Examples of weathering and deterioration of Tertiary building stones at St. Michaels Cathedral in Cluj-Napoca (Romania)

St. Michaels Cathedral is one of the oldest Gothic architectural monuments in Cluj. It is built predominantly of Cenozoic (Eocene) limestones which were deposited on a shallow carbonate platform. They are composed of different facies and microfacies types with varying amounts of particles, matrix and cement. Limestones from the Baci quarry (Cluj Limestone), which is situated about 3 km from the centre of Cluj, were commonly used. The weathering features correspond to the microfacies types, to the position of the stones in the walls and to the exposure (east, south, west, and north), which controls the moisture/dry cycles. This interrelationship is documented by macroscopic and microscopic examples of the decay of samples analyzed from the lower part of the walls. General parameters for a correct classification of carbonate rocks and their importance for weathering are discussed. The general mechanisms of weathering (thermal expansion, water uptake, freezing, chemical weathering) are described. The migration of moisture (capillarity) and the formation of crusts of varying mineralogy on the surface of building stones are documented. Macroscopic description of damage includes the decay into plates and flakes, the formation of crusts, the formation of fractures, and the growth of lichens and microorganisms. The microscopic analysis documents characteristic damage in detail: fractures parallel to the surface of the stone, internal cementation of fossil chambers (micro-nodules), repeated formation of crusts in varying generations, fracturing in intensively lithified, rigid limestones, and settling of lichens in different positions. Furthermore, different historical mortars and modern “concrete-mortars” and their behaviour in comparison to the adjacent limestones are briefly discussed. The possible conservation and restoration of weathered limestones in St. Michaels Cathedral are discussed with regard to the most recent conservation methods. The advantages and disadvantages of surface conservation by silica ester, hydrophobic substances as well as by epoxy resin are also dealt with. Special methods for conservation (acrylic total impregnation, lime slurry, biological lime slurry) and DIN-methods as well as analysis of the success of the methods used are presented in a short summary