**1. Introduction**

Yellow dyes were used in artworks for millennia up until the advances in modern chemistry. *Reseda luteola* L., or weld, was one of the most important dyes in Europe up until the 19th century, and the primary source for organic yellows [1]. These were used in the textile industry as a source of yellow and green colors and prepared as artists' pigments to create precious masterpieces [1–3].

Although weld was possibly identified in textiles from Xinjiang [4,5], in 17th-century Arraiolos carpets, Portugal [6] and in Southern Swedish painted wall hangings from the 18th–19th centuries [7], assessing its conservation condition and the causes of degradation in artworks is still in its early stages. To understand the degradation mechanisms that are in play in such complex matrices as found in our cultural heritage, it is necessary to have reference materials prepared with as much historical accuracy as possible. These are used to assess the natural evolution of these colors and simulate by accelerated ageing experiments with a limited number of variables the aging of these systems.

**Citation:** Veneno, M.; Nabais, P.; Otero, V.; Clemente, A.; Oliveira, M.C.; Melo, M.J. Yellow Lake Pigments from Weld in Art: Investigating the Winsor & Newton 19th Century Archive. *Heritage* **2021**, *4*, 422–436. https://doi.org/10.3390/ heritage4010026

Academic Editor: Diego Tamburini

Received: 1 February 2021 Accepted: 21 February 2021 Published: 25 February 2021

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Methanol:water extracts of *Reseda luteola*, which gave the highest flavonoid yield, have shown that the main chromophore is luteolin 7-*O*-glucoside (Lut-7-*O*-glu), followed by luteolin 3 ,7-*O*-glucoside (Lut-3',7-*O*-glu). Luteolin (Lut), apigenin 7-*O*-glucoside (Api-7- *O*-glu), chrysoeriol glycoside (Chry-gly), luteolin 4 -*O*-glucoside (Lut-4'-*O*-glu), are also found, with apigenin (Api), apigenin-6,8-di-*C*-glucoside (Api-6,8-*C*-glu), and a luteolin di-*O*-glucoside (Lut-di-*O*-glu) found in lower amounts [6,8–10]. When analyzing dyed textiles, weld is identified as the yellow used by the presence of "luteolin-type" flavonoids. In 17th-century Arraiolos carpets, the yellow historical samples analyzed with LC–MS contained primarily Lut-7-*O*-glu, small amounts of apigenin-6,8-di-*C*-glucoside, Lut-3,7-*O*glu and its isomer, as well as Api-7-*O*-glu and Lut, see Figure 1 [6]. Moreover, for the dyed textiles from Xinjiang, the identification of Lut-7-*O*-glu, along with other "luteolin-type" and "apigenin-type" flavonoids, led to the proposal of the use of weld in the samples analyzed [4,5].

**Figure 1.** Collection of *Reseda luteola* in its native environment; structures for the main chromophores found in *Reseda luteola* yellows: luteolin, apigenin, luteolin 7-*O*-glucoside, luteolin 3 ,7-di-*O*-glucoside, and apigenin 7-*O*-glucoside.

Generally, most dyes were applied as lake pigments, formed by the colorant's precipitation with a complexing agent, such as alum, hence becoming a non-soluble pigment, in a process analogous to the mordanting of textiles [2]. Although these dye-metal complexes' exact structure is still unknown for most lake pigments, there are some proposals for luteolin-metal complexes [11–13]. Following a DFT/TDDFT study of the complexation sites of luteolin and apigenin, Amat et al. proposed that luteolin is preferentially co-precipitated or absorbed with Al3+, or other metals in a bi-dentate mode involving the 4-keto-5-hydroxy site and with a Al:Luteolin 1:1 stoichiometry [11]. The same structure was proposed by Gao et al. for luteolin-Cr(III) complexes, while Dong et al. proposed the same coordination sites for complexation with manganese (II), although with an Mg:Luteolin 1:2 stoichiometry, which means the existence of a complexation network is expected involving the hydroxyl groups [12,13]. On the other hand, Smith et al. found that the aluminium ion-flavonoids complexes (present in dyed textiles and lake pigments) prevent the natural efficient and non-degradative dissipation of excitation energy by an intermolecular proton transfer involving the 5-OH and the 4=O groups, hence are more susceptible to degradation [14].

Within an interdisciplinary team of chemists, botanists, and heritage scientists, with 20 years of experience in studying and retrieving the "lost knowledge" on natural dyes found in historical documents and artworks [15–19], this work will be the first step of a systematic study on the technology used in the past to produce weld lake pigments. For this first approach, we investigated recipes found in the Winsor & Newton (W&N) 19th century Archive Database, a unique primary documentary source covering handwritten formulation instructions and workshop notes of a leading artists' colormen that supplied prominent painters. The W&N Archive Database comprises a summary index-linked to digitalized page-images of 85 manuscript books (corresponding to 15.003 database records) and a digital collection of 47 W&N 19th-century trade and retail catalogues [20–22].

In a time of chemical development, especially of artificial dyestuffs, it is very interesting to note that W&N was producing at an industrial scale and selling natural yellow lake pigments during the 19th century [23]. In previous studies, we have proven that W&N was committed to primarily selling the most high-quality and durable products [18,24,25]. More importantly, we have demonstrated that research on the W&N Database enables pigment reconstructions with as much historical accuracy as possible. These references will be fundamental to advance analytical methodologies on the identification of weld lake pigments in artworks. In this work, for the first time, we disclose the infrared bands of weld lake pigments, complemented by their chromatographic profiles.

#### **2. Materials and Methods**
