Time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging using cluster primary ion beams is used for the identification of a green painting layer on the scene The Angels Concert from the Issenheim Altarpiece (painted in 1516) from a German Renaissance painter, Matthias Grünewald. Copper carboxylate clusters inside a basic copper chloride (atacamite) layer have been identified and located in the copper green layer. The mechanisms of transformation of atacamite into copper carboxylates may be initiated by an aging of the paint layers. The combination of the high mass resolution of the technique together with a micrometer spatial resolution and the possibility to simultaneously identify both minerals and organics, has proven to be the method of choice for the study of the stratigraphy of a paint cross-section.