In the wood industry, it is desirable to have woods that are of a dark color or to substantially preserve the color of white woods. With respect to dark woods, for example, dark wood veneers are particularly valued in a number of different applications including furniture, architectural woodwork, flooring, paneling, plywood, cabinets, novelties, caskets, toys, and musical instruments.
Of the wood veneers produced, black walnut (Juglans nigra L.) has one of the highest market values, due to its relative scarcity and highly aesthetic dark, rich color. The cost of handling black walnut and processing it to obtain the highly prized dark color is high. In order to obtain the desired color, cut walnut trees have to be maintained wet, then steamed or cooked for long periods of time in order to soften the walnut for cutting, as well as to facilitate the process of color darkening in the wood itself. Further, black walnut must undergo further manipulation in the manufacturing process in order to achieve full dark color. Once cut, the sliced veneers are set aside for 24-48 hours after cutting, and maintained in a wet condition, for the final darkening of the wood to occur. Care must be taken when handling the wood. For instance, walnut that is taken straight from cutting into an oven for drying does not darken and is fixed into its native green state. Compared to other woods, that do not need substantial processing and handling, preparing black walnut is costly due to the added processing steps.
A great deal of research has been aimed at physically treating black walnut logs to obtain a dark, rich color, thereby minimizing or optimizing the expensive procedures that are used. In general researchers have approached the problem in terms of materials processing, and have focused on physiochemical approaches to change the walnut by cooking it. Brauner, A. B., Connway, E. M. (1964), “Steaming Walnut for Color,” Forest Products Journal: 425-427; Brauner, A. B., Loos, W. E. (1968), “Color Changes in Black Walnut as a Function of Temperature, Time, and Two Moisture Conditions,” Forest Products Journal 18(8): 29-34; Chen, P. Y. S. (1975), “The Effect of Steaming Time and Temperature on the Longitudinal Permeability of Black Walnut,” Wood and Fiber 7(3): 222-225; and Chen, P. Y. S., Workman, E. C. (1980), “Effect of Steaming on Some Physical and Chemical Properties of Black Walnut Heartwood,” Wood and Fiber: 218-227. The results of this research have been the acceptance and adaptation of physical processing schemes that are currently used to modulate color of black walnut during processing. This includes controlled environment storage, long term cooking of whole logs in large scale vats, and delayed multistage handling schemes to promote final stages of color maturation. None of these studies approached the problem based on the analysis of a biological/biochemical system.
Approaches taking advantage of biochemical and molecular mechanisms include techniques to control fruit ripening and mediate damage responses in cut flowers and other vegetables. Since the 1950s, the control of post-harvest physiology in fruits, vegetables, and cut flowers using such techniques that take advantage of intrinsic biochemical signaling systems in plants has revolutionized that industry. For example, during shipment of green bananas ripening is inhibited by cold temperatures. Once in the mark the ripening process is “switched on” by treatment with the plant hormone ethylene.
More specifically, in herbaceous plants there has been a large amount of research on responses to herbivore, pathogens, and abiotic stress. Secondary plant compounds, those not known to be needed for basic metabolism of the plant, are now an important and growing area of research. Research has shown that there are complex interactions between chemicals metabolized by the plants, insect herbivores, parasitoids of herbivores, and plant pathogens. For example, it has been shown that chemical pathways are induced in plants in response to attack by insect herbivores and pathogens. The physical damage to the plant tissue induces a signaling response that leads to the production and activation of compounds such as proteinase inhibitors, polyphenol oxidase and steroid glycoalkaloids that appear to contribute to plant resistance against many insect attackers and some pathogens. This understanding has led to an expanding awareness of the importance of biological mechanisms to control plant pests in crop systems to enhance agricultural productivity.
To date, despite over 50 years of work with plants, these biochemical techniques and mechanisms have not been applied to wood. Various embodiments of the inventions described here provide novel and unobvious ways to modify the color of wood.