Procedure

A uniform coat was applied to each test block. One half of the surface of each wood test block was then covered with a wood panel to prevent light from hitting the stained surface. The covered section thus served as a control for comparison later on. The blocks of wood were placed on the inside of a window ledge that had northern exposure, meaning it received only indirect sunlight most of the year. Although the amount of natural light on the test block varied considerably throughout the day and from season to season, on a clear summer day it measured 1,400 foot-candles, as measured by a luxmeter.

Figure 1. The 10-year test results of the National Park Service's Wood Stain Durability Test are shown. The upper half of the test board was exposed in a window to north light. The bottom was covered during the test period. Comparison of the corresponding exposed and unexposed areas reveals the extreme susceptibility of certain commonly used wood stains to damage by light. Photo: Mike Wiltshire, National Park Service

Test Results

The two aniline dyes proved to be extremely light sensitive with noticeable fading even on a year-to-year basis. After 10 years, the aniline dye in both the water stain and the alcohol stain had faded so completely that the natural color of the wood predominated. Similar results occurred with the aniline dye coated with two coats of orange shellac.

The UF-3 Plexiglas filter had only negligible effect on the durability of the aniline dyes. The dyes in this case had nearly completely faded out as well.

Tests were run using a Crawford UV monitor to determine whether the UF- 3 Plexiglas was still performing at the same level as when first used. Readings taken on a clear summer day through the glass panes of the window indicated that approximately 450 microwatts per lumen were landing on the wood blocks. With the UF-3, 1/8" Plexiglas, most of the ultraviolet rays coming through the window were being filtered out, as the reading of about 50 microwatts indicated. Also of interest in regards to the long-term effectiveness of such filters, both the 10-year old UF-3 Plexiglas used on the wood test blocks and new UF-3 Plexiglas registered the same ultraviolet reading.

Figure 2. A veneer repair patch, located on the front of the third drawer, has faded from light. This extreme fading is typical of areas where aniline stains have been applied. Photo: Mike Wiltshire, National Park Service.

The UF-3 Plexiglas and orange shellac seemingly offered insufficient protection against the harmful natural light during the 10-year test period. In stark contrast with the two aniline dyes, the oil stain was only minimally affected by the light. (This is not to imply, however, that over a much longer period noticeable differences might not occur with the oil stain.) Although further testing would be needed to establish broader findings regarding the durability of stains when, exposed to natural light, the 10-year test supports the following conclusions:

1. Natural light is damaging to certain types of wood finishes, even in cases of only indirect light exposure.
2. Although only a few aniline dyes were tested, the results strongly indicate that they are extremely light sensitive (see figure 2).
3. Certain oil colors (earthen pigments ground in oil) appear to produce a stable stain that resists fading.
4. UF-3 Plexiglas may shield out most of the ultraviolet light, yet visible light is left largely unfiltered and can be extremely damaging to light-sensitive objects and architectural woodwork.