A review of timber discoloration during drying and its impact on appearance-based products: a case study using Tasmanian Blackwood

  • YEAR
    Tanton, David
    Dewsbury, Mark
    Wood, Kyra
    2021 Symposium Abstracts
    Conference Papers


Timber is a natural product and is subject to natural defects that are either inherent to the material or derived from non-ideal conditions during processing. One defect that is of particular importance to timber products that are marketed primarily based on their appearance is discoloration, which can reduce the marketable grade of the timber where it occurs, to the point where it is no longer suitable for that market (Luostarinen and Luostarinen 2001, Sandoval-Torres et al. 2010). Reduction of timber grade can result in increased production quotas to meet demand which is a concern from environmental, sustainability and financial perspectives, but are primarily driven by architectural market preferences. This study examines process induced timber discoloration found in existing literature, and presents initial conclusions from a case study examining process induced discoloration in Tasmanian Blackwood (Acacia melanoxylon (R.Br)).

An international literature review was conducted to establish the causes and types of discoloration that had been observed from old growth and plantation forests harvested timber , as well as if and how those discoloration types had been mitigated or eliminated. Concurrent to the literature review, site-based rack surveys were conducted during final processing of Tasmanian Blackwood (Acacia melanoxylon (R.Br)) timber, to establish the location and severity of discoloration within each board that may be resulting from the drying process. The rack surveys were conducted over a full year in order to determine whether seasonal variability is a factor and included data on location of each discoloured board, location and orientation of each rack, duration of drying and season when milling occurred.

The literature review established five causes of observed discoloration, those being; biotic induced, light induced, chemical induced, resin exudation and extractive movement (Hon and Minemura 2000, Scheepers 2006). Several of these types of stain, were divided into multiple sub-categories with specific causes, appearances and mitigation measures.
The site analysis has identified two dominant types of discoloration, which appear to be associated with the timber slats (called stickers) used to separate layers of timber during the drying process. Both types of discoloration were found to increase in frequency and severity with decreasing cross-sectional area of board but increased surface area.

The preliminary analysis of the site-based observed discolouration has started to identify trends that may include rack orientation, drying duration and log history. The next stage of the research will include an in-depth statistical analysis of the multi-variant data that has been collected form the site-based sawmill surveys for each individual board. It is hoped that the results of this rack survey will lead to an understanding of what factors are contributing to process induced discoloration and contribute to solutions to existing problems, particularly in Blackwood and similar species where visual appearance is of importance in the marketability of that timber.


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