Color Changes in Black Walnut as a Function of Temperature, Time, and Two Moisture Conditions

The change in color from sapwood to heartwood in Juglans nigra L. is one of the most extreme of all commercial woods. It is known that the color after change is most uniform if the lumber is steamed before drying. Many furniture makers use steaming to control color, preferring this method to staining. In this study, the effects of temperature, moisture, and time on the development of color were examined. Color development was measured spectrally at 30-mu intervals from 400mu-700mu. Accurate description of color required definition of three variables, lightness or darkness, dominant wavelength (hue), and purity or saturation (freedom from grayness). The spectral reflectance curves for each sample were obtained before and after treatment. It was found that heating ovendried samples caused no significant changes in color. All color changes produced by heating water-saturated samples were significant with the rate of change increasing with temperature. These results are as follows: 1) 75?C.and 100?C. did not significantly change purity whereas 125?C. did after 8 hours of heating. 2) darkening at 75?C. and 100?C. was about the same but was much higher at 125?C. with most darkening occurring in the first 8 hours of heating. 3) dominant wavelength changes were significant in the first few hours of heating with the largest changes at 125?C. and those at 75?C. and 100?C. about equal but lower. 4) treatment at 125?C. produced a change in color similar to natural heartwood. Overall, the color changes produced by heating are an increase in purity, a decrease in lightness (increase in darkness), and a decrease in dominant wavelength.

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