Quality assessment of replacement stones for the Cologne Cathedral: mineralogical and petrophysical requirements

Abstract

Owing to its long building history, different types of building stones comprised the construction of the Cologne Cathedral. Severe damage is observed on the different stones, e.g., sandstones, carbonate, and volcanic rocks, especially when the different stone materials neighbor the medieval ''Drachenfels trachyte'' from the ''Siebengebirge''. The question arises, ''Is the insufficient compatibility of the implemented building materials causatively related to the strong decay of the Drachenfels trachyte?'' The present investigations focus on the petrography and mineralogical composition of eight different stones from the Cologne Cathedral. Petrophysical data, i.e., phase content, moisture and thermal characteristics as well as strength properties are determined and discussed in correlation to each other, showing that not only in terms of lithology great differences exist, but also the petrophysical properties strongly diverge. The ascertained parameters are discussed in view of the deterioration behavior and decay mechanisms of the different stones. To evaluate the compatibility of original, replacement and modern building materials, the properties of the investigated stones are compared to those of Drachenfels trachyte by means of constraints given in the literature. Besides optical properties, petrophysical criteria are also defined as well as strength values. It could be shown that primarily moisture properties, i.e., capillary and sorptive water uptake, water saturation, drying processes and moisture dilatation can be addressed to the deterioration processes.

The effect of air pollution on stone decay: the decay of the Drachenfels trachyte in industrial, urban, and rural environments—a case study of the Cologne, Altenberg and Xanten cathedrals

“…In particular, the Drachenfels trachyte shows severe deterioration in the form of cracks, surface deterioration, and backweathering coexisting with flaking, exfoliation, and structural disintegration to crumbling and the massive formation of gypsum crusts ( Fig. 4a) (Graue et al 2011). The flaking can occur in a very pronounced fashion (von PlehweLeisen et al 2007), which eventually leads to structural disintegration and complete material loss (Fig.…”

“…The trachyte has a medium porosity of 11.9-13.4 % and is of light gray color (Graue et al 2011). Two different varieties of Drachenfels trachyte can be distinguished macroscopically: a partially pale grayish one and a yellowish reddish porphyritic trachyte (Simper 1990).…”

“…14 %) leading to characteristic disintegration in the form of scaling, flaking, and granular disintegration into sand (Lukas 1990). The main deterioration phenomenon of Stenzelberg latite is a typical formation of scales with a thickness of 2-3 mm (Graue et al 2011). Obernkirchner sandstone has a high weather resistance, but shows black crusts and the formation of gypsum crusts in posterior and sheltered areas.…”

“…The building stones show severe deterioration phenomena, especially the Drachenfels trachyte (Graue et al 2011). Thin laminar and black framboidal crusts, which incorporate particles from the pollution fluxes, cover the building stones.…”

“…On the Krensheimer Muschelkalk, the crusts seem to temporarily stabilize the stone surface (Siegesmund et al 2007). On surfaces exposed to rain, solution phenomena can be observed, e.g., microkarst (Graue et al 2011).…”