It can be seen from the image above that wood, in the form of vertical stakes, forms the foundation of Venice. Wood, which was used to solidify the marshy foundation of Venice, undergoes various chemical and physical processes that lead a course for conservation work.
Timber Usage in Venice
Timber was a popular commodity in Renaissance Venice because it was used for shipbuilding, house carpentry, and making oak and pine piles, or tolpi, which were staked vertically into the ground to create a solid foundation for building heavy structures.
Wood usage was especially widespread in Venice because the nobility owned much of the forests of the Terraferma, or mainland, and realized its economic potential.1 This reliance on the Terraferma was not a new concept for Venetians: these island-dwellers had to reap the bulk of their agricultural commodities and building materials, like wood and stone, from the mainland. The importance of timber in Venetian society is reinforced by the choice to depict timber yards in Jacopo de’ Barbari’s map of 1500, depicted below.
This immense usage of wood now has Venice dealing with various conservation issues regarding the high potential of wood decay in areas of variable water levels.
What is Wood?
In order to understand the decay processes of wood, one must know the basics of its physical makeup. Wood is composed of three main substances: cellulose, hemicellulose, and lignin. These substances are polymers, long chained molecules of repeating units with high molecular weight, meaning each molecule has tens of thousands of atoms. Aside from cellulose, hemicelluloses, and lignin, a variety of additional materials are present in low abundance, including nitrogenous materials, pectin, starch, low molecular weight sugars, and minerals, such as iron, magnesium, and manganese. In wood, the cellulose molecules, rather than piling into disorganized bundles, are able to arrange themselves in linear bundles due to hydrogen bonding between the chains.2 This bundle of molecules, in turn, makes up the cell walls of wood’s cellular structure, as seen below.3
The long, bound chains of molecules make wood a sturdy building material, but its organic makeup also leads to relatively quick weathering and decay processes.
Like marble and brick, wood is susceptible to weathering. Wood’s surface can be damaged by ultraviolet rays from the sun, so that the lignin and cellulose molecules break down and are washed away by rain and flooding.4 Water in its liquid and gaseous forms can be drawn into wood through tiny holes in its exterior, and the water proceeds to be absorbed into the cell wells, causing the wood to swell.5 Constant wetting and drying causes wood to shrink and swell, leading to cupping and warping of the wood.6 This damaging cycle is only the case for wood above water. The oak and pine piles on which Venice’s foundation is built are embedded in airless, muddy soil, which naturally preserves the wood.7 While piles under Venice are safe from the frequent flooding, the wood above the water level faces wetting and drying and shrinking and swelling issues.
In all environments, wood is attacked by microbiological organisms. While plants are able to receive nourishment from self-produced chlorophyll, bacteria and fungi reap energy from outside organic sources, such as wood, by producing an enzyme to break down the molecules of the material into simple sugars. In the case of wood, microorganisms break down the cellulose, hemicellulose, and lignin into sugars, from which they obtain energy by absorbing.8 While bacteria will slightly weaken wood’s structure, fungi decays the wood, and is therefore is of much more consequence.9 Serious damage can only be caused by fungi if the moisture content of the wood is above the fiber-saturation level, which occurs if water adds approximately 30% to the wood’s weight.10 This fiber-saturation level can not occur solely from a humid environment: wood must come into contact with water.11 In the case of Venice, with its consistent flooding, this is a significant issue. In the case of piers, submerged wood is immune to this fungal attack, with the exception of areas near the surface, due to fungi’s inability to survive without oxygen.12 Mold is a widespread problem in Venice, though its effects are not nearly as degradative to wood’s structure as decay-causing fungi. Though mold is a type of fungi, it consumes sugars already in the wood, leaving cellulose, hemicellulose, and lignin, the more important structural features, intact.13 While wood under water is essentially safe from decay, structural wood in buildings above water level undergo destructive interactions with fungi in wet environments like Venice.
Conservation of Wood
Conservators must first diagnose wood ailments in order to take action in repairing, cleaning, consolidating and preserving the wood. Conservators recognize physical weathering through several clues. Frequent water contact leads to warping, while ultraviolet light damage leads to a color change from yellowing or browning which will eventually lead to graying on the outer 0.05-2.5 millimeters of the exposed wood.14 Conservators recognize decay in wood visually by noting penetrated deterioration, reduced strength, and abnormal coloration.15 Brown rot, where cellulose is attacked leaving high levels of lignin, is noted by cracks, shrinkage, and an abnormal, charred shade of brown, while white rot breaks down both lignin and cellulose leading to an off-white shade with black lines throughout.16 White rots can also be recognized by small dips in the surface of the wood and should be dealt with immediately, as the wood will retain its structure until it collapses.17 Once the conservator has identified the causes of damage in wood, some causes can be removed from the buildings environment, but many have to be prevented with special treatments.
A basic, yet important, aspect of conserving buildings is prevention. In the case of weathering caused by ultraviolet light, a simple coat of paint will obstruct the sunlight’s rays.18 As fungi are the most detrimental microorganism to wood, the application of fungicides is a common preservative treatment. Fungicides will either make the fungi unable to digest wood, inhibit their growth, or kill fungal spores to halt reproduction.19 Another method of prevention for fungal decay as well as water damage is to exclude water from the wood by using water repellents, plastics, and resins. Water repellents drastically slow the rate at which water can enter wood. This method, while not successful in locations with constant moisture, would have significant results in locations with Venice’s intermittent flooding. Plastic coatings can be used if fungicide has not been applied, but these coatings can easily be damaged. A better remedy is to use a penetrating finish, where a water-repellent and fungicide can be mixed with a resin and solvent to form what is named a water-repellent preservative.20 A more costly method is to introduce the small molecule pieces of the polymer chain into the wood and polymerize them in the wood.21 This method, while successful, is too expensive for use throughout an entire city. Another expensive, yet highly effective method involves impregnating the wood with a cell-wall-penetrating resin, letting the resin dry, and heat curing the wood. This prevents water from penetrating to a water-level where fungus is able to attack.22
After the causes of the problems have been removed if possible, the effects must be dealt with. This includes the restoration and conservation aspects of replacement, reinforcement, and consolidation. If the wood is so deteriorated that it jeopardizes the structure of the building, it should be partially or completely replaced by new timbers of the same species, quality, cut, color, finish, and grain direction and pattern. If the old wood is still sturdy, it can be mechanically reinforced with wooden, metal, or glass fiber dowels or pegs reinforced with plastic, but care must be taken not to restrict the natural shrinking and swelling of wood. Timbers can also be flanked with new timbers, steel, or plastic, which are bolted in place, as shown below.23
This is particularly useful if portions of rotten wood had to be removed. Wood can also be strengthened by consolidation through impregnation, which helps bond the wood back together using synthetic resins or molten wax. Consolidation work should a number of conservation guidelines: the consolidants should penetrate deeply into the wood, not shrink, bond well to the wood, be removable, and should not under any circumstances damage the wood through chemical reactions. Because epoxy resins are irreversible, acrylic resins are generally preferred as they can be reversed. Two common methods involve using synthetic resins plus steel reinforcement: Wood Epoxy Reinforcement (WER) and the BETA system, both of which allow for slightly damaged wood to be able to carry load-bearing weights.24
In the WER system, shown to the left, a deep groove is made through the center of an old wooden beam, which is then filled with epoxy, followed by a steel rod, followed by more epoxy. This treatment preserves three of the four faces of the timber. In the BETA system, a decayed end of a beam is removed, holes are drilled in the remaining lower portion of the beam to fit plastic rods which are reinforced with glass fiber, and the rods are then used to support an epoxy mortar which is cast to take the shape of the removed portion of the beam, as shown below.25
An Example of Wood Restoration
An example of restoration work of wood in Venice, rather than conservation work, is the 1973-1977 work on the Rialto bridge, shown below. The bridge was cleaned, the Istrian stone facing was restored, and the lead roofing, structural beams and sustaining structures were repaired and parts were reconstructed.26 It seems structural repair of wood in Venice lends itself better to restoration than conservation.27 This is due to the fact that conservation work is generally best-suited for areas which must remain visually coherent with original appearances, and wood was used as a structural element in Venetian architecture, rather than an aesthetic addition.
1. R. J. Goy, Building Renaissance Venice: Patrons, Architects and Builders c. 1430-1500, 85-86.
2. T. K. Kirk, “The Chemistry and Biochemistry of Decay” in Wood Deterioration and Its Prevention by Preservative Treatments, Vol 1, Ed. Nicholas, Darrel D. (New York: Syracuse University Press, 1973), 149-150.
3. W. C. Feist, “Outdoor Wood Weathering and Protection”, in Archaeological Wood: Properties, Chemistry, and Prevention, Ed. By R. M. Rowell and R. J. Barbour, (Washington, D.C.: American Chemical Society, 1990), 266.
4. I. S. Goldstein and W. E. Loos, “Special Treatments” in Wood Deterioration and Its Prevention by Preservative Treatments, Vol 1, Ed. Nicholas, Darrel D. (New York: Syracuse University Press, 1973), 354.
5. W. C. Feist, “Outdoor Wood Weathering and Protection”, in Archaeological Wood: Properties, Chemistry, and Prevention, 269.
6. I. S. Goldstein and W. E. Loos, “Special Treatments” in Wood Deterioration and Its Prevention by Preservative Treatments, Vol 1, Ed. Nicholas, Darrel D., 354.
7. C. Fletcher and J. Da Mosto, The Science of Saving Venice (Torino, Italy: Umberto Allemandi & C., 2004), 35.
8. T. C. Scheffer, “Microbiological Degradation and the Causal Organism”, in Wood Deterioration and Its Prevention by Preservative Treatments, Vol 1, Ed. Nicholas, Darrel D. (New York: Syracuse University Press, 1973), 32.
10. Ibid, 40.
12. Ibid., 42-43.
13. T. C. Scheffer, “Microbiological Degradation and the Causal Organism”, in Wood Deterioration and Its Prevention by Preservative Treatments, 84.
14. W. C. Feist, “Outdoor Wood Weathering and Protection”, in Archaeological Wood: Properties, Chemistry, and Prevention, 269.
15. Ibid, 46.
16. T. C. Scheffer, “Microbiological Degradation and the Causal Organism”, in Wood Deterioration and Its Prevention by Preservative Treatments, 46-48.
17. M. E. Weaver, Conserving Buildings: A Manual of Techniques and Materials, Revised Edition, (New York: John Wiley & Sons, Inc., 1997). 40-41.
18. I. S. Goldstein and W. E. Loos, “Special Treatments” in Wood Deterioration and Its Prevention by Preservative Treatments, Vol 1, Ed. Nicholas, Darrel D. (New York: Syracuse University Press, 1973), 354.
19. T. K. Kirk, “The Chemistry and Biochemistry of Decay”, 185.
20. W. C. Feist, “Outdoor Wood Weathering and Protection”, in Archaeological Wood: Properties, Chemistry, and Prevention, 288.
21. T. K. Kirk, “The Chemistry and Biochemistry of Decay”, 208.
23. M. E. Weaver, Conserving Buildings: A Manual of Techniques and Materials, Revised Edition, 40-41.
24. Ibid., 42-43.
26. Unesco, Venice Restored, 86.