The influence of light and relative humidity on the formation of epsomite in cadmium yellow and French ultramarine modern oil paints

Heritage Science

Table 2 Composition of the W&N AOC tube paints used

Tube paint	SEM EDX	ATR-FTIR (see Figs. 2a, 7a)
Tube paint	SEM EDX	Absorption bands (cm⁻¹)	Assignment
Cadmium yellow	C, O, Cd, S, Al, Mg, Zn	3400 [–OH stretch]; 2924, 2854 [CH stretch], 1740 [ester carbonyl], 1379 [CH₃ umbrella mode] 1244 [C–O ester]; 1160 [C–O ester], 1100 [C–O ester], 722 [CH₂ rock]	Oil
		3649 [Mg(OH)₂; O–H vibration]; 1484, 1421; [CO₃²⁻/HCO₃⁻; v₃ asymmetric stretching vibration] 884 [v₂ carbonate], 853 [v₂ carbonate] 800 [CO₃²⁻ bending vibrations]	Hydromagnesite
		Broad band at 3400, 1624, 980	Possible alumina hydrate (hydrated oxide of aluminium)?
French ultramarine	C, O, K, Na, Mg, Al, Si, S	3340 [–OH stretch]; 2927, 2853 [CH stretch]; 1740 [ester carbonyl]; 1162 [C–O ester]; 1095 [C–O ester]	Oil
		3649, [Mg(OH)₂; O–H vibration]; 3515, 3448, 1483, 1420 [CO₃ ²⁻/HCO₃⁻ v₃ asymmetric stretching vibration]; 884 [v₂ carbonate]; 854 [v₂ carbonate] 800 [CO₃²⁻ bending vibrations]	Hydromagnesite
		~ 1006 (Si–O–Si asymmetric stretch); 692, 657 (Si–O vibrations)	Ultramarine
		3692 [Al≡O–H stretch], 3619 [OH stretch]; 1027 [Si–O–Si]; 1006 [Si–O–Si], 913 [Al–O–H], 692	Probable kaolinite (an aluminosilicate)
		Broad band at ~ 3400 (–OH stretch), 1624 broad band	Possible alumina hydrate (hydrated oxide of aluminium)?