NOTES

1. Tg or glass transition temperature is measured as that temperature at which the available thermal energy is smaller than the forces holding molecules together. At lower temperatures, very little molecular adjustment is possible. Below its Tg, an amorphous polymer is brittle and hard; above its Tg, it is softer and can be dissolved more easily.

2. Polar is descriptive of a molecule in which the positive and negative electrical charges are permanently separated, as opposed to nonpolar molecules in which the charges coincide. Polar molecules ionize in solution and impart electrical conductivity. Water, alcohol, and sulfuric acid are polar in nature; most hydrocarbon liquids are not. Carboxyl and hydroxl groups often exhibit an electric charge. The formation of emulsions and the action of detergents are dependent on this behavior.

3. Refraction is the change in direction (apparent bending) of a light ray passing from one medium to another of different density, as from air to water or glass. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is the index of refraction of the second medium. Index of refraction of a substance may also be expressed as a ratio of the velocity of light in a vacuum to its velocity in the substance. It varies with the wave length of the incident light, temperature, and pressure. The usual light source is the D line of sodium, the standard temperature being 20�C. The expression of refractive index is nD20.

4. Horie (1987) describes the chemistry of p-xylene, which evaporates more slowly during the initial wet stage of drying but is released freely during the later stages. This characteristic is ascribed to the difference in polarity in the solvents.

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AUTHOR INFORMATION

SASHA CHAPMAN originally trained as an archaeologist specializing in recording of buildings. She joined English Heritage (the government's statutory advisor on archaeology and the historic enviornment in England) in 1993 and was involved in coordinating research and technical advice in several areas, particularly graffiti removal. She is a former chair of the United Kingdom Institute for Conservation of Historic and Artistic Works, Stone and Wall Paintings Section.

DAVID MASON trained in fine art, worked as a stone conservator, and gained a Ph.D. from De Montfort University in Leicester, England, where he specialized in the history and theory of conservation. He joined the Building Conservation and Research Team at English Heritage in 1997. He has coordinated BCRT technical research and was editor of English Heritage Research Transactions from 1999 to 2001.