Zprávy památkové péče 2019, 79(3):341-346 | DOI: 10.56112/zpp.2019.3.12

Stratigraphic survey of facade colors

Petr Kuneš

NPÚ GnŘ

The stratigraphic survey of surface layers, i.e. an examination of a section perpendicular to the layer of paints using a microscope, is an important source of knowledge about the older rendering of historical faćades. As with other methods of study, the possibilities of stratigraphy are limited both technically and interpretatively. A meaningful use of the results of stratigraphic research in practice is therefore conditioned by a knowledge of the possibilities and limits of this method. A stratigraphic survey should be considered as an extension and complementation of findings obtained in situ by a facade survey. Such a survey determines the choice of sites as well as the quantity and method of sampling for stratigraphy, and it usually also decides on the further usability of the obtained laboratory results. The selection of suitable sampling sites is also crucial for their quality. The appropriate number of samples taken for stratigraphy should always be assessed individually, taking into account the complexity and layered facade; it is also largely dependent on the experience of the researcher and should therefore be left to his/her discretion. A sampling must always be based on specific and explicitly stated reasons for taking a sample from that particular site. The sampling itself is most often carried out by carefully cutting a small cube of the base plaster with a sharp chisel; a sample with an area of about 1 cm2 is ideal, and it is essential to capture the entire relevant composite of layers. Appropriate storage of the sample, marking the sampling site in the plan with field notes, and photographic documentation of the sampling site should be a matter of course. In addition to a description of the collection site, it is advisable to supplement the samples with simply phrased descriptions of the finding situation. The actual stratigraphic survey consists of observing a cut section perpendicular to the plane of the faćade surface with modifications using an optical microscope in reflected white light. Further information about individual layers can be obtained from observing fluorescence induced by UV radiation, or through the use of more demanding instrumentation. Observation of the perpendicular cross-section allows for a very clear observation of the chronology of the layer modifications, their thickness, homogeneity, condition, and surface alteration or contamination. On the other hand, the properties of the surface treatments which may be equally important for the general surface appearance, such as the translucence of the treatments, the optical behavior of the coatings in the surface, the presence of shading, glazing, drawings, etc., may be more difficult to read from the section, if at all. The tonality and luminosity of the treatment on the sample can also be significantly influenced by embedding the sample in a resin and the different effect of the layer coating when viewed in on the overall surface and on the cut. These changes are mainly due to the replacement of air in the pores of the coating with a liquid resin, which is a material with a significantly higher refractive index than air. For this reason, the stated optical shifts are significant in the case of porous (highly absorbent) coatings, typically on lime coatings, which are the most common modifications of historic facades. In addition to observing the individual coatings, the interface between the coatings can also be well observed on the sample. This interface may reflect the exposure of the building to weather, i.e. in particular degradation and contamination of the coating, and thus serve to distinguish the temporal succession of the treatments. A stratigraphic survey of surface treatments plays an irreplaceable role in the investigation of historical buildings; it brings a significant amount of unique knowledge about the development of facades, and it contributes to the objectivity of color schemes considered in their subsequent renovations. At the same time, however, this is always a partial documentation that must be evaluated individually in the context of each specific building, its construction development, and of the objectives of its renovation.

Keywords: facade survey, color survey, stratigraphy, color scheme of facades, microscopy, sample, surface treatments

Published: September 1, 2019  Show citation