Weathering is the process by which stones and metals are broken down over time. As with any other piece of human architecture, monuments are exposed to several different types of weathering. Without intervention, even the sturdiest monuments will eventually be ground down to soil and dust by nature's elements.
Ice crystallization, often referred to as "ice wedging" or "frost wedging," is a form of physical weathering that occurs often in cold climates. Ice crystallization occurs when water seeps into a crack or hole in the surface of a monument and then freezes. Because water expands when it freezes, it widens the crack in the monument and can eventually fracture and break off pieces, altering the shape of the monument and necessitating costly restorations and repairs. The more porous the material the monument is built of, the more susceptible it will be to ice crystallization.
Another form of physical weathering that takes a toll on man-made monuments is salt crystallization. Salt crystallization occurs when a saline solution enters a pore or crack in a monument and then crystallizes due to a change in temperature. As with freezing water, salt expands when it crystallizes, which expands fractures in monuments and eventually breaks pieces off. As with ice crystallization, the more porous a material is, the more susceptible it is to salt crystallization. Often, salt crystallization and ice crystallization work together to weather monuments. Salt crystallization is more common in coastal areas and hot, dry climates. Examples of weathering by salt crystallization are the brick monuments in Ayutthaya, Thailand, and many limestone monuments in Cairo, Egypt.
A third form of physical weathering that can have a devastating effect on man-made monuments is "root wedging." Root wedging happens when the root of a plant begins to grow into a crack in a monument. Over time, as the plant grows larger, so does the root. Eventually, the root will become so large that it will widen the cracks in the monument and most likely break pieces of it off. Root wedging is mostly a problem for monuments that are no longer taken care of; monuments that are regularly cared for will have any plants removed before root wedging can occur.
In addition to physical weathering, many monuments are susceptible to chemical weathering as well. "Acid rain," or rainwater that is more acidic than normal due to high levels of pollution, can seriously corrode monuments over time. Different building materials are subject to different types of chemical weathering. Metals such as bronze or copper are susceptible to corrosion by interaction with the elements. They are also likely to form a thin layer of "patina," a by-product of chemical weathering that has a green hue, and can be seen on the Statue of Liberty and the roof of the Chateau Frontenac in Quebec City, for example.
The Statue of Liberty
A good case study in monuments and weathering is the Statue of Liberty. As soon as the statue was erected in 1886, it began to undergo the natural processes of physical and chemical weathering. The salt water of New York's harbor began to weather it through the processes of ice and salt crystallization, and salt water also acted as a chemical agent that exacerbated the corrosion of Lady Liberty's copper skin. After nearly 100 years, the statue needed extensive repairs, which were undertaken from 1984 to 1986 and cost millions of dollars. If the statue is to exist for future generations, it will need periodic restoration, because the processes of weathering are relentless. All monuments must be cared for if they are to survive.