It is anatomical structure which determines liquid flow rate within different timber species. This cannot be deduced from properties such as density or rate of growth, and can only be found by experiment. A standard test has been in use at the Princes Risborough Laboratory for many years, employing a pressure-impregnation treatment, and a large number of timbers have been studied. The test consists of impregnating timber specimens by a full-cell treatment with a standard grade of coal-tar creosote. Heartwood sticks 50x50x1100 mm³ are selected from various positions in different logs; where possible, specimens comprising wholly or partly of sapwood are also tested. The uptake of creosote is determined be weighing each specimen before and after treatment: penetration is measured after the specimens have been cut longitudinally and transversely. It will be clear from published results that in both softwoods and hardwoods there can be considerable variation in absorption by different samples of the same species. Variation is more obvious in timbers that are difficult to treat; permeable species are more consistent in their behaviour. Sapwood and heartwood frequently differ greatly in permeability and their response to treatment must always be considered separately. Sapwood generally can be impregnated easily, even when the heartwood of the same species is extremely resistant to treatment. A characteristic feature of many softwoods is that penetration occurs more easily in the late wood than in the early wood. In resistant softwoods often the only lateral penetration is that occurring tangentially along the late wood rings. Consequently, many softwoods exhibit a streaky pattern of penetration when examined. In the pines however lateral penetration also occurs in the radial direction through the rays and a 'solid' zone of treated wood is thus obtained. The type of penetration obtained also varies in hardwoods. In some, eg beech, lime, ramin, both the vessels and the fibres are penetrated throughout to give complete impregnation. In others, eg celtis, gurgun, keruing, all the vessels are penetrated with creosote, but none of the fibres. Creosote is often only loosely held by these woods and a considerable amount may drain out if they stand upright for any length of time. The preferred pathway for penetration into hardwoods is in the longitudinal direction of the wood; little or no liquid movement takes place laterally even in permeable material.

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