2. DETERIORATION OF BUILDING STONE

The processes leading to the deterioration of building stone have been the subject of numerous publications [11, 14-16, 21-23]. Therefore, stone deterioration is only briefly reviewed herein for the purpose of providing a basis for understanding the performances of stone consolidants. The factors considered to be among the leading causes of building stone deterioration include salt crystallization, aqueous dissolution, frost damage, microbiological growth, human contact, and original construction. In this review, however, only a few cases were found in which the cause of stone deterioration was unequivocally determined. All too often a speculative approach was used in the analysis of stone deterioration, in place of a more scientific diagnostic method.

2.1 Salt Crystallization

Crystallization of salts within the pores of stones can generate sufficient stresses to cause the cracking of stone, often into powder fragments. This process is considered to be the major cause of stone deterioration in many parts of Western Europe [24-26]. Closely related to the crystallization of salt is damage caused by salt hydration and by differential thermal expansion of salts [14]. The resistance of stone to salt damage is dependent on the pore size distribution and decreases as the proportion of fine pores increases [11]. Crystallization damage caused by highly soluble salts, such as sodium chloride and sodium sulfate, is usually manifested by powdering and crumbling of the stone's surface [11]. Less soluble salts such as calcium sulfate form glassy, adherent films which cause spalling of a stone's surface [16].

A major source of salts in urban environments is the reaction between air pollutants and stone. For example, limestone can react with sulfur dioxide to ultimately produce calcium sulfate. Other sources of salts include ground water [27], airborne salts [27], sea spray [28], chemical cleaners [29], and deicing salts [30].

2.2 Aqueous Dissolution

Carbonate sedimentary stones e.g., limestone and dolostone), carbonate-cemented sandstone, and marbles are types of stone that are susceptible to dissolution by water acidified with dissolved carbon dioxide, sulfur dioxide, and nitrogen oxides [31]. It has been reported [32, 51] that the rainwaters in many urban areas in the United States and Europe are sufficiently acidic to accelerate the weathering of exposed building stone. In areas where the rainwater is relatively free from pollutants, the dissolution of most common building stones is usually not a serious problem [30].

2.3 Frost Damage

Certain stones which are exposed to freezing temperatures and wet conditions may undergo frost damage. The frost susceptibility of a stone is largely controlled by its porosity and pore size distribution [33,34]. Of stones with a given porosity, those with the smallest mean pore size will generally be the most susceptible to frost damage. Frost resistance also generally decreases with increased available porosity [15], i.e., pore volume which is accessible to water. The frost resistance of a stone is often assessed from its saturation coefficient,(2) with stones having saturation coefficients less than 0.8 being generally immune to frost damage [35].

Some European stone conservators [11,26] believe that in their countries frost damage is not an important process in the deterioration of stone. They regard frost damage as a secondary process, e.g., frost damage may be responsible for the final fragmentation of stone damaged by other processes, such as salt crystallization. However, because of the use of possibly more frost-susceptible stone and more severe climates, frost damage may be an important factor in the northern part of the United States [36,37].

2.4 Microbiological Growth

The attack of stone by a variety of plants and animals has been reported [14] including roots of plants, ivy vines, microorganisms, boring animals, and birds. Of these, microorganisms appear to be the most destructive. Some types of bacteria, fungi, algae, and lichens produce acids and other chemicals which can attack carbonate and silicate minerals [14, 38]. It appears that under certain environmental conditions attack by microorganisms can be a serious problem [39-41]. However, it seems that many conservators feel that such instances are uncommon and that microorganism growth usually takes place in stone which had been partially deteriorated by other processes.

2.5 Human Contact

Because of an increasing interest by the public in historic structures, the effects of human contact upon the condition of stone, as well as all other building materials, is of growing concern. For example, stone floors are gradually worn by foot traffic, stones are damaged by people either collecting souvenirs or poking into soft stone [28], and graffiti removal has become an important maintenance problem [42-48]. It is conceivable that human contact may become a major problem challenging the ingenuity of both stone conservators and maintenance specialists.

2.6 Original Construction

The durability of stone structures also depends on factors encountered during their original construction including proper design, good construction practices, and proper selection of materials. Unfortunately, these are factors over which the preservation scientist has no control. However, the same mistakes should not be repeated in repairing or restoring historic structures. For example, normal steel and cast iron anchors, dowels, reinforcing rods, etc., were often used in the construction or repair of stone structures. Certain ferrous metals are susceptible to corrosion which can lead to the cracking and spalling of stonework [11]. Therefore, noncorroding material should be selected, e.g., epoxy-coated steel [44], certain types of stainless steel [45], or non-corroding non-ferrous alloys [11].

A large portion of stone durability problems are the consequence of using poor quality stone in the original construction. Riederer has suggested [46, 47] that air pollution is often blamed for stone deterioration in Germany which actually should be attributed to the natural weathering of poor quality stone. The use of poor quality sandstone in completing the Cathedral of Cologne [48,49] is presenting acute conservation problems. In another example, a poor quality dolomite limestone was used in the construction of the British Houses of Parliament [50] which eventually had to be replaced with a more durable limestone [15]. It is doubtful that a stone consolidant can make a poor quality stone durable and, as with the above example, stone replacement can be at times the most rational approach [52].