The Virgin and Child with a Bunch of Grapes

Light microscopy, SEM / EDX, HPLC

Cross-section examination:

Sample No.: PO5/1

Sample location: 16.6 cm from bottom/0.2 cm from the right

Brief description of paint: red

Description:

1. White ground: SEM/EDX: O, Ca (chalk)

2. Red layer(s): comment: in the backscattering image three discrete and very thin layers (in UV all dark) can be distinguished in the cross-section that is only 5.5 µm thick. The lower one could be an isolating layer consisting of organic material; SEM/EDX(bottom): O, Ca, Fe, Na, Si, Al, Cl, K, S, Mg, Pb, Cu, [P, Mn]; SEM/EDX (top): O, Fe, Si, Al, Mn, P, Ca, S, [K]; HPLC: the sample was placed in a 20 µl BF³/MeOH [5%] solution and left overnight. An octadecyl column (Type C18: Hyperclone 5 u ODS [C18] 100 x 2.00 mm 5 micron, Phenonemex) was used for the separation method. The phase was a mixture of (a) distilled water with 10% MeOH, (B) MeOH and (C) 2% solution of formic acid in water(60:30:10, isocratic for 2 min). The ratio was changed within 23 mins to 0:90:10, this was maintained for c. 10 mins and within 2 min returned to 60:30:20. The flow rate was 0.175 ml/min. Retention time [Rt=20.0 min] and the UV-Vis-Spectrum of the peaks [Absmax=248, 280, 430) identified alizarin, and purpurin [Rt=24.0 min, Absmax=257, 295, 480, 520 nm) in very small quantities. (red ochre [containing Mn], madder lake [alizarin, Al-, large quantity of P, small amount of S], chalk

Comment:

The lake particle appears dark in UV. The percentage of phosphorous is very high; the Al:P ratio was always c. 1.5-2 [in At%). It could therefore be kermes or cochenille. HPLC analysis could not confirm this. The chromatogram did exhibit a peak at Rt =14.63 min – a retention time characteristic for carminic acid – however the concentration is too small to be able to identify it on the corresponding UV/Vis-spectrum. No glass or quartz particles were found.

- earliest felling date: 1520; earliest date of production: 1522

[Martin Schawe, Alte Pinakothek Munich, 26.03.2010]