Sources of Deterioration

While vandalism and accidental damage are quite common, the environment is actually the main cause for deterioration in art. Chemical reactions with different factors take place immediately after the artwork is created. To prevent these reactions from happening and to minimize the damage, it is important to first understand how the art interacts with the environment and what kinds of reactions take place.


Figure A: Linseed oil and pigment

Paints are composed of the colored pigment and a vehicle that spreads the pigment. In oil-paints, the pigment is a "drying oil". A drying oil is a polyunsaturated triglyceride, a type of triglyceride two or more hydrogen atoms short of a saturated fatty-acid chain.

When spread in thin layers, like in paint, the unsaturated oil reacts with oxygen in the air to polymerize, a process commonly called drying. The paint turns into a solid, thus fixing and protecting the pigment.1 More unsaturated oil results in a faster polymerization rate. Monosaturated or saturated triglycerides like olive oil and coconut oil are then unsuitable because they take too long to polymerize, thus classified as "nondrying oils". Linseed oil is one of the popular choices for vehicles because it does not crack or yellow as much as other oils, and it polymerizes quickly.1

Typically, oil paintings are varnished with natural resins to enhance the colors and make the surface glossy. The varnish is applied to the surface in a thin layer, and it protects the painting. The natural resins are formed from hydrocarbon secretions of plants. They are oxidized by oxygen in the air. When this occurs, they become dried and cracked and form a yellow film that dulls the originally enhanced colors.1

Figure B: Pigments and the minerals they are formed from, such as malachite and azurite

Air Pollutants

The primary cause of air pollutants is the burning of fossil fuels. Sulfur and nitrogen oxides in the air can dramatically deteriorate artwork as they react with metals in pigments.

Lead was commonly used in paints such as white lead (PbCO3, lead (II) carbonate) and chrome yellow (PbCrO4, lead (II) chromate).1 Sulfides in the air react with the lead to form lead (II) sulfide, a black pigment that cause graying in white colored areas of a painting.1

Pb2+ + S2- --> PbS

Similarly, blue azurite (2CuCO3 · Cu(OH)2) turns brown or dark green with the formation of copper sulfides (black).1

Figure C: A stone sculpture damaged by acid rain

Stone and metal sculptures in open air may react with air pollutants, as sulfur oxides and moisture to form sulfuric acid in acid rain. Marble, made of calcium carbonate (CaCO3), reacts with sulfuric acid to form calcium sulfate, or gypsum (CaSO4). The powdery chalk-like calcium sulfate absorbs water from rain, turning into a soluble hydrated salt that is then washed away by rain.1 This deteriorates the marble over time.

SO2 + ½O2 + H2O --> H2SO4
H2SO4 + CaCO3 --> CaSO4 + CO2 + H2O
CaSO4 + 2H2O --> CaSO4 · 2H2O

Iron, copper, and bronze sculptures also corrode by sulfuric acid from acid rain. Other copper sculptures react with oxygen to form copper oxides, which then turn into bronchantite in the presence of sulfur oxides from pollution.1

4CuO + ½O2 + SO2 + 3H2O --> CuSO4 · 3Cu(OH)2

In addition, carbon dioxide dissolves in rain to produce carbonic acid, which reacts with copper to form malachite (CuCO3 · Cu(OH)2) or azurite (2CuCO3 · Cu(OH)2).1 Both of these cause copper's blue-green verdigris patina.

The green patina on the surface of copper protects the metal underneath. Cracks in the patina allow for further corrosion of the metal inside. Chlorides in the atmosphere, from sea salt or industrial waste, cause "bronze disease".1 The chloride ion replaces the hydroxide ion to form a soluble metal chloride. It reacts with water to form hydrochloric acid, creating pits and eventually leading to the complete destruction of the metal.1

Figure D: Chalcocite (Cu2S) on left, Brochantite (CuSO4 · 3Cu(OH)2) in center, Malachite (CuCO3 · Cu(OH)2) patina on right


Light, a form of energy, can change the composition and structure of chemicals it interacts with. Many pigments and dyes fade in the presence of ultraviolet light. Ultraviolet light also speeds up yellowing and cracking in varnishes.1 In paper manuscripts and textiles, light weakens the natural cellulose fibers, causing paper to become yellow and brittle. Damage from light is cumulative. To control the effects of light, museums prohibit flash photography and regulate their lighting to be free of UV light.2

Figure E: The Charters of Freedom, preserved in the National Archives in controlled light and humidity

Temperature and Humidity

Canvases expand when they absorb moisture and shrink when they dry. With unregulated temperature, the repeated expansion and shrinking cracks the paint, causing the painting to flake.1

Humidity is also a major factor in deterioration of paper works. Since the 1750s, certain chemicals have been added to paper to prevent ink from spreading and make the paper fit for printing.1 Aluminum sulfate, Al2(SO4)3, added to the paper can create a hydration complex when aluminum ions react with water from moisture.1 The hydration complex then reacts to produce the hydronium ion, which is acidic.

Al3+ + 6H2O --> [Al(H2O)6]3+
[Al(H2O)6]3+ + H2O --> [Al(H2O)5OH]2+ + H3O+

The acid produced breaks apart the cellulose fibers in the paper, causing the paper to become brittle and crumble.

Plastics also lead to deterioration of paper documents. In the beginning of the 20th century, cellulose nitrate was a polymer commonly used to make different plastics.1 Under light, cellulose nitrate decomposes to form nitrogen oxides. Nitrogen oxides react with water to form nitric acid, which decomposes and destroys the paper.1


1. Kabbani, Raifah M. "Conservation A Collaboration Between Art and Science." The Chemical Educator 2.1 (1997): 1-18. Print.

2. McGlinchey, Christopher. "Color and Light in the Museum Environment" In The Changing Image: Studies in Paintings Conservation; The Metropolitan Museum: New York, 1994; pp. 44-52.


A. Handwerker. Linseed oil paint. Digital image. Wikipedia. Wikipedia, 14 Jan. 2012. Web. 18 Apr. 2014. <>.

B. Eboix. Pigments. Digital image. Wikipedia. Wikipedia, 30 April 2011. Web. 18 Apr. 2014. <>.

C. Barbieri, Nino. Acid rain damaged gargoyle. Digital image. Wikipedia. Wikipedia, Sept. 2006. Web. 18 Apr. 2014. <>.

D. Left: Lavinski, Rob. Chalcocite. Digital image. Wikipedia, originally Wikipedia, March 2010. Web. 18 Apr. 2014. <>.
D. Middle: Lavinski, Rob. Brochantite. Digital image. Wikipedia, originally Wikipedia, March 2010. Web. 18 Apr. 2014. <>.
D. Right: Lavinski, Rob. Copper. Digital image. Wikipedia, originally Wikipedia, March 2010. Web. 18 Apr. 2014. <>.

E. Kay, Kelvin. Rotunda for the Charters of Freedom. Digital image. Wikipedia. Wikipedia, 21 May 2004. Web. 18 Apr. 2014. <>.