Table 3 Key findings resulting from the analysis of W&N AOC cadmium yellow (CY) and French ultramarine (FU) tube paint samples

CY

75% RH, elevated light

Diamond-shaped and hexagonal-shaped crystals observed on the paint surface.

Samples aged at 75% RH/elevated light conditions presented a superficial surface layer positioned underneath/surrounding seeded crystals. Both seeded crystals and the superficial layer were rich in Mg, S and O

The superficial surface layer appeared similar to the layer observed after ageing at 75% RH/near-dark conditions, which lacked seeded crystals.

The emergence of SO₄²⁻ absorption bands at 1087 cm⁻¹ after ageing, is consistent with the formation of a sulphate salt.

Samples aged at 75% RH/elevated light contained a higher abundance of SO₄²⁻ species relative to the sample aged at 50% RH/ambient light.

The emergence of additional diffraction patterns after ageing supports the crystals to be MgSO₄·6H₂O

75% RH, near-dark

Colourless superficial dendritic layer visible across the paint sample surface, but no large surface crystals.

N/A

N/A

FU

75% RH, elevated light

Rod-shaped and hexagonal crystals observed on the paint samples after ageing at 75% RH under both elevated light and near-dark conditions, but crystals were more abundant for elevated light aged samples;

“cross-shaped” radial crystals only seen for samples on acrylic-primed canvas.

A superficial surface layer rich in Mg, S and O (indicating magnesium sulphate) underneath/surrounding seeded radial crystals rich in Na, S, and O (indicating sodium sulphate) was observed for samples on acrylic primed canvas, aged at 75% RH/elevated light.

The surface layer appeared similar to that observed in cadmium yellow paints after ageing at 75% RH.

The emergence of SO₄²⁻ absorption bands at 1100 cm⁻¹ after ageing at 75% RH indicates the formation of sulphate salts. This may arise from the sodium enriched seeded crystals theorised to be Na₂SO₄, or the Mg enriched surface layer likely to be MgSO₄ 

The emergence of SO₄²⁻ species is evidenced by changes in O 1s and S 2p environments after ageing at 75% RH/ elevated light relative to ageing at 50% RH/ ambient light

A stronger Na 1s signal also suggests an increase in Na species at the sample surface after ageing at 75% RH/ elevated light

See Section 3 in Additional file 1

75% RH, near-dark

Small hexagonal surface crystals were observed, however these were generally less abundant relative to the samples aged at 75% RH/elevated light.

N/A

N/A