Drying

Air drying alone was initially used by the industry, generally taking from one to three years depending on thickness and stacking pattern. Unfortunately the need for great care in carrying out this seemingly simple operation was often neglected and so much distorted or incompletely dried product found its way to the market that it developed a bad reputation vis-a-vis imported softwood that took many years to overcome.

Kiln drying was introduced about 1880. Various methods were used -the compartment system in which the timber was held stationary and the temperature and relative humidity of the atmosphere varied, the progressive system (such as the House system developed in 1912 by the Victorian Forests Department) where the timber was moved first through an atmosphere of steam and then through one of hot, dry air, and the Erith system in which it was moved counter-current to an incoming stream of hot, dry air. Although these methods reduced drying times down to a few weeks lack of adequate control and proper understanding gave an end result that was generally far from satisfactory. In other countries too the kiln-drying of hardwoods presented a problem and it was not until towards the latter part of the First World War that sound kiln-drying principles and practice were established for hardwoods, largely due to the work of Tiemann in USA.

With some Australian hardwoods, notably those of lower density, a further problem often arose during the early stages of drying, namely that of 'collapse', caused by buckling and flattening of the fibre walls and producing high and irregular shrinkage, surface distortions and internal checking. About 1920, however, two Victorian sawmillers, J. and G. Grant, accidentally discovered that collapse could be permanently removed by steaming after drying, a finding that was greeted by some scepticism at the time. Shortly afterwards, Tiemann was invited to Australia by the Forests Commission of Victoria and among other activities made an investigation of the Grants' process which confirmed its claims. After the CSIR Division of Forest Products came into being in 1928 the process, which by then was only in use in a few mills, was further studied and practical recommendations evolved for its application.[49] This led to its widespread adoption and it has since become part of the standard drying technology for many eucalypts.

Prior to the Second World War CSIR started a very successful program to up-grade the standard of kiln-drying by establishing recommended conditions for various woods, testing and modifying the performance of commercial kilns and training operators in proper control practices. At the same time it continued to seek ways to improve kiln-drying practice and this led to the development and wide acceptance of air drying to fibre saturation moisture content (25-30 per cent) followed by kiln-drying to equilibrium moisture (10-15 per cent) as a more economical and readily controlled drying process for the eucalypts. Since the early 1950s simple pre-driers have been introduced in some mills to dry to fibre saturation point or below without climatic constraints.[50] The two stage drying process, interposed with steaming (re-conditioning) to remove collapse and followed by a short high humidity treatment to relieve stress, has, together with the introduction of better control methods and kiln design, done much to improve the image of kiln-dried timber as a quality product in Australia. A variation employed in some Tasmanian and Victorian mills involving pre-steaming the green sawn timber was developed by Campbell[51] to increase drying rate and reduce checking.

Australian Academy of Technological Sciences and Engineering