About 40 years ago, cement was considered a durable solution for historic buildings. Today, however, the Protestant town church in Schönau in the Odenwald shows that this assessment is not always accurate. Joints containing cement led to stress, moisture damage, and damage to the natural stone. During the current restoration, the conservators are therefore once again relying on historic lime mortar and traditional craftsmanship to preserve the historic church for the long term.

The Schönau City Church is one of the most significant medieval structures in Baden-Württemberg. It is the largest surviving building of the Cistercian monastery founded by Bishop Burkhard II of Worms in 1142. Until the year 1400, the monastery served as the family monastery and preferred burial site of the Counts Palatine of the Rhine. Consequently, the requirements for the masonry restoration were exceptionally high, as the goal was to preserve the historic structure while simultaneously ensuring its long-term protection.
About four decades after the last major renovation, significant damage had developed in the natural stone masonry. This was primarily caused by cement-based joint mortar, which was considered durable and resistant in the 1980s. Today, however, it is known that cement is often unsuitable for historic natural stone masonry. Its high strength and low permeability prevent the natural exchange of moisture between the STEIN and the joint. This creates stress, causes moisture to accumulate in the masonry, and increases frost damage. The result was cracks, spalling, sand loss, and extensive moisture damage affecting approximately 97 percent of the facade.
The goal of the current masonry restoration was therefore to completely remove the damaged cement joints and replace them with historic trass lime mortar. At the same time, the natural stone facade was to be cleaned and damaged areas professionally repaired. The client was the Steinachtal parish, while the Schönau Foundation provided funding for the project. The work was commissioned to Dengel-Bau, a Schöntal-based company specializing in historic preservation. The church’s roof had already been renovated by a specialized contractor the previous year.

Historical Materials Instead of Modern Standard Solutions

Before the actual work could begin, the restoration experts first analyzed historical mortar samples from various areas of the church. They examined binders, aggregates, and grain distributions in order to replicate the original recipes as accurately as possible. Particular attention was paid to reddish-colored joints, which presumably date back to a renovation in 1882. This later phase of construction was intentionally left visible and was therefore also taken into account in the material development. Based on the analyses, four different lime mortar formulations were developed. Regionally available sands, trass lime as a binder, and brick dust were used to produce the characteristic reddish coloration of individual joint areas. Before large-scale work began, extensive testing was conducted directly on the building. Only after the color effect, surface texture, and integration into the existing structure were deemed satisfactory were the final mixtures approved.
A particular challenge was that the church is constructed from three different types of masonry. The ashlar masonry has extremely narrow rub-joint joints and required particularly fine-grained lime mortar. Medium-grained mortars were used in the layered masonry, while the rubble masonry required significantly coarser aggregates to fill larger joints with minimal stress. This material-specific coordination is an essential feature of professional masonry restoration and contributes significantly to the longevity of historic buildings.
The actual work on the facade was carried out systematically from top to bottom. First, the experts carefully cleaned the natural stone surfaces. They then removed all damaged cement joints and repointed the masonry with the specially formulated trass lime mortars. In addition, larger gaps were filled with natural stone blocks, damaged edges were repaired with restoration mortar, and cracks were stabilized using injections. The restorers carefully backfilled hollow areas in the masonry to permanently secure the structure.

Precise craftsmanship down to the smallest detail

Numerous natural stone details also required the highest level of craftsmanship and precision. On the historic rosette window, the stonemasons restored damaged areas and carefully replaced missing pieces. Particularly challenging was the reconstruction of weathered corbel heads, where foliage and volutes were added based on historical models. The goal was always to preserve as much of the original material as possible while faithfully maintaining the historical design. Replacing the damaged gable stone on the roof presented an extraordinary challenge. Water infiltration and corroded metal fasteners had caused the natural stone near the weather vane to spall. In the workshop, the stonemasons faithfully recreated a new gable stone from Odenwald red sandstone. Using a chain hoist, the heavy stone was then lifted onto the roof and installed—following nearly the same principles as in the 12th century. The restoration also focused on the historic metal fasteners. Many of the centuries-old iron clamps were cleaned and preserved. However, severely damaged examples had to be replaced. Since the grade of iron originally used is no longer available today, the experts opted for custom-made stainless steel clamps, which a blacksmith crafted individually based on historical models. This ensured that the structure’s design and appearance were preserved.
A special finale to the work was the traditional lead casting of the clamp joints. This historic technique is now mastered by only a few specialists. After carefully cleaning the joints, the craftsmen modeled lateral boundaries out of clay and heated smelted lead directly on the scaffolding. The molten metal then had to be precisely poured into the prepared joints within a matter of seconds before it solidified again. After cooling, the lead was trimmed flush with the stone surface. This historic technique contributes to the permanent sealing and stabilization of the structure.

Heritage-Conscious Masonry Restoration for Future Generations

The work on the Schönau City Church demonstrates how important professional masonry restoration is for the long-term preservation of historic buildings. Instead of repairing individual areas of damage, the damaged sections of the facade were comprehensively restored. This involved the use of historic materials, carefully formulated mortar mixtures, and traditional craftsmanship techniques. The restoration combines modern damage analysis with historical knowledge and artisanal expertise. It demonstrates the contribution that material-appropriate masonry restoration can make to the preservation of historic buildings. At the same time, the project makes it clear that supposedly durable building materials such as cement are not always the right choice for historic structures. With the consistent return to lime mortars, masonry restoration is once again guided by the materials and construction methods that have proven themselves over centuries and are better suited to the requirements of historic natural stone facades.

Read more: The Restoration of the Buch Castle Church.