Istrian stone is one of the most durable materials in Venetian architecture due to its nonporous physical and chemical makeup. Because of this durability, it is well-worth the time and costs of conservation work, as the cleaning and consolidation will enhance the stone’s durability.
Istrian Stone Use in Venice
Istrian stone is by far the most prevalent stone in Venetian architecture, as it accounts for more than 90 percent of the stone used in Venice. Its white color made it a popular stone to contrast with colored marbles, as can be seen on the facade of the Palazzo Ducale, shown above. The stone was quarried outside of Venetian territory from the city Trieste, located on the west shores of Istria.1
The famed architect Jacopo Sansovino (1486-1570) wrote about the qualities of the stone:
It is white in colour, and similar to marble, but firm and strong in character such that it endures for a very long time against the ice and the sun; from it can be made statues, which, when polished with felt like marble…have the appearance of marble.2
Chemical Makeup of Istrian Stone
Sansovino was correct in describing Istrian stone as having qualities of marble while not technically being marble. It is in fact a type of dense limestone. Limestone and marble are both different forms of calcium carbonate (CaCO3).3 Each block of Istrian stone is composed of millions of CaCO3 molecules, and each CaCO3 molecule has one calcium atom, one carbon atom, and three oxygen atoms. Limestone is a porous version of CaCO3, meaning the limestone mass is filled with holes throughout and has less molecules per cubic area than marble, which is a dense version of CaCO3 with more molecules per cubic area. Istrian stone, though technically a limestone, is dense like marble, as noted by Sansovino, and therefore wears like a marble due to decay processes involving CaCO3 and water.
Decay of Istrian Stone
While limestone wears at a faster rate than marble, both undergo the same chemical decay processes. Limestone and marble can slowly wear away in the presence of water. This is due to the fact that high levels of carbon dioxide (CO2) are present in the air due to animals and humans exhaling, organic matter such as leaves and plants decomposing, and carbon and oil burning from coal plants and vehicles. As rain droplets fall, they absorb this carbon dioxide from the atmosphere, therefore becoming slightly acidic. The rain, though slightly acidic with a pH of 5.6, is more acidic than pure water with a pH of 7.5, but less acidic than acid rain with a pH below 4.4 When the slightly acidic rain comes into contact with CaCO3, a slow reaction begins which is notated as the chemical reaction:
CaCO3 + H20 + CO2 → Ca2+ + 2HCO3–,
where the CaCO3, H20, CO2 combine to break apart the CaCO3 molecule into two pieces: calcium ions, Ca2+, and bicarbonate ions, HCO3–, which are washed away by water, as illustrated in Figure below.5
While Istrian stone is more durable than regular limestone, it is slightly less durable than true marble. This is because Istrian stone is so dense and has so few pores, the water is not able to penetrate into the stone to the degree it can penetrate into limestone. Overall, Istrian stone is a very durable material that can withstand Venetian flooding and rain quite well.
Unfortunately, Venetian industrial pollution has both aesthetically and physically devastating effects on marble. The introduction of fossil fuels in the 18th century has lead to high levels of atmospheric sulfur dioxide (SO2) and nitrous oxide (NO2). SO2 reacts with CaCO3 to produce a gypsum crust (CaSO4·2H20) as follows:
CaCO3 + SO2 + ½ H20 → CaSO4·2H20.
The CaCO3 will be converted into a gypsum crust, producing a black exterior, as illustrated below.
This crust will then peel off, leaving a new layer of exposed CaCO3, or the Istrian stone, limestone, or marble, to which more damage can be done.6
General Stone Conservation Procedure
Conservation is a multi-faceted process, where the causes of damage must be diagnosed, the building must be treated in an attempt to bring the stone back to its original condition, highly damaged portions should be restored, and the damages of possible future decay should be reduced.7 The first step to conservation work is to pinpoint and eliminate the cause of decay or damage. In the case of natural flooding and environmental pollution, little can be done, but areas of overloading, leaking, plant growth, insect infestation, and rusting from iron pieces can all be handled.8 Obvious causes can be dealt with in a straightforward manner, but further testing should be performed to acquire a complete diagnosis. This diagnosis results from the removal of some of the weathered structure, the crust, if one has formed, and the sound structure to serve as comparison. Ideally, this is drawn from crumbled pieces, but drilling into the building and extracting samples is usually necessary, as is removing a section perpendicular to the structure to analyze the stone’s sectioned layers.9 The sectioned piece can be polished and examined via X-ray diffraction, which will identify the minerals present, and scanning electron microscopy, which will result in a highly magnified image of the sample. The smaller pieces can be evaluated by dissolving water-soluble minerals in water, which will be examined with ion chromatography, whereas the residue can be evaluated with scanning electron microscopy.10
Once a diagnosis has been determined, solutions can be planned to aide in the prevention of deteriorating Istrian stone. In the case of Venice, authorities realized the dangers of air pollution on the Istrian stone surfaces and became one of the first Italian cities to convert to natural gas for power.11 Natural gas contains no sulfur impurities and causes no soot; therefore, the creation of gypsum crusts and soiled stone has been greatly limited. Even so, air pollution can travel large distances, so in theory, the cities and countries in the vicinity of Venice would need to limit their coal and oil burning as well. It would be ideal if UNESCO and Italian authorities could begin a campaign to promote a conversion to clean energy in all surrounding cities.
Several factors should be taken into account before the plan for conservation is generated: all treatment should be reversible, any consolidant used should reach inside the stone deeper than the layer of weathering, the stone should retain its ability to breath so no water is trapped inside, the material used for treatment should not absorb ultraviolet radiation or atmospheric gases, and the treatment material should be used as sparingly as possible.12 General practice today for restoring stonework is either to replace or repair it. If the stone has weathered significantly, new stone may be inserted for structural reasons; on the other hand, pores may be filled in with a polymer.13
Another physical step of conservation is generally the cleaning of the stone structure. This cleaning has two purposes, with two different methods: the cleaning of the surface, and the removal of salts from within the stone. In order to clean the surface, chemical, mechanical, and biochemical methods may be employed. Chemical methods are generally avoided, as the usage of acids and alkalies can dissolve part of the stone. Mechanical methods are gentler and include the use of pH neutral substances, such as air, water, steam, and solvents, to dissolve soiling.14 In order to clean the gypsum crust that can form on the surface, water washing is utilized, as gypsum is partially water soluble. The water can be applied as a spray, poultice (paste), or with pressure and can be paired with brushing the surface to remove more durable encrustations.15
Protection and consolidation are two important factors in conservation. Traditional water repellents for stone are not generally useful and applying protective or sacrificial plasterwork entirely changes the appearance of the structure. Films and consolidants, on the other hand, are widely used in conserving stone. Consolidants such as alkoxysilanes are good consolidants and some provide water-repellent effects.16
Two Examples of Stone Conservation: One a Porous Marble, One Istrian Stone
Architectural facades in Venice often employ various types of marble, making conservation a strenuous, lengthy, expensive task. The 1972 conservation of Pyrgotelis’s (?-1531) sculptural Virgin and Child on the facade of S. Maria Dei Miracoli and Sansovino’s Loggetta (1537-1549, rebuilt in 1912 after collapse of the Campanile) of the campanile in Venice involved the cleaning of the surface and the application of two polymer consolidants: Maraset and X54-802. To plan a conservation treatment, conservators not only have to consider the surface they are treating, but the environment of the work, the equipment available to them, the allotted budget, and the amount of time required to complete the task.
In the case of cleaning the porous Carrara-type marble Virgin and Child, shown above, air-abrasive cleaning was preferred but not used, as Venice was not able to acquire the equipment until later in the conservation process; instead, the work was manually cleaned with pumice powder.17 To clean the non-porous Istrian stone on the Loggetta of the Campanile, shown below, tap water was sprayed on the surface, allowing the sulphate and dirt to soften, which were then removed with the aide of air abrasive techniques, followed by drying and warming with an infrared lamp.18 In air abrasion, a stream of compressed air is used to propel abrasive particles at the surface of the building material. While this method is one of the fastest and least expensive ways to remove surface impurities, its drawback is it also removes a thin layer of the material.19
The Carrara-type marble of the Virgin and Child and the Istrian stone of the Loggetta of the campanile were not only cleaned, but also consolidated and/or protected. The Virgin and Child sculpture was heated to 70-80ºC by creating an insulated tent around the sculpture and heating overnight with electric heaters. This heat ensures the stone is dry and the resin is able to penetrate deep into the stone and polymerize. A resin was created by combining 100g Maraset X555 to 7g H555 catalyst, the mix of which was then applied with a brush until saturated in the stone. By closely covering the stone with a Melinex sheet for a few months, the stone was protected from rain and oxygen, which can whiten the resin and cause the sculpture to look significantly different than its original state. Covering the Virgin and Child for months also allowed the resin to polymerize within the stone’s structure. After the sheet was removed, a coat of Cosmolloid wax was applied for further protection of the surface.20 As the Istrian stone on the Loggetta of the Campanile has a smooth, non-porous surface, the conservation involved protecting only the surface from decay, which does not require a consolidant, but rather requires only a surface coating. As with the Virgin and Child, Cosmolloid wax in a slurry with solvent white spirit was brushed on to the surface, the solvent was allowed to dry, and the wax was softened with a butane flame to ensure even film.21
Istrian stone is a durable architectural material. Although it is technically a limestone, its nonporous surface slows wear, causing Istrian stone to wear more like a marble. Even so, Istrian stone is an expensive building material, which should be carefully conserved to preserve its integrity. Because the stone is so durable, sculptural works of Istrian stone generally do not require consolidation, but should be cleaned and protected. On the other hand, more porous stones used in Venice do require cleaning, consolidation, and protection.
1. R. J. Goy, Building Renaissance Venice: Patrons, Architects and Builders c. 1430-1500, (New Haven: Yale University
Press, 2006), 79.
2. F. Sansovino and G. Martinioni, Venetia, citta nobilissima et singolare…, vol 1 (Venice, 1968), 383, in R. J. Goy, Building Renaissance Venice: Patrons, Architects and Builders c. 1430-1500, (New Haven: Yale University Press, 2006), 80.
3. K. L. Gauri and J. K. Bandyopadhyay, Carbonate Stone: Chemical Behavior, Durability, and Conservation (New York: John Wiley & Sons, Inc, 1999), 74.
4. Ibid., 74, 101.
5. Ibid., 74.
6. Ibid., 99-105.
7. Ibid., 212.
8. I. Bristow, “An Introduction to the Restoration, Conservation, and Repair of Stone”, Conservation of Building and Decorative Stone, Vol 1 (London: Butterworth-Heinemann, 1990, 1.
9. K. L. Gauri and J. K. Bandyopadhyay, Carbonate Stone: Chemical Behavior, Durability, and Conservation, 212.
10. Ibid., 213.
11. C. Fletcher and J. Da Mosto, The Science of Saving Venice (Torino, Italy: Umberto Allemandi & C., 2004), 42.
12. K. L. Gauri and J. K. Bandyopadhyay, Carbonate Stone: Chemical Behavior, Durability, and Conservation , 213-214.
13. I. Bristow, “And Introduction to the Restoration, Conservation, and Repair of Stone”, Conservation of Building and Decorative Stone, Vol 1 (London: Butterworth-Heinemann), 1990, 13.
14. K. L. Gauri and J. K. Bandyopadhyay, Carbonate Stone: Chemical Behavior, Durability, and Conservation, 218-219.
15. N. Ashurst, Cleaning Historic Buildings, Vol 2: Cleaning Materials and Processes (London: Donhead, 1994), 17.
16. J. Ashurst, “Method of Repairing and Consolidating Stone Buildings,” Conservation of Building and Decorative Stone, Vol 2, (London: Butterworth-Heinemann, 1990), 3.
17. A. Moncrieff and K. F. B. Hempel, “Conservation of Sculptural Stonework: Virgin & Child on S. Maria dei Miracoli and the Loggetta of the Campanile, Venice,” Studies in Conservation, Vol. 22, No. 1, 1977, 2.
18. Ibid., 8.
19. N. Ashurst, Cleaning Historic Buildings, Vol 2: Cleaning Materials and Processes, 33-34.
20. A. Moncrieff and K. F. B. Hempel, “Conservation of Sculptural Stonework: Virgin & Child on S. Maria dei Miracoli and the Loggetta of the Campanile, Venice,” Studies in Conservation, Vol. 22, No. 1, 1977, 2-3.
21. Ibid., 8.