Drying Eucalyptus nitens: Screening for checking and collapse
By Rosie Sargent, Doug Gaunt, Steve Riley, Toby Stovold, Mari Suontama, Grant Emms, June 2017.
Download SWP-T022 (pdf)
Eucalyptus nitens frequently develops within-ring checks and collapse (washboarding) during drying, making the wood unsuitable for use as sawn timber. Numerous studies have tried altering drying conditions to reduce levels of checking and collapse, but this has not been completely successful, and low recovery due to drying defects remains a major barrier to its use for sawn timber.
Previous nitens drying research at Scion aimed to see if pre-treatments could be used prior to drying to reduce levels of checking and collapse in dried timber, but this work highlighted issues around log selection and drying technique, which may have led to low levels of and collapse
The current study aimed to ensure that boards used in drying studies had a wide range of checking behaviours, and that the effect of lab-scale drying methods on final levels of degrade could be understood. For this, 200 trees were screened using a variety of non-destructive measures, and these were compared to numbers of checks counted in discs cut from 100 of the trees. From this data the numbers of checks expected in the remaining 100 trees were predicted, and this was used to select 30 trees to be sawn into boards for a drying study. Discs cut from the 30 selected trees showed very little correlation between the number of checks predicted and the number actually counted. For discs cut at breast height the correlation was slightly stronger than that for discs cut 3m up the tree (at the height the logs were cut).
The sawn boards were dried using 8 different drying techniques, and following drying the boards were docked at once end and assessed for checking and collapse (grouped as ‘degrade’ for the purposes of this report).
Due to delays in shipping discs and logs to Rotorua, currently (30 May 2017) the drying study got underway later than planned. Six of the eight drying schedules are complete and the final two drying treatments are expected to be complete in late 2017 or early 2018.
Drying temperature had a significant effect on levels of degrade seen in the boards - boards dried at 50°C had significantly higher levels of degrade (53% of boards with degrade) compared to boards dried at 20 or 25°C (~15% of boards degraded). Boards that had been frozen showed no improvement in degrade compared to boards that had not been frozen. Boards dried with varying temperatures (air drying) had slightly higher degrade than boards dried at a constant temperature. In future, drying work should use drying conditions that change between day and night because this will better replicate drying done in industry. Despite the differences between drying methods, some logs produced boards that consistently had very low or very high levels of degrade, even in the very severe, or very mild drying methods.
To understand the relationship between the non-destructive screening measures and the check propensity of each log, the levels of degrade were averaged across all the boards cut from each log. This ‘log condition’ value was used to create a linear model linking to predict log condition from the non-destructive measurements. This model was reasonably effective at predicting the log condition (R2 = 0.44). This suggests potential to screen standing trees to select those which are less likely to develop degrade during drying.
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