PRESERVATION
Tech Notes

MUSEUM COLLECTIONS
NUMBER 2

Reducing Visible and Ultraviolet Light Damage to Interior Wood Finishes

Ron Sheetz
Division of Conservation
Harpers Ferry Center
National Park Service

Charles Fisher
Preservation Assistance Division
National Park Service

One of the major sources of damage to finishes of historic millwork and furniture is light--especially sunlight. Sunlight can cause finishes to crack or flake and paints and stains to fade or disappear altogether. Even the cellular structure of the wood can be damaged, especially once the finish has broken down. With historic wood stains, the result can be a change in coloration-- or worse, its complete loss. Even after wood had been refinished in the course of restoration, continued, unprotected exposure to sunlight may necessitate additional conservation work. As early as the 19th century, people were aware of the destructive nature of sunlight to interior furnishings. Curtains, Venetian blinds, and interior and exterior shutters were often used not only to keep rooms cool but to lessen color fading on the interiors of many buildings. With the advent of mechanical air-conditioning, the reliance on these traditional heat and light reducing devices has lessened, exposing historic interiors to increased risks of damage.

Museums with particularly sensitive materials, such as paintings and fabric, have addressed the problem in a variety of ways. One approach over the past 20 years has been the use of ultraviolet (UV) light filters in windows. These filters are effective in largely blocking one type of harmful rays--those of ultraviolet radiation. A 10-year fade test of wood stains by the National Park Service, however, suggest that visible light passing through windows protected with UV filters can cause significant damage to certain types of stains. This Tech Note covers the 10-year fade test and also discusses steps that can be taken to protect sensitive furnishings, even where UV window filters exist.

Problem

Radiated energy from the sun travels in different wavelengths, including ultraviolet light and visible light. Many museums have taken measures only to reduce the damage caused by ultraviolet light. While information is readily available on the effect of ultraviolet light on historic interiors, the photochemical damage to historic objects caused by visible light is less well known. In 1978, the Division of Conservation at the National Park Service's Harpers Ferry Center began a long-term test of the durability of different types of stains popularly used in furniture restoration work when walnut exposed to ultraviolet and visible light.

Test Variables

To determine their durability, three stains were applied to clear white pine and then exposed to natural light over a 10-year period (see figure 1). Each of stain was tested under three conditions:

Condition 1-One coat of each stain was applied and left exposed to natural light

Condition 2-One coat of each stain was applied and then protected by UF-3 Plexiglas to filter out ultraviolet light, leaving the stain exposed to visible light

Condition 3-One coat of each stain was applied followed by 2 coats of orange shellac (shellac being a traditional finish found on period furniture)

The stains tested consisted of two aniline dyes and one oil color mix. (Aniline dyes have been used since the 1850s because of their reasonable cost and ease of application.) The two aniline dyes used in the study were walnut colors, each from different manufacturers. As directed, one of the aniline dyes was mixed in warm water and the other in Solex-denatured alcohol. Equal concentrations of the aniline dyes were used in the two stains. The third stain tested consisted of two oil colors (burnt umber and burnt sienna) combined to create a mahogany walnut color and mixed in turpentine. From the outset, it was recognized that certain colors tend to be more stable than others; however, for the purpose of this test stability was not a selection factor. These three stains were selected because they were in common use at that time in the National Park Service's Conservation Laboratory for wood restoration on historic furnishings.