Now showing 1 - 2 of 2
ItemAn evaluation of timber drying problems in terms of permeability and fine structureKininmonth, John Alexander (1931-) ( 1970)The relationships of difference in rate of drying to permeability and wood structure were determined for two angiosperms and one gymnosperm. These investigations took two particular drying problems as a basis for study and attempted to explain why: - heartwood of Nothofagus fusca (red beech) takes many times longer to dry than sapwood. - green sapwood of Pinus radiata (radiata pine) dries readily but, if dried and pressure-treated with water-borne preservatives, its subsequent drying is greatly retarded. Test material was used from 14 trees of N.fusca from New Zealand, four trees of Eucalyptus regnans (mountain ash) and seven trees of P.radiata from Victoria, Australia and the experimental work was carried out under three headings: (a) Unidirectional drying. Small specimens, sealed on all except one pair of grain faces, were dried in a laboratory kiln at temperatures up to 60C. Comparisons were made between radial and tangential drying in sapwood and heartwood or in green and resaturated specimens; effects of treatments such as steaming were also assessed. Moisture gradients were determined to show the contribution of free water movement to overall drying. (b) Permeability studies. A method was developed to measure the transverse permeability of green wood to the flow of micro-filtered water; established methods were used for longitudinal permeability. Data for P.radiata met the requirements allowing application of Darcy's Law for flow of fluids through inert porous media and N.fusca approximated them. Pathways of flow were determined with chemical stains. (c) Wood structure. The transmission electron microscope was used to compare the appearance of pit membranes and the cell walls in sapwood and heartwood of N.fusca. In P.radiata, emphasis was on determining the percentage of bordered pits that were aspirated in sapwood - green, after drying and resaturation and after various treatments - and relating this to differences in drying and permeability. The main conclusions drawn from this study are: (a) The green sapwood of N.fusca and E.regnans is permeable to micro-filtered water in the radial and tangential directions. After drying and resaturation, the permeability of N.fusca is unchanged but that of E.regnans is drastically reduced, particularly in the tangential direction. The heartwood of both species is impermeable when tested at a pressure differential of 40 cm.Hg. (b) Differences in the permeability of N.fusca can be explained by differences in the appearance of pit membranes in sapwood and heartwood: in heartwood, the membrane surfaces are usually completely occluded when viewed as replicas in a transmission electron microscope; in sapwood, the surfaces are always less occluded often exhibiting a clean primary well texture. It is inferred from studying the effects of various extraction treatments that the pit membrane surfaces in sapwood are less occluded than indicated by the appearance of replicas. (c) Plasmodesmata may provide pathways for mass movement of liquids in the radial direction in the wood, but, in other pits, without obvious pores, permeability probably results from movement through the general structure of the pit membrane. (d) Heartwood of N.fusca takes several times longer to dry than sapwood because of its reduced permeability coupled with lower rates of moisture diffusion. (e) Contrary to previous reports, at least 80 percent of the bordered pits in green sapwood of P.radiata are open, irrespective of distance from the outside of the tree. After drying and resaturation most pits are aspirated and the wood is much less permeable than in the green state. (f) The condition of the bordered pits has an effect on the rate of drying in the tangential direction - causing a marked reduction in resaturated material - but has no appreciable effect on radial drying which is little different in green or resaturated wood.
ItemStudies of the natural and chemically-enhanced resistance of wood to decayDa Costa, Edwin Warner Brandon ( 1977)The researches described in the attached papers followed a prolonged period of change and development. The initial purpose was to evaluate in the laboratory the probable durability of Australian timbers in service as poles, posts, sleepers and so on. (Papers A1, A2, A3, A4). These papers were at the time of publication more rigorous and comprehensive than any published tests. (In this connection, it should be noted that the “Progress Report” series of the Division of Forest Products were distributed to all known major research workers in the field, abstracted in major abstracting journals, and freely cited in publications. Divisional policy was at that time opposed to publication of results in scientific journals, rather than specialized reports with “targeted” distribution). They were especially notable in the variety of test fungi used; in the development of testing techniques which would allow decay in dense timbers (based on extensive so far unpublished experiments on varying technique parameters); in the use of statistical analyses; and in recognition that no single value can be assigned to “natural durability”, a theme developed in A14, my final definitive paper on natural durability, which should be a seminal paper for future work in this field. A major contribution has been the clear understanding that weight losses in decay tests cannot have any absolute value and that tests are essentially comparative. Classification of natural durability is therefore best obtained, not by arbitrary classification on weight losses, but by close comparison with “yardstick” timbers whose durability is thoroughly familiar. (A11, A12, A14). These papers, and several unpublished experiments of the same nature, contributed largely to the lists of natural durability of timbers published by the Division of Forest Products and used in Australian Standards (e.g. As 1604 – 1974, on preservation of sawn timber). Similar inter-species comparisons were later made of Papua New Guinea timbers (A11) where information on natural durability, vital to effective utilization of mixed hardwood rain forests, was virtually absent. From this work on inter-species variation in durability, there came an interest in the causes of this variation and this was investigated for several years in collaboration with Dr. P. Rudman (A3, A4, A7, A9, A10). The initial and critical paper in this series (A3) was a seminal paper in being the first paper to adopt a “toxicity balance” approach, in which the decay resistance of untreated wood, of extracted wood, and of susceptible wood containing these extractives was measured. This was important because previous workers had concentrated on the toxicity of specific extractives as the explanation of durability, neglecting the possibility that the wood might still be durable after complete removal of these extractives, due to other extractives or to physical factors. My approach also took account of the detoxification of extractives during removal and of the effect of their original distribution in the wood. The paper also contained the first description of a reliable and precise technique for determining decay resistance of wood meal of a variety of species. Since adequate extraction required conversion of wood to a finely-divided form, this technique was essential and was unexpectedly difficult to develop. The general design of the work the decay technique, the decay tests and interpretation of the results were the work of the senior author. This line of work on chemical reasons for decay resistance was gradually transferred to Dr. Rudman. Because of the reputation acquired from earlier work on natural durability, work on natural durability of teak (Tectona grandee) was requested by the Food and Agriculture Organization (FAO) of the United Nations. This was concerned with relative durability of plantation-grown and natural teak and with possible detrimental effects of high growth rates on durability. The resultant work (A5, A6, A7, A8) showed that the widely held belief that fast frown timber was less durable was, at best, only partially true. The papers were important in describing some of the few attempts to test this experimentally and in pioneering multiple regression analyses to assess the relative importance of silvicultural, factors in durability of teak and the potential importance of decay resistance testing in selection and breeding of teak and other timbers. The early stages of my research in wood pathology were confined to natural durability, but with the establishment and rapid growth of a pressure-impregnation wood preservation industry in Australia, and the world-wide need for more sophisticated treatments, my research activities changed to study of chemically enhanced decay resistance. After some preliminary investigations to solve urgent problems (B1, B2, B3), they took the form of an intensive study of water-borne preservatives (especially copper-chrome-arsenic preservatives or “CCA”) and factors affecting their efficiency (B4, B5, B6, B7, B16, B19); and also a study of factors affecting the performance of Australian [?] (B8, B10, B11, B12, B18, B20). CCA studies were important as being the most comprehensive studies to that date of fixed water-borne preservatives (B6) and as the first to discuss the effect of wood substrate on preservative performance (B7), a topic of international recognition in the 1970’s. My demonstration of the enormous variation in CCA tolerance of basidiomycete wood-destroying fungi (C1) led to a fundamental study of fungal tolerance (c4, C5, C6). This included the original discovery that basidiomycete cultures could be dedikaryotized by toxic agents (C2, C3) a discovery of considerable importance to general biology and experimental fungal taxonomy, as well as to wood pathology (C5). My later discovery that the toxic effects of arsenic could be antidote by phosphate (B16, C7, C8, B19, B21) is also one of fundamental importance to general biology, as well as to wood pathology, especially to techniques of standard testing of wood preservatives (C7, C8). The research on creosote established conclusively that removal of phenols from low temperature creosotes lowered their efficiency and led directly to a revision of the Standards Association of Australia specification for creosote (despite considerable opposition from manufacturers). The work on use of propylene oxide for sterilization (B10, B11) had important implications in view of its widespread use in laboratory tearing (e.g. in British Standard 338; 1961). I was probably the first worker to use propylene oxide as a sterilant for wood specimens (B3, C1) and have much unpublished data on its use, as well as the best statement to date (B11) of its limitations. Apart from work on CCA and on creosotes, I studied various organic solvent preservatives (B9, B13< B14) and also studied the use of special techniques for preservation of plywood (B15, B17). In general, these papers are regarded as making a substantial contribution to the science of wood pathology and of biodeterioration as well as having had some influence in the application of wood preservation technology in Australia and overseas. Many of the papers (e.g. A1, A3, A8, A10, A11, A14, B6, B9, B17, B20, C1, C2, C5, C6, C7) introduced novel concepts and techniques.