Why do marble statues corrode when kept in open for long time?
Marble is susceptible to chemical weathering. Marble holds up better than limestone in both acid and unpolluted rainwater, but it will show signs of weathering in time. Water enters small holes in the marble at this point, worsening the weathering. This phenomenon is known as surface flaking. Weathering is the natural breakdown of rocks into smaller particle sizes and different forms. Marble is a metamorphic rock that was once a limestone or dolomite rock; therefore, the properties of marble and limestone are similar. Chemical weathering causes a rock to dissolve or alter its form from one rock type to another. Over time, a marble or limestone rock may be subject to physical weathering as well, depending on its location. Temperature swings from hot to cold can form cracks in limestone or marble as the rock expands and contracts. Over time, these cracks will increase in size and weaken the rock, causing it to crack and break into smaller pieces. In addition, ice crystals and minerals, such as quartz and calcite, can precipitate in the cracks of the limestone and marble rocks and cause them to expand and break as the crystals grow. Marble and limestone rocks are very susceptible to erosion by water and acid rain. Water from rainfall, streams and rivers pouring over and hitting the rocks will slowly wear away the surface of the stones. Over time, the marble and limestone will become smooth and decrease in size. Marble and limestone river rocks form from the action of water on the rock as they move down the river. Acid rain has a significant effect on marble and limestone. Carbonic, sulfuric or nitric acid are components of acid rain that will react with the calcium carbonate in the marble and limestone to pit and dissolve the rock over time.
Marble is metamorphosed limestone. It is a carbonate rock that reacts with low pH rain water (elevated levels of H ions). Weak concentrations of Hydrochloric acid (HCl) occur naturally in rain water or Nitric or Carbonic acid is found in elevated amounts (acid rain). It is the result of industry releasing carbon dioxide and nitrogen oxide into the atmosphere.
