By far the most extensive literature on accelerated aging deals with the chemical stability or chemical resistance of specific materials. One may regard this emphasis as a fundamental approach, for it is generally held that it is chemical changes that are ultimately responsible for the physical disintegration and failure of materials. Much more rarely do conservators focus their attention on the physical stresses of use unless they are concerned with the maintenance of monuments and historic sites, the ruggedness of bookbindings, or the design of shipping containers and earthquakeresistant display cases.

This review primarily discusses the chemical aspects of photochemical stability, particularly the ways in which the oxidative deterioration induced by exposure to visible and near-ultraviolet radiation takes place and the ways in which the underlying chemical processes can be speeded up in order to estimate the long-term serviceability of materials. Acceleration of thermally initiated deterioration will also be discussed briefly.

Despite being given a relatively ineffective and somewhat belated administrative and legal framework, the prestige and popularity of medieval monuments were sufficient to ensure a significant number of restoration works after 1840. Literature on art, the press and manuscript sources of the period, sought to identify the principles to underlie the idea of restoration in Portugal between 1839 and 1925, as compared to models adopted throughout Europe. Although Portuguese artistic culture lagged far behind other European nations, the restoration of medieval religious architecture was common practice in Portugal at that time, both due to the idea that unity had always been one of the fundamental principles of architecture and because the cult of monuments stemmed from a mythical and symbolic production of national identity.

Rehabilitating a historic commercial building is important because it Enhances its marketability as distinctive retail or office space, Mitigates damage cause by inappropriate modifications and Restores functional elements of a building.

Those who take on responsibility for the upkeep of the fabric of war memorials often have little or no experience of the conservation and repair of historic buildings, and do not know who to turn to for advice. Although the local small building firm or monumental mason may have experience in producing new work and carrying out general household repairs, they often lack the specialised knowledge and skill which are essential for good specification and practice in building conservation work.

During some periods in the history of architecture, the roof imparts much of the architectural character. It defines the style and contributes to the building's aesthetics. The hipped roofs of Georgian architecture, the turrets of Queen Anne, the Mansard roofs, and the graceful slopes of the Shingle Style and Bungalow designs are examples of the use of roofing as a major design feature.

But no matter how decorative the patterning or how compelling the form, the roof is a highly vulnerable element of a shelter that will inevitably fail. A poor roof will permit the accelerated deterioration of historic building materials-- masonry, wood, plaster, paint--and will cause general disintegration of the basic structure. Furthermore, there is an urgency involved in repairing a leaky roof since such repair costs will quickly become prohibitive. Although such action is desirable as soon as a failure is discovered, temporary patching methods should be carefully chosen to prevent inadvertent damage to sound or historic roofing materials and related features. Before any repair work is performed, the historic value of the materials used on the roof should be understood. Then a complete internal and external inspection of the roof should be planned to determine all the causes of failure and to identify the alternatives for repair or replacement of the roofing.

As more and more people come to recognize and appreciate the architectural heritage of America's downtowns, however, a growing interest can be seen in preserving the historic character of commercial buildings. The sensitive rehabilitation of storefronts can result not only in increased business for the owner but can also provide evidence that downtown revitalization efforts are succeeding.

Great care must be taken if substitute materials are used on the exteriors of historic buildings. Ultraviolet light, moisture penetration behind joints, and stresses caused by changing temperatures can greatly impair the performance of substitute materials over time. Only after consideration of all options, in consultation with qualified professionals, experienced fabricators and contractors, and development of carefully written specifications should this work be undertaken.