The liquid permeability of Japanese larch (Larix kaempferi) wood is extremely low, especially in the heartwood. It is difficult to achieve the penetration of wood preservatives, which is specified by standards, with pressure treatment of waterborne preservatives. Recently, the protection treatment with an oilborne preservative called “deeper penetration treatment” is commercially available, which achieve enough penetration even in the larch heartwood. In this method, the oilborne preservative is only sprayed to incised materials with some post-treatments. To find out the penetration mechanism of the deeper penetration treatment, we tried to visualize the penetration pathway of the oilborne preservative using the larch heartwood samples with and without pre-steaming. The pieces of larch heartwood, which were soaked the oilborne preservative colored with Sudan black B and the water colored with acid fuchsin under atmospheric pressure, were observed with a stereo microscope and a light microscope. The affinity between the larch resin and the oilborne preservative was investigated by mixing them. The acidic fuchsin aqueous solution hardly penetrated into the larch heartwood. The oilborne preservative colored with Sudan black B relatively well penetrated into the samples. The penetration traces of the oilborne preservative were observed in the lumen of tracheid, ray parenchyma, ray tracheid, and inside the axial and radial resin canal. Many of them were observed in the latewood and inside the axial and radial resin canal. The oilborne preservative quickly reached the opposite side through the axial resin canals connecting to the opposite side. The affinity between the resin and the oilborne preservative was high. It is thought that the oilborne preservative traveled quickly through the resin canals as the penetration route. The results of our present study indicate that the resin canal network is one of the factors of the high permeability of the oilborne preservative in larch wood.

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