CA-OXALATE FILMS AND MICROBIOLOGICAL INVESTIGATIONS OF THE INFLUENCE OF ANCIENT PIGMENTS ON THE GROWTH OF LICHENS - THERMOGRAVIMETRIC THERMOMICROSCOPIC ANALYSES/

Simultaneous thermogravimetry and thermomicroscopy were performed on C a-oxalates which play an important role in the appearance of oxalate f ilms connected with lichens superficially growing on human artefacts. Ca-oxalate exists in two well-described modifications: as the more sta ble monoclinic monohydrate whewellite and the less stable tetragonal d ihydrate weddelite. Weddelite serves for lichens as a water absorbing and accumulating substrate which transforms to whewellite when humidit y drops. To follow these morphological changes optically and the water loss gravimetrically at the same time, a combination of thermomicrosc opy and thermogravimetric analysis was constructed. The reflection lig ht microscope was connected to a video camera and computer so that the morphological changes and the thermogravimetric curves could be shown simultaneously on the screen as function of time and temperature. Lic hens - double organisms composed of algae and fungi - form surface lay ers not only on natural organic and inorganic material but also on hum an artefacts. In calcareous artefacts such as the famous Chinese terra cotta soldiers or Egyptian epigraphs they lead to a destruction of the surface by forming Ca-oxalate layers and thus to a deterioration of t he historian work of art. But in places where the surface is covered b y some blue colours (Egyptian and Chinese Blue, Chinese Purple) the gr owth of lichens is inhibited and the artefracts are well preserved. Th e copper ion contained in the pigments is responsible for this effect since copper is a strong poison for microorganism. As lichens exhibit an extremely slow growth under natural and laboratory conditions the t wo Lichen components: algae and fungi were investigated separately. Th e three mentioned ancient pigments have very low solubility products a nd thus do not act on the quickly growing fungi. But under special exp erimental conditions Egyptian Blue formed clear halos on the growth pl ates of the algae Chlorella minutissima and Trebouxia glomerata. These halos were compared with those of the easily soluble copper sulphate as a standard.