The essential feature required of stone used for any construction is to be long-lived. Usually, this property depends upon the strength and resistance for weathering of a rock. According to data reported by A.M.Victorov and L.A.Victorova in the book “Natural Stone in Architecture” (M., 1983) longevity of granitic articles approximates 500 years, while the life of products made of sandstone approaches 100 years.
The following values of ageing of stone were given in “Architect’s Handbook” (vol.XIV, M.,1952):
These estimates are relative and may be variable under conditions of different cities. The great number of data on the state of numerous buildings and monuments in St.Petersburg have permitted V.G.Pudovkin to find considerably lower values for early evidence of facing stone disintegration. Thus, Serdobol Granite used in facing shows first marks of destruction in 150-170 years, evidence of disintegration of gneissoid red Valaam Granite appears in 70-80 years, while Ruskeal Marble begins to decay in 30 years and loses its lustre in 69 years. Factors decreasing longevity of stone applied in building and for sculptures are simple, but difficult to eliminate.
Mechanical forces, physical weathering, activity of microorganisms, chemical reactions of stone with the highly polluted air and chemically active waters of the city, and various foulings of surfaces can be listed here. Very often man’s activity brings the lest predicted, or prognosticated effect.
Mechanical forces terminate in opening of natural fractures and in formation of new cracks in stone. Original jointing was found, for instance, during finishing of blocks of Shoksha Quartzite used for the pedestal of the monument to Nickolas I. The fractures were very thin and did not penetrate deep into the rock. They were thus led to reject 20 defective blocks prepared for the monument. Nonuniform load is an example of mechanical forces. It was a cause for formation of cracks at the bottom of columns of the western portico of St.Isaac’s Cathedral. The figures of Atlases adoring the portico of the New Hermitage experienced (their legs) the analogical forces. Strains producing cracks in a stone around the places of fixation of iron and cast-iron fastenings of fences, grills and holders are a special case.
Percussive methods of surface finishing of facing slabs exert an adverse effect on the stone rigidity. Right there V.G.Pudovkin sees the reason for exfoliation of slabs of Serdobol Granite facing the basement of the Engineers’ Castle and used for bases of hermae embellishing the house N 43 in Great Naval Street.
Among mechanical affectings are damages originated from shell-and-bomb-splinters during the siege of Leningrad in 1941-1944. Such dints can be seen at columns of the western portico of St.Isaac’s Cathedral, at the plinth, platbands and outside window frames of the building of the General Headquarters. Marks of such disturbances are also preserved at columns of the house N 7 in Pochtamtskaya (Post office) Street. Analogical, at first glance, damages of the same mechanical type take place on quite different occasions. Thus, there are dints in stone of walls left after setting of signboards, lamps, lanterns and so on. Among other such things, deep lacerated holes speckle stone slabs facing the facade of the house N 1 in Nevsky Prospekt. Many round openings that are traces of the vanished fastenings are seen at granitic walls of the house N 4 located in Gorokhovaya (Pea) Street. Unfortunately, cases of barbarous treatment of stone are numerous.
Physical weathering proceeds under the action of climatic conditions. It leads to mechanical disruption – fracturing, formation of flakes, pulling off, crumbling of rocks, and to chemical transformations of their surfaces – loss in lustre and changes in colour. The nature, intensity and rate of weathering depend on predominant agents of the process, on their activity and aggressiveness and on limits of daily and seasonal changes. In St.Petersburg, as in other industrial cities and regions, chemically agressive urbanistic atmosphere and surface- and ground-waters add to influence of climatic factors of weathering.
As to climatic conditions, St.Petersburg lies in the area that is exposed to the influence both of the Baltic Sea and Arctic Region (Arctic Ocean). Abundance of the air masses brought to the city by winds from the seas causes relatively mild winter with frequent thaws and moderately warm, or cool summer. High cyclonic activity of the air masses and the widest swings in weathering are characteristic of any time of year. The mean monthly temperature of the coldest month (January or February) is about -8o, or -8.5oC, and for the warmest month (July) it approximates +17oC. St.Petersburg has a fairy moist climate. The relative humidity of the air is rather consistently high, as in summer, so in winter – about 80% for 146 days a year. On the average, the annual precipitation fluctuates from 500 to 650 mm and number of rainy days exceeds half a year. In winter, temperatures above the freezing point and below it alternate very often. Rains and snow, freezing and melting of ice, covering pavements, walls of buildings, sculptures and other urbanistic constructions standing in the open air, take turns correspondingly. It is apparent that such conditions are less favourable to preserve stone in the city environment as compared with known centers of architecture in Western Europe. St.Petersburg’s surroundings are similar to conditions of the contemporary Arctic megalopolis – Murmansk (Tables).
Systematic investigations of properties and resistance of stone derived from deposits of the North-Western Russia under conditions of the damp northern climate and agressive surroundings have been performed both at laboratories and in the field in Monchegorsk city (Murmansky region). A stand with polished plates of the facing stones was exposed to the open air within 2 km of the metallurgic copper-nickel integrated works. The standard plates were kept on deposit at room conditions. Marble from the two well-known deposits – Belogorsky (Tivdiysky) from Karelia and Koyelginsky from the Urals – was studied. Correlating those experimental results with data of the field study and theoretical calculations, V.V.Lashchuk (1996) gives characteristics of the stone longevity at the agressive weather and atmospheric conditions of the Murmansky region. Analogous figures for St.Petersburg’s surroundings would be larger. However, there are no such investigations for the city. In any case the longevity of stone in St.Petersburg and Murmansk is considerably less, than average indexes. It is connected with hard climatic conditions and atmospheric contaminations in these cities.
Clearly the resistance of stone to weathering and its longevity depend on mineralogical composition of rocks, on their structure, jointing and porosity. Pores may be of various origin. Some of them are very thin intergranular spaces of different shape, others are represented by thin fractures inside grains of minerals, the shape of small interstices can be rounded, tabular, etc. Pores are subdivided into capillary - with conventional, that is, averaged radii under 10 microns, and those with radii more than 10 microns.
Capillary properties (capillary condensation of moisture and its exhalation) are most conspicuous in stone with the conventional pore radii which are variable from 0,01 to 0,1 microns. Such kinds of stone in St.Petersburg are sandstones, limestones, tufa, marbles, namely, many-coloured sandstones from Poland and Germany, Putilovo Slab and other limestone slab, marbled limestones and dolomites from Estonia, marbles from Ruskeala and Tivdiya, Pudost Stones. The porosity of stone of these kinds, together with macroheterogeneity and, every so often, jointing, causes intensive frost weathering at climatic conditions of St.Petersburg due to frequent alteration of snowfalls and winter rains, freezing and thawing of water.
Frost weathering of stone used for outside decoration of Peterburg’s buildings can be schematically described as processes of two types, or categories. The first type represents capillary phenomena. Moisture fills pores. At the expense of rise of H2O volume during freezing, the pressure on the ore sides increses and stone is subjected to internal mechanical stresses. Simultaneously, the locked-up stresses between grains of different minerals arise from variations in their temperature coefficient of linear extention-contraction. On thawing of water in capillaries, H2O volume decreases and a new portion of water is absorbed into capillaries. During freezing a new cycle of the same processes begins and in the consequence the mechanical disruption of the stone grows time and again. The second category of the processes is connected with freezing and melting of moisture inside large cracks and other defects in stone. Unfortunately they are very numerous nowadays.
The processes of the physical weathering outlined above are synchronous, therefore the harm of them is very considerable every so often. To cite examples rather important for St.Petersburg, some following facts should be mentioned. The best known example of this type of destruction is vertical twisting fractures at the Alexander Column. In 1963, by the time of the first restoration, there were about 100 cracks at the Column. They ranged in length from several centimetres to first decemetres. However, five fractures were more extensive, one of them being 4.5 m in length. They proved to be rather thin, attaining a thickness of 6 mm, and shallow. As is evident from the literature and old engineer descriptions, those cracks appeared already in four years after the Column had been put up. The The first information on them goes back to 1836. The cracks might be present in the stone even in the period when it was in situ. Later they could widen by physical weathering at nonuniform temperature drops. The cracks were considered by M.F.Bronnitsky to be the result of stress decrease during the cutting of the monolith out of depth. He called those fractures “a payment for very large size of the Column”.
For the first time, those cracks had been skilfully done up with mastic before the Column was set up, a granitic inset 26 x 17 cm2in size had been installed in one place. By 1860 the most part of the mastic had been fallen out, though it was preserved intact on the north-eastern side of the Column. In 1912 a scaffold arose around the Column. Small fractures were closed up with Swedish three-crowned portland-ce-ment. The cracks of width up to 6 mm were filled with granitic bars with gutters and notches. The bars called “cocks” (“roosters”) were fixed with portland-cement that should be wet, so that it would harden well. The place of the inset was covered with cardboard. Sawdust constantly moistened with water was placed between the stone and cardboard. Prominent parts of the bars were ground off with use of wooden circles edged with rems of fusion of lead, tin and antimony and by circles made up of thick copper sheets. The circles were rotated by electro-motors with belt-drives. Then, the repaired place was ground with carborundum and emery and after it was polished with powder of tin oxide by hand. At last, all cement junctions were thrice perfected with special aqueous solution (Kessler’s liquid). All the surface of the Column was rubbed up with linseed-oil for three times. In 1963 a scaffold was built around the Alexander Column again and the new restoration was carried out. At this time cracks were puttied with epoxidic pitch (tar) with addition of granitic crumb. The surface of the Column was machined by electro-drill: at the beginning, the rough dry peeling took place, after it, the Column was thrice processed with abrasives different in grain sizes. Later on, the Column was polished with special powders and glossed and glazed by felt circles with pouring of carborundum. At the end, the Column was washed and dried with rags.
Processes of the physical weathering have left marked changes at the walls of St.Isaac’s Cathedral. Many slabs of Ruskeal Marble characterized by inequigranular structure were disrupted rapidly and in 30 years already they had to be replaced by slabs of Florentine Marble Bardiglio. Sculptures carved from limestone weather still easier. The layering comes into particular prominence here, therefore many sculptures must be puttied and covered with paints.
Putilovo Slab, Pudost Stone and other limestones are extremely pliable to physical weathering. Being arranged vertically in outside constructions and walls of buildings, they start to peel, split to plates, crumble along bedding. However, the same stone serves for a long time without any defects, if it is laid so that the bedding and tabularity prove to be horizontal, and such is indeed the case when it comes to basements, steps and overhead covers. Thus, the secret of the stone is very simple, and we need only think of it more often today. But basements of newly erected buildings of St.Petersburg are rather frequently faced with limestone slabs that have vertical orientation. Therefore they exfoliate fast and begin to crumble.
Yet another stone undergoes extremely intensive physical weathering in St.Petersburg. This stone is travertine, or Pudost Tuff of which the fronton and columns of the Kazansky Cathedral are made. The stone is porous, brittle. It is easily broken up by frost weathering. Abundant coverns in the travertine of the facades of the Kazansky Cathedral had to be closed up with cement during the restoration of 1963. As a result, the facing of the cathedral had lost its initial appearance and had become particoloured, spotty.
Biological processes of stone weathering in St.Petersburg have been left beyond attention of researchers and restorers up till now. Processes of this type manifest themselves in an explicit form as growing green of granite of outside staircases in the yard of the Engineers’ Castle. A relatively higher dampness in the yard is favourable to the development of colonies of microorganisms on the granite.
Urbanistic atmosphere has harmful chemical effect upon any rock containing carbonates, e.g. marbles, limestones, sandstones with carbonaceous cement. Of gases existing in the air of an industrial city, carbon dioxide and sulphurous gase are the most ruinous of stone. They dissolve in the air moisture and rain-water to form carbonic acid and sulphureous gases which are destructive of stone both on its surface and inside, as they seep into the pores, capillaries and fine cracks. Carbon dioxide dissolves carbonates causing pores and cracks to increase. Sulphuric acid, interacting with calcite, changes it into gypsum. As this takes place, magnesium carbonates are turned to epsomite. Other hydrous sulphates may arise here too. The harm of such new formations is of two kinds.
In the first place, they are well dissolved in water, migrate through pores of stone and change them due to a redeposition. As a result, the porosity decreases in superficial layers of stone and increases inside. A packed, thickened crust develops on the surface.
In the second place, hydrous sulphates have larger molecular volume when compared to volume of calcium and magnesium carbonates: the volume of formulated unit of gypsum is 1.78 times greater than this parameter of calcite. Therefore crystals of sulphates formed in pores and cracks of a carbonaceous rock exert pressure on surrounding grains. The pressure (being repeated over and over again) is sufficient to diminish the strength of the rock and even to cause its complete disintegration. places The stone distends in places. The superficial layer begins to exfoliate, flake and falls off in the end. A marly, unconsolidated lower layer reveals itself and starts to crumble in turn.
In such a manner marble and limestone used in facing of buildings and for sculptures fall into decay rather quickly. Disappeared first and foremost are fine, delicate details and carving, or fretworks
Oxidation of small crystals of iron sulphide, that is pyrite, frequently entering into the composition of carbonaceous rocks has an analogical destructive effect. Pyrite transforms initially in iron sulphate and then it changes to limonite, or brown iron ore. Owing to this a volume of the above-mentioned inclusions increases and the stone cracks. Furthermore, ugly, dirty brown spots and damp stains emerge on its surface.
Stone used in St.Petersburg undergoes considerable surface contamination as well. Dust and soot from the air accumulate on the surfaces and, being adsorbed by pores, cover the stone with black greasy film. Ferrugination, i.e. apperance of rusty spots around droinpipes, benevth windows, pitches and cornices, or ledges, may be added to erdinary urbanistic pollution of stone. Sometimes bluish-green stains arise by oxidation of copper and bronze articles. A special investigation of the stains seen on the metal of the monument to Nickolas I and responsable for a growing green of Carrarian Marble that makes up the pedestal have been carried out by I.V. and A.V.Mikheyevs and S.G.Tuchinsky. They have determined antlerite Cu3(SO4) (OH)4, brochantite Cu4(SO4)(OH)6, atacamite Cu2Cl (OH)3 in those efflorescences on the bronze.
St.Petersburg lies on the same latitude as Stockholm, Turku and Helsinki – the cities of the common sea basin. Climatic conditions in Helsinki and St.Petersburg are almost identical. Correlating the state of stone in architectural decor of these Baltic cities, one is forced to accept the fact that St.Petersburg is in the saddest situation on this point. The degree of preservation of stone in St.Petersburg is adversely affected, among other things, by the social factors of 1914-1980-s. In the middle 1990s active works on restoration of the historical centre of the city began.
To the beginning