In order to determine the liquid absorption and the penetration different heartwood species were treated with a boric acid containing solution. Heartwood of six wood species (Fir, Spruce, Douglas fir, Scots Pine, Oak and Black locust) were used in these investigations whereby the dimension corresponds to the EN 252 format. One half of the stakes were end sealed before treatment. All stakes were impregnated by means of two vacuum pressure processes whereby the short process lasts two hours and long one 24 hours. Retention was determined by weighing while penetration of boron was measured using a two component system consisting of salicylic acid and cucurmin. Following ranking regarding the retention was found independent of test parameters: Fir >> Scots Pine > Douglas fir  Oak  Spruce >> Black locust It could be shown that remarkable higher retentions were determined for Douglas fir compared to Black locust although both heartwoods are classified as “extremely difficult to treat” according to DIN EN 350-2 (1994). From the practical point of view it is recommended that information concerning penetration as well as liquid absorption should be published together in future.

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