Table 2 Binder of the intermediate layers used in the reconstructions

Animal glue solution

10 g rabbit-skin glue in plates + 90 ml distilled water

Animal glue plates immersed in water overnight. Heated in a water bath to a maximum of 40–50 ºC

KREMER cold-pressed linseed oil

Cold-pressed linseed oil from Sweden (Kremer, Ref. 73020)

HART untreated linseed oil^a

Manually cold-pressed linseed oil (Sofie Linseeds, Netherlands)

Oil pressed on the 05. 06. 2019. No treatment to eliminate sediment or mucilage. Left to settle, decanted oil used (See Additional file 1)

HART drying linseed oil^a

250 ml manually cold-pressed linseed oil (Sofie Linseeds, Netherlands) + 12.5 g minium + 12.5 g litharge

Oil prepared according to Recipe P1P327 from the Winsor & Newton Archive Database (17. 06. 2019)

Dry powders: c. 10% of the solution

The dry powders were mixed together and added to the oil

The mixture was heated to 93.3 ºC (200 ºF) and stirred continuously for 5h 35 min

Left to cool, decanted oil used (See Additional file 1)

HART untreated linseed oil  +  HART drying linseed oil (1/1)

Equal parts of each oil were manually mixed in a glass jar

KREMER cold-pressed linseed oil  +  turpentine

The brush was first dipped in turpentine and only then slightly mixed in a glazed ceramic bowl containing the oil binder (~c. 30–50% turpentine)

For the imprimatura paints, the brush was dipped in turpentine and directly mixed with the paint on the glass slab

The paint became quite liquid (~c. 30–50% turpentine)

The percentage of turpentine was assessed by eye

HART untreated linseed oil  +  HART drying linseed oil (1/1)  +  turpentine

The brush was first dipped in turpentine and only then slightly mixed in a glazed ceramic bowl containing the oil binder (~c. 30–50% turpentine)

For the imprimatura paints, the brush was dipped in turpentine and directly mixed with the paint on the glass slab

The paint became quite liquid (~c. 30–50% turpentine)

The percentage of turpentine was assessed by eye