6 RECENT ANALYSIS OF THE MEDIUM

The 20 Dijsselhof tubes and the sap green belonging to Van Beek were examined by J. J. Boon of the FOM Institute for Atomic and Molecular Physics in Amsterdam. They were analyzed with direct temperature-resolved mass-spectrometry with an JEOL SX-102A double-focusing mass-spectrometer (Boon 1992). With this method of analysis one obtains a picture of the oils, waxes, and resins in the paint in addition to information about polymer fractions such as the oil paint network polymer, proteins, polysaccharides, and metals like lead, cadmium, mercury, and iron (Boon et al 1995; Boon and Van Och 1996). The paint samples were dissolved and suspended in toluene:ethanol:ethyl acetate (40:5:5), whereupon a few micrograms of the paint suspension were placed on a platina/ rhodium filament probe and heated to a high temperature (800�C). The evaporation and pyrolysis products were analyzed using mass-spectrometry with low electron volt (eV) electron impact ionization (16 eV) at the mass range of 20–1000 atomic mass unit (M/Z).

Drying oil appears to be the main component in the 21 samples studied, albeit its precise nature is not clear because classification is normally based on saturated fatty acids in hardened paint while the samples had to be taken from not yet hardened paint (Mills and White 1994). In some cases irregularities were found that would indicate unusual oil components. The possibility that these were oxidation products of triglycerides is being studied further by Boon. In one semihardened sample taken from the outside edge of one tube, the determined palmitate/stearate (P/S) ratio (1:4) indicated linseed oil as the medium. In some tubes minimal amounts of beeswax and triterpenoid resin were indicated. Notable was the presence of zinc in a few cases, the purpose of which requires further research (zinc palmitate, zinc stearate, or zinc white as additives perhaps). Beeswax and zinc salts are both mentioned in the literature as preservatives for paint in tubes (Mayer 1991).