The same processes that cause heartwood densification (improving durability) also prevent the penetration of preservative solution. This is not too much of a problem if the heartwood has a ‘good’ durability classification (Class 3 or better e.g. ‘old growth’ pine, larch, douglas fir; see Module 5), because it will resist decay in ground contact for 10 – 15 years anyway. But it may be a problem if the heartwood is less durable (Class 4; may last less than 10 years untreated). In fact, practical experience over many decades has indicated that even those softwoods with lower durability heartwood (Class 4 e.g. Norway spruce and most UK grown pine/larch/douglas fir) can still perform well in the field even though preservative penetration is limited.
The part of the wood which always needs to be preserved is the sapwood. Well-seasoned sapwood will always absorb preservative relatively easily via the end grain i.e. via the exposed open-ends of cells/tracheids. However, we also need to achieve good penetration/uptake laterally (through the sides of the conductive cells). This can happen because all xylem cells are connected to each other by a large number of holes called pits but in seasoned wood these pits may or may not remain ‘open’.
In general, the ease with which preservative can penetrate through the pits depends on how big they are (this is a simplification, the whole process is still not completely understood). The important thing to remember is that some species (for example Scots pine) are easier to treat than others (for example, Spruce). This feature of wood is called its treatability. The treatability of both sapwood and heartwood of many commonly used species is listed in BS EN 350 and some examples are given in Module 5 where its practical effect on timber treatment are described and explained. Another key factor limiting the treatment of wood is its moisture content.