for a wide number of shades. The so obtained spectral patterns were examined, compared to reference spectra of pure colorants and lake pigments, divided into different groups, and thus discussed.
Spectroscopic analysis of Winsor & Newton color washes allowed us to deepen our knowledge about the variety of pigments in use during the modern age, and the results obtained provide valuable reference data for dating, as well as in authentication and identification studies. While lakes prepared from plant and insect dyes, mostly madder and cochineal, are predominant among the colors offered by Winsor & Newton in the 19th century, some tints based on synthetic dyes such as alizarin were also found in the 20th century edition, often accompanied by commentary on their poor lightfastness. The present Raman and SERS study allowed us to formulate some hypotheses concerning the main constituents of those shades for which a description is not available in the handbooks, such as dragons’ blood and most of the alizarin-based pigments. In addition, we could correct erroneous indications provided by Winsor & Newton on some tints, as in the case of violet carmine, and study how the formulation of certain pigments has been modified over the time.
This project was supported by NSF SCIART Award CHE-1041832. We also acknowledge Award No. 2006-DN-BX-K034, National Institute of Justice, Office of Justice Programs, US Department of Justice. The opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect those of the Department of Justice. A special thanks to Dr. Jacob Simon (National Portrait Gallery, London, UK) for useful discussion about dating of Winsor & Newton historical handbooks.
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