*3.1. Weld Lake Pigment Recipes in the Winsor & Newton 19th Century Archive Database*

From a total of 1511 database records for yellow pigments, 42% pertains to yellow lakes. Although the majority of these records pertain to quercitron-based products, 12 of these mention weld, as referred above. Interestingly, the five recipes to prepared weld lake pigments were discovered in manuscript books belonging to the founder Henry Charles Newton and his son Arthur Henry Newton. The recipes were found under the names: "Yellow from weld", "Yellow Lake. Cool tint." and "Weld Yellow", as described in Tables 1 and 2. It is important to note that the pigments prepared from the first three recipes (*WL1*, *WL2* and *WL3*) were originally produced with 12.7 kgs of weld plants, whereas those from the last recipes ((*WL4* and *WL5*) were produced with 0.9 kg, which suggests the latter were experiments. Nonetheless, all recipes were reproduced, and the pHs of the extraction solution and after precipitation of the weld lake pigments obtained are also presented in Table 2.

As Table 1 shows, W&N was extracting weld in a neutral media for most recipes, excluding *WL3* recipe, which involved a basic media. This was accomplished by the addition of carbonate compounds ((KHCO3 and K2CO3) that "assist the extraction of the colouring matter" as stated in the recipe *Yellow from Weld (WL1* and *WL2*). The latter recipe also includes the preparation of what W&N called the "body" of the pigment formulation, which involves mixing calcium carbonate (CaCO3) and alum (KAl(SO4)2·12H2O. *WL1* and *WL2* differ in the order of addition of these ingredients. According to W&N, the pigment resulting from *WL1* was "rather pale because the body was not thoroughly homogeneous" and the improved process *WL2* resulted in a color "deeper & looked brighter". Experimentally, we observe a yellow with a stronger red component, see Table 3. Curiously, they also refer "the quantity of yellow was less" for the *WL2*, however, we did not obtain this result as the yields are very similar, as presented in Table 2. In fact, those that experimentally presented the best yield of all five recipes were *WL1* and *WL2*, while *WL4* had the worst yield. Regarding recipe *WL3*, the yield experimentally obtained was very similar to W&N.

According to them, this recipe produced "a lively kind of yellow lake for sale &c. Greenish in hue, bright in the drop & full coloured" and when "tried in oil it produces a very beautiful yellow lake\_very cool\_bright & strong". Although the resulting pigment presents one of the lowest red component values (a\* ≈ 2), it did not show a greenish hue. This may be related to incomplete precipitation of all coloring matter as in the original recipe (see Table S1) is claimed that the quantity of alum used by W&N "was found to precipitate the colour entirely, leaving only a very faint tinge of yellow in the supernatant". This was not observed in our experiment and will also be addressed in future work.


**Table 2.** Production name, ingredients, synthesis methods, final pHs and yields for W&N's weld lake pigments.

<sup>1</sup> The pH extraction is related to the pH after extracting the plant material and subsequent filtration. <sup>2</sup> The observed yield (η) was calculated as follows: (*final lake amount*) ÷ (*weld amount*) × 100. ηW&N–observed yield calculated by the quantities given by W&N, see Table S1; ηEXP–observed yield calculated by the quantities obtained experimentally.

**Table 3.** Colorimetry, HPLC chromatograms and infrared spectra of the weld lake pigments synthesized applied over filter paper, with gum arabic media. HPLC chromatograms were obtained from the extracts of the lake pigments. For more details, please see text.

z

z

z

**Table 3.** *Cont.*

The complexing agent always used was Al3+ in the form of alum (KAl(SO4)2·12H2O) in the majority of the recipes and hydrated alumina (Al(OH)3) in *WL5* recipe; however,

complexation with Ca2+ cannot be excluded as this has been observed for W&N 19th century cochineal lake pigments [18]. The addition of borax (Na2B4O7) was also experimented with alum in *WL4* recipe. The pH after precipitation was slightly acidic, between 3 and 6, but always resulted in bright yellow lake pigments, as shown in Table 3. The reasoning for the production methods found is discussed below.
