The portals on the brick building: Construction of St. Mary’s Church in Rostock began in the 13th century and the sacred brick Gothic building was completed in the middle of the 15th century. Two portals are made of limestone. They were restored between 2009 and 2011. In contrast to the other portals and window pinnacles, the south and north portals of the large transept of St. Mary’s Church in Rostock are made of Gotland limestone. The north portal is made of light beige crinoid limestone with gypsum backfill mortar, while the south portal is made of gray-blue peloidal limestone with backfill mortar based on swamp lime. This makes them a special feature compared to the otherwise continuous brick construction and emphasizes the building’s characteristic transept.
The portals, which are cut deep into the structure, are divided by pilaster strips and archivolts. The north portal also features ornamental floral decoration. Additions and adaptations used in connection with the assembly are clearly visible. Bricks embedded in plaster mortar complete the portal. As the pointed arch portals were installed as a second use, it can be assumed that, due to their quality, they were removed from the previous building and later reassembled in the transept. This is reflected in the north portal by the different stone materials of the inner wall and the outer archivolts. The strong joints at the south portal. The limestone portals superficially showed a cementitious, obscuring reworking of the heavily damaged natural stone surfaces. This layer was characterized by cracks, was hollow in places and showed flaking and spalling. The physical properties of the extremely hard reworking layer and the building-damaging chlorides and nitrates embedded in it led to the decision to remove this layer.
Damage caused by salt and acid formation due to the permanent build-up of moisture was noticeable underneath. This led to structural destruction and fissuring. The damage is caused by the absorption of water from the soil and air. The water transports various chemical compounds and these are deposited on the surface or in the resulting cavities. Existing salts originate from deposits such as dirt (soot, grease, bird droppings, microorganisms, etc.) and building materials containing cement. The former “protective sludge” also causes subsequent damage to the original substance from top to bottom. The north portal mainly showed a strongly softened, sanding surface, while the south portal showed flaking, shell-shaped surface lifting combined with cavities. The restorers removed the cement slurry from the limestone in order to reduce the potential for damage to the natural stone surfaces and thus preserve the underlying limestone in the long term. They treated this slurry dry using the low-pressure particle blasting method. They manually thinned out any remaining residue. Depending on the size and layer thickness, they used compressed air and compressed air micro-chisels or the low-pressure blasting method to restore open-pored surfaces. Grout containing cement mortar, putty and rusting iron parts were removed. Due to the different types of stone on both portals, suitable supplementary materials were developed in the laboratory and later tested in test areas on site.
Read more about the restorers’ approach to the restoration of the limestone portals of St. Mary’s Church in Rostock in STEIN in February 2014.
Pictures: Hans Leisen, Marcus Mannewitz and Boris Frohberg