*2.2. Synthesis Methods for Weld Lake Pigments*

Research on the W&N 19th century Archive Database was carried out under the sub-topic weld, which resulted in 12 database records. Among these, it was possible to identify seven records for the production of weld lake pigments, however, there are only five recipes as 3 of the records are copies, see Table 1. The remaining five database records include two notes on weld, two experiments to extract weld, and one recipe to prepare Yellow Carmine from weld, Persian berries and quercitron bark. The transcription of the production records used in this work may be consulted in Table S1, and the synthesis methods reproduced are described in Table 1. Interpretation of the materials used was based on our previous works [18,24,25], which allowed us to infer that the term 'whiting' used in the recipe *Yellow from Weld* corresponds to calcium carbonate (CaCO3), 'Sub. Carb. Pot', used in the same recipe, is potassium bicarbonate (KHCO3) and 'Pearlash', used in the other recipes, is potassium carbonate (K2CO3). It is important to note that the recipe *Yellow Lake. Cool tint.* refers to the use of alum (ammonia sort), however, it was chosen to use common alum (KAl(SO4)2·12H2O) in all recipes to facilitate a first comparison between them. For this reason, we also decided always to use weld flowers. The introduction of experimental variants such as using ammonium alum (NH4Al(SO4)2·12H2O) will be investigated in the future. All materials were scaled-down from industrial to laboratory scale, and quantities in British measures were converted to SI units [22,23].

pH measurements were acquired throughout the syntheses. After 1 day left to precipitate, the lakes were centrifuged for 10 min at 2400 rpm, washed with distilled water, and centrifuged again for 5 min at 3000 rpm. The lakes were air-dried and ground in an agate mortar for 15 min each.


**Table 1.** Production name, recipe and pigment code of the synthesis methods for weld lake pigments, adapted from the original text transcribed in Table S1.

§ The unique recipe code is the code from the W&N Database that identifies a database record. ¥ Although this record is a copy, its title is "Experiment with Weld for Yellow Lake for Water Colours" and is dated 6 October 1854.

#### *2.3. Paint References*

Paint references were prepared using gum arabic as a 20% solution; the pieces were ground and then added to pure water. The lake pigments were first ground in a glass mortar with pure water and then ground with the binder. The paints were applied on filter paper with a paintbrush and allowed to dry. Filter paper was selected because no additives are present such as brighteners; this was confirmed by checking the filter paper under an UV-lamp (280 nm). The paint references were analyzed by colorimetry.

#### *2.4. Equipment and Characterization Methods*
