Reduce cement crusts on marble

The marble epitaph of Konrad Popp from the late 16th century, which was attached to the southern outer wall of the parish church of St. Leonhard in Lavanttal (Carinthia), was examined and restored at the Institute for Conservation and Restoration at the University of Applied Arts Vienna due to its endangered condition.
At the time of dismantling from the wall structure, some areas of the object exhibited severe sugar-grained decay, which in some cases was accompanied by pronounced biogenic growth. Various forms of crust were found in the central area, which were identified and reduced in further steps. One crust, which was most likely formed by cement dust, and its reduction will be the focus of the investigation.

The memorial consists of eight white marble blocks (H:271 cm x W:125 cm x D: 40 cm) and shows the founder Konrad Popp and his family in the central field in an adorative pose. The relief rests on an inscription cartouche that reveals demographic information about the donor and is surmounted by a triangular pediment showing the coat of arms of the Popp family. The formal language is characterized by the volutes, expressive faces and the disproportionately and exaggeratedly depicted bodies, which appear mannerist and point to the end of the Renaissance. Like the design of the front side, the back of the marble blocks is highly interesting due to its shape: the blocks are not cut straight on the back and show almost no traces of carving.
To determine the stone more precisely, samples were taken and thin sections made which were examined under a light microscope and a scanning electron microscope (SEM-EDX). The medium-grained marble (crystals up to 2 mm) shows a high dolomite content (about 65 percent), as well as a high proportion of silicate minerals (about five percent) for marble, including the accessory phlogopite (Figure 2). This can be seen macroscopically as golden-yellow platelets in the marble and, in combination with the high dolomite content, is characteristic of marble deposits in the nearby Koralpe.

The installation situation of the epitaph was analyzed in order to determine the damage mechanisms. The object was sunk into a buttress on the southern outer wall, which is why it was aligned to the east (Figure 3). The installation situation can be defined as the main cause of the damage due to a structural-physical problem in the church building and the orientation. The recent façade plaster was identified as cement plaster and can be traced back to the façade renovation of the 1970s by consulting existing archival documents. The plaster lies on a natural stone masonry, which was covered with a lime plaster. This poses a problem, as cement plasters have a low water vapor diffusion capacity and do not dissipate the soil moisture absorbed by the natural stone masonry to the outside, which leads to damage on the inside of the church wall. The problem is exacerbated by the non-functioning rainwater drainage in the area of the buttresses, which is why some of the water was drained away via the façade and, at the same time, via the epitaph. The orientation of the epitaph to the east can also be described as problematic in the context of the thermal dilatation of marble. If the marble is warmed by the morning sun within a short period of time, a large temperature difference can be reached or a freeze-thaw cycle can occur more frequently. In combination with the high and permanent availability of water and the protected areas in the relief of the object, the orientation towards the morning sun optimally fulfils the growth conditions for biogenic colonization.

The marble shows a loosened structure in the exposed areas as a result of sugary-grained decay, which has been able to progress far, particularly due to a combined effect with the biogenic growth. The biogenic growth comprises a wide range of organisms, including fungi, lichens and bacteria. In addition to the weakened areas, there are also areas compacted by crusts, which can be divided into lime sinter, gypsum and cement crusts. The cement crust is a rare phenomenon and could only be determined by sampling and analyzing the cross-sections. Macroscopically, it appeared as a hard and brittle ochre-colored crust with a “pustular” surface morphology (Figure 4). Under the scanning electron microscope, a multi-layered structure of the crust was revealed: a hydraulic lime crust was present on the marble, which was firmly bonded to the rock. The remains of a hydraulic binder were found above this, which originated from a Portland cement and were covered by a gypsum crust (Figure 5). Due to the shape of the crust, it could be ruled out that this could have reached the surface as mortar splashes when the façade plaster was applied. The stratigraphy of the crust and the affected areas indicate that the formation can be attributed to cement dust, although the church is located in a rural area. It can therefore be assumed that the cement reached the surface of the building with the wind during the renovation of the façade in the 1970s and was able to form a crust in combination with moisture.

Cleaning and exposing the surface of microbiogenic growth and the various crusts was the focus of the concept of measures in addition to securing the existing structure by strengthening and reinforcing it. The treatment of the cement crust posed a particular challenge that required an unusual approach. Initial attempts to reduce this mechanically using fine tools or lasers did not produce satisfactory results, which is why the microparticle blasting method was used. Here, too, it was found that the hardness and irregularity of the crust meant that there was a high risk of the underlying marble also being processed. Microparticle blasting is a linear process in which a uniform amount of blasting material hits the treated area at a constant pressure. On a harder surface, the process is automatically slower and less effective. Due to the irregular surface morphology, thinner areas of the crust had already been removed within a short time and the marble had already been treated, while in other areas the crust was still almost in its original layer thickness. In order to apply the process as gently as possible, it was adapted as follows. A leveling layer was created to level the crust and imitate its hardness. A test series was created in advance to test the suitability of different materials as a leveling layer. Cyclododecane, gypsum and Roman cement were included in the test series. The materials were applied evenly with a brush so that the highest points of the crust were visible on the surface and the rest was covered under the leveling layer. Cyclododecane was not convincing due to its insufficient hardness and surface adhesion. Gypsum and Roman cement proved to be good leveling layers: Both materials were easy to apply and hardened quickly. In this case, Roman cement was convincing due to its higher hardness and also due to its color, as the areas to be treated stood out more clearly from the stone surface.

In addition to the material of the leveling layer, two different types of blasting material and various combinations of pressure and quantity of blasting material were tested. Another test area was used to determine the effect of the blasting angle on the degree of invasion of the method, which was applied to a polished marble slab. A flat angle of incidence of the blasting material could enable gentler work than a steep one, which is why a flat angle was chosen as the angle of incidence.
After the adaptations had been determined by test series, the crust on the relief was covered with the leveling layer and the Roman cement was covered with damp cloths for 24 hours to harden. The blasting process could begin the next day. The process is time-consuming due to the removal of the additional material, but the results and ease of application are impressive.

The condition of the marble epitaph was characterized by sugar-grained decay, biogenic growth and various types of crust. In particular, the removal of an unusual cement crust was a challenge that was met by adapting the microparticle blasting process. The process, which has been adapted using an equalizing layer, is a gentle way of reducing hard and, in particular, uneven deposits, while at the same time allowing the blasting progress to be easily observed. The technique is easy to use, makes it easier to control the blasting progress and makes the process safer. Furthermore, it should be considered that other materials could also be suitable in addition to the use of Roman cement. Through further application and analysis or evaluation, there is the potential to perfect the technique and adapt it for other situations in order to find a broader range of applications and further dissemination.

The investigation and accompanying restoration was carried out as part of Sarah Moyschewitz’s diploma thesis “The marble epitaph of Konrad Popp from the parish church of St. Leonhard in Lavanttal. On the problems of a marble with sugar-grained decay and biogenic growth” at the Institute for Conservation and Restoration (headed by Prof. Dr. Gabriela Krist), University of Applied Arts Vienna in cooperation with the Diocese of Gurk and the Austrian Federal Monuments Office, Carinthian State Conservation Office.

It is assumed that the marble blocks could have been foundlings or blocks that had been left behind in a quarry for some time.

The investigations were carried out under the guidance of sen. Lect. Dr. Farkas Pintér at the Institute for Conservation and Restoration (headed by Prof. Dr. Gabriela Krist), University of Applied Arts Vienna.

⁴Calcite powder and micro glass beads (75-125 micrometers).

⁵The following settings were selected: Micro glass beads 75-125 micrometers, 2.5-3 bar and the emergence angle was kept as flat as possible.

Read more: The Basel Historical Museum embarked on a “mammoth task” and carried out a general inventory of all objects.