Restoring
Kristiansten Fort
to its 1684 condition
Anne Berit Bjørken
Trondheim's Kristiansten Fort is being restored with mortar copied from the lime mortar used by the monks who built the cloister on Tautra in the Trondheim Fjord in the 13th century.
We are removing all the plaster and stone that has been added to Kristiansten Fort since 1684", says stonemason Ståle Storås.
"We are not just doing this for fun, but because we have realised that the high-cement mortars used to repair old buildings and historic monuments have done more damage than good," says Alf Waldum, project manager of the Norwegian part of the international Eurolime programme.
The aim of the programme is to develop lime-based mortars and methods suitable for restoring old buildings. Since the war, our old buildings have been repaired using high-cement mortars, which has caused direct damage to our ancient monuments. Elements of the programme consist of surveying types of plaster and their condition, testing their durability, studying their lifetime and developing and manufacturing new mortars. The three Norwegian Eurolime pilot projects are Kristiansten Fort, Jomfruland Lighthouse and an elderly tenement building in the centre of Oslo.
Blood and bones
In the search for the secrets of the good old-fashioned lime mortar, thorough chemical analyses at NTH have revealed a number of special "additives" in the mortars used all the way back to the 13th century.Traces of bull's blood and the remains of the bones of cats and pigs have been found in the lime mortar in the joints of the Tautra cloister, in the Nidaros Cathedral in Trondheim and in the ruins of Hamar Cathedral.
"Such finds are so common that there is every reason to believe that blood, bone and other types of organic material were deliberately added during lime production in order to improve the quality of the mortar. Additives of this sort can produce air pores which increase the workability of the mortar and its ability to withstand frost. We already know that horsehair was used as fibre," says Waldum of the Norwegian Building Research Institute.
Kristiansten Fortress
"We are not adding blood or bone to our mortars today, but we do add other materials that will serve the same purpose. Old types of mortar were often extremely rich in lime. A calcium content of as much as 30 - 40% of the weight of the mortar was quite normal. Such a high proportion of lime is not used at Kristiansten, but a more highly graded sand. Wet-slaked Norwegian lime is the only binder. Wet-slaked lime has been unknown on Norwegian building sites for the past 50 years," says Waldum.The restoration of Kristiansten Fort is a very thorough process. In principle, all stone and plaster added since 1684 are being removed. All earlier mortar repairs, including stones that have been put in place, are taken out. Two facades of the fortress tower were finished in 1993, and the other two were rehabilitated in summer 1994.
The craftsmen have been on a course, where they learned the working techniques that were used in the old days, and they also use contemporary tools. Nor is electrical equipment permitted when they are chipping out old plaster. All the work is done with hammer and chisel.
From lime to cement
One of the main problems caused by using high-cement mortars to repair old stone buildings is that they are too stiff or inelastic to follow the movements of the building. Modern mortars also dry so slowly that the humidity balance of the walls is changed for the worse.Instead of slowing the process of disintegration, these repairs caused an accelerated degradation of historical buildings. Until 1910 - 1920, lime was more or less exclusively used as a binder in masonry and plastering mortars. At about the time of the First World War a mixture of Portland cement and lime was introduced in mortars for external use. One of the main reasons for this was to reduce the curing time required before the mortar acquired the necessary hardness and durability. Until 1970, such mortars were the most common type for external masonry and plastering. During this period there was a trend in the direction of more and more high-cement mixtures. Lime was gradually regarded as an additive that merely enabled the mortar to be worked. Since 1970, masonry cement has dominated the market. In some parts of the mortar industry lime has completely gone out of use.
A study of rehabilitated tenement blocks has shown that in 22 of 26 projects, the facades had to be repaired only a few years after rehabilitation as a result of the use of the wrong materials and techniques. These tenements were restored in the 80s. Loose stone and plaster on a number of facades are a source of danger to passers-by. These problems are due to faulty plaster and paint products, work of inadequate standard, poor maintenance of gutters, downpipes, and leaky seals between facades and the ground.