**3. Results and Discussion**

As a pre-interpretation step of the Raman spectra acquired from the Kerdijk paint-outs, production recipes collected at the Talens' archives were studied and evaluated and a list of expected SOPs was made [4]. Where possible, the commercial names used in the recipes were related to the Colour Index definitions, e.g., the pigment Hansa Geel 10 [Hansa Yellow 10] corresponds to PY3. Then, the acquired Raman spectra were compared with the reference spectra of the expected SOPs.

Table 2 gives a detailed overview of the analysed paint-outs and their interpretation. The expected SOPs according to the recipes are reported. The settings for acquisition refer to the averaged settings yielding the best result. It was found that the handheld Raman produces readable and interpretable spectra with defined peaks at almost constant settings of 150–200 ms exposure time and 100–200 acquisitions.

#### *3.1. Identified SOPs*

The Talens Rembrandt oil paints of the Kerdijk colour charts were revealed to contain the SOPs of eight different pigment groups. Furthermore, it was also possible to identify paints that contained a mixture of SOPs (Table 2).

Some of these SOPs belong to groups that are already well known to have been used in fine artists' materials and can be expected in this time period. This concerns SOPs belonging to the BONA, monoazo, and phthalocyanine group as well as the ß-Naphthol pigments PR3 and PO5, which have indeed been found in artworks from before 1950 [30]. In other cases, the Kraplak [madder], Karmijn [carmine], and Alizarine [alizarine] paintouts provided well defined spectra, but the SOPs have not yet been identified and are labeled as "unknowns".

Among the identified SOPs, there are also some pigments that were not yet known to have been used for the production of fine artists' paints dated before 1950. The occurrence of the mordant dyes and triarylcarbonium pigments are among the most surprising results of this study and emphasizes the importance of searching unexpected possible pigment groups in pre-1950 artists' materials. Three examples are discussed in more detail.

The paint-out Gamboge from 1932 was revealed to contain Mordant Yellow 1. This SOP was discovered by Nietzki in 1887 and is also known as Alizaringelb GG [Alizarine Yellow GG] or Beizengelb TG [Mordant Yellow TG] [45]. Wagner (1928) reports that this pigment was not very popular because of the elaborate preparation needed to obtain a pure product. It is therefore surprising that, according to the recipe records, Talens has used this SOP for fine artists' material production for almost 20 years [34,46]. Figure 2 displays the handheld Raman spectrum of the paint-out Gamboge and the microRaman spectrum of Mordant Yellow 1 from the in-house reference collection. The handheld Raman spectrum displays the peaks that allow an interpretation, but also contains artifacts.

Within the paint-out Talens Groen Licht [Talens Green Light] the pigment PG9, a SOP belonging to the group of Indamine, was detected. This pigment, a barium salt of an iron complex, was discovered in 1909 and labelled as Hansagrün GS [45]. It was recommended by the Colour Index to be used "particularly in mixture with CI PY 1 or 3" [45–47]. Schultz (1931) describes the pigment as a "very lively green when mixed with Hansa Yellow for all purposes (almost as fiery as Schweinfurter Green)" [48]. This suggests its use in artists' colours in mixture with monoazo yellows as a substitute of the toxic inorganic pigment and fits therefore to Talens' application, documented in the recipe of Talens Green Light.

Figure 3 shows the handheld Raman spectrum of the paint-out Talens Green Light and, for comparison, the microRaman spectrum of a Hansagrün GS reference (paint-out from Wagner, 1928). The peaks of the PY3 component in the reference spectrum are very faint, which can be related to different PY3 and PG9 ratios within the samples.


*Heritage* **2021**

 oil paint-outs dated 1932–1950.

**Table 2.**

Summary of the results obtained with the handheld Raman device on Kerdijk colour charts with Talens' Rembrandt

, *4*


*Heritage* **2021** , *4*

**Table 2.** *Cont.*


*Heritage* **2021**, *4*

**Table 2.** *Cont.*


**Table 2.** *Cont.*

are not yet interpreted.

**Figure 2.** Handheld Raman spectrum of the paint-out Gamboge (K32) and the microRaman spectrum of Mordant Yellow 1, labelled as Alizarine Yellow GG (reference collection of the Cultural Heritage Agency of the Netherlands, Amsterdam, The Netherlands).

**Figure 3.** Handheld Raman spectrum of paint-out Talens Green Light (K32) and the microRaman spectrum of reference Hansagreen GS (Wagner, 1927), mixed with barium sulphate (987 cm<sup>−</sup>1), both showing the characteristic peaks of PG9 in mixture with PY3. The peaks that clearly correspond to the KIK-IRPA reference for PY3 are highlighted with a star.

Two paints of the Talens in-house brand, labelled as "Talens paints", showed triarylcarbonium pigments. In the paint-out Talens Groen Donker [Talens Green Dark] of the 1941–1950 editions, the green pigment PG2 was identified. PG2, brought to the market under the product name Fanalgelbgrün [Fanal Yellow Green], is defined as either a mixture of Basic Green 1 and Basic Yellow 1, precipitated with phosphotungstomolybdic acid (PTMA), or Basic Green 1 with Auramin O, precipitated with PTMA [31,45,48].

The other triarylcarbonium pigment that was identified is PB8. It was detected in the paint-out Talens Green Dark (1932), contradicting the recipe (Table 2), and in the Talens Groen Blauw [Talens Green Blue] in the editions from 1932 and 1937. PB8, labelled in the Talens recipes as Fanalbremerblau G neu [Fanal Bremer Blue G new], is a mixture of Basic Blue 5 and Basic Green 1, precipitated with PTMA [17]. Figure 4 displays the handheld Raman spectrum of Talens Green Blue and the reference microRaman spectrum of a reference sample, labelled as a mixture of Basic Blue 5 and Basic Green 1 precipitated with PTMA, provided from the Historic dye collection of the technical university in Dresden.

**Figure 4.** Handheld Raman Spectrum of paint-out Talens Green Blue (K32) and the microRaman spectrum of a reference sample containing Basic Blue 5 and Basic Green 1, precipitated with PTMA (Historic Dye Collection, Dresden, Germany).

Both PB8 and PG2 belong to the Fanal brand, which was introduced by I.G. Farben in 1924 [31]. Lomax et al. [1] point out that "the major use of these pigments [triarylcarbonium] is in the printing ink industry" and it seems surprising that Talens chose these SOPs for their fine artists' oil paint brand. However, Steger et al. detected two pigments (PV2 and PG1), belonging to this group, in a reverse glass painting from 1945 [49].
