On the use of miniaturized wood specimens in fungal decay experiments - mini-blocks versus EN 113 test specimens

Preliminary evaluation and classification of wood durability against decay fungi using laboratory monoculture experiments has been a longstanding practice because these tests offer quicker results than field tests. Various methods, including miniaturized specimen formats and different incubation periods, have been explored to expedite decay processes and assess wood preservation efficacy. The 'mini-block' format, suggested in 1978, has gained popularity due to its rapid decay and suitability for testing. However, debates persist regarding incubation times and specimen sizes impacting decay rates and wood preservative evaluations.

This study aimed to compare decay and durability classifications between standard specimens and mini-blocks across different incubation times for various wood species and fungi. The experiment utilized two specimen formats: standard EN 113-2 (2021) specimens (15 x 25 x 50 mm) and mini-blocks (5 x 10 x 30 mm) exposed to two fungi (Coniophora puteana and Trametes versicolor) for varying incubation periods (6 to 16 weeks). A total of ten wood species were tested and the durability of wood species was classified per EN 350 (2016) standard based on median mass loss percentages.

This study clearly demonstrated that mini-block trials yield dissimilar outcomes compared to the European standard test (i.e. EN 113-2, 2021) at 6, 8, or 10 weeks of incubation. This discrepancy extends to both durability classifications based on median percentage mass loss and those based on x-values. When incubated with T. versicolor, the data provided a more consistent comparison between the two methods than incubation with C. puteana. Hence, while the mini-block tests can be considered useful as a rapid initial screening tool, e.g. for evaluating new wood protection systems, caution is warranted for its use in durability classification due to the potential for generating misleading results. Therefore, the main conclusion drawn from this research is that the mini-block test is not advisable as a substitute for conventional durability classification assessments.