Liquid water flow and discolouration of wood during kiln drying
Date
2006-03
Authors
Scheepers, G.C.
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch: University of Stellenbosch
Abstract
The discolouration of South African softwood during kiln drying can reduce the value of furniture grade
lumber. Thermal discolouration of wood, as found due to heat treatment, produces a homogeneously
browner colour in wood than is normally expected. This type of discolouration is attributed to reactions of
the macromolecules present in wood and is found in both hard- and softwoods. Yellow stain and kiln brown
stain can severely alter the colour of the outer few millimeters of a wooden board and is attributed to the
reaction of water-soluble sugars and nitrogenous compounds, present in the wood sap, after deposition at
the wood surface due to liquid or capillary water flow during drying.
A discussion of the mechanism of discolouration due to yellow stain and kiln brown stain would be
incomplete without a good understanding of the liquid flow of water during drying above fibre saturation
point. This thesis brings the two concepts of liquid water flow and discolouration in context and is presented
in four chapters:
• an introduction motivating the aims of the investigation (Chapter 1);
• a literature review of factors which may influence discolouration and liquid water flow during
drying (Chapter 2);
• original manuscripts describing the discolouration of South African softwood and liquid water flow
in hard- and softwood (Chapter 3); and
• a final conclusion that links up the results from the investigations (Chapter 4).
The investigations into the occurrence of yellow stain and kiln brown stain showed that the intensity of these
types of discolouration was influenced by geographical origin (and/or climate), tree species, planing depth
of dried lumber, and kiln schedule parameters like dry- and wet bulb temperature and time. The
characteristic discolouration pattern of yellow stain and kiln brown stain indicated that this stain type was
related to the wetline phenomenon that is found during the liquid water flow phase of drying wet wood.
Thermal discolouration, on the other hand, occurred homogeneously throughout the volume of lumber and
is, therefore, not related to free water flow, but to chemical changes of the macromolecules in wood.
The results of the liquid water flow investigations support the invasion percolation theory of drying that
states that the largest meniscus will retract into a drying liquid-filled capillary network until it is not the
largest meniscus anymore. Fluctuations in the rate of moisture loss from the cores of wood pieces above
fibre saturation point were also found. The pattern of fluctuation differed appreciably between Betula
verrucosa and Pinus radiata. In both cases, the start of the last phase in rate of moisture loss from the core
coincided with a reduction in the cross-sectional area of the drying wood piece. This behaviour is explained
by the hypothesis that distinct capillary size classes are emptied of free water, in order, from large to small.
As smaller capillaries are emptied, the capillary forces become greater, to the point where the forces are
great enough to cause permanent or temporary deformation of the remaining water-filled capillaries. Classification and regression tree analysis was a useful statistical technique to analyse a large multivariate
dataset. The importance of kiln schedule temperatures and planing depth to control yellow stain and kiln
brown stain was clearly pointed out by the technique, which can help to simplify the control of colour quality
during the industrial processing of wood.
Description
Thesis (PhD (Forest and Wood Science))--University of Stellenbosch, 2006.
Keywords
Theses -- Forest and wood science, Dissertations -- Forest and wood science