White Spruce Sapwood – a Tricky Beast to Dry!
Our technical experts offer some advice on how to handle this common drying issue.
by Marc Savard and Peter Garrahan
he question often arises when visiting a sawmill processing white spruce: What should we do with heavy sapwood pieces? Should we dry them with fir or should we keep them with our regular spruce? Our immediate reaction is that the heavier, wetter, sapwood pieces will take longer to dry than the lighter, drier, heartwood pieces. If the mill is set up to sort by board weight, these pieces will get put into a slow drying category. Is this the best solution? If your mill processes any amount of white spruce you are no doubt faced with this question. The following information may help explain how and why this material is different and how it can be handled to minimize the problems it poses during drying.
White Spruce
White spruce has a large geographic distribution across the country, as shown in Figure 1. The size and form of white spruce varies significantly across this range from very large and fast-grown to small and non-merchantable. White spruce is typically cut and processed with other spruce species such as red spruce in the Maritimes and Engelmann spruce in B.C. It commonly grows in the same area as balsam or amabilis fir and is therefore often mixed with one or the other of these species for processing.
On its own, white spruce can be dried on relatively aggressive schedules with good results. Final moisture content uniformity is always a concern when drying any spruce species, including white spruce. This is due to the significant variability in initial moisture content (MC), which is mainly caused by heartwood and sapwood differences in moisture content within this species.
As an example, the FPInnovations manual “Drying Spruce-Pine-Fir Lumber,” shows the results of a sample taken at one B.C. sawmill where the initial moisture content of lumber pieces varied from 30% to 255%! Other samplings conducted across the country have shown that green moisture content of white spruce sapwood averages 144% versus 38% for the heartwood part of the logs. This is bad news when you are a kiln operator and there is no specific sorting strategy developed at your mill. How can we expect lumber with such differences to dry in the same time? In most species, the differences between heartwood and sapwood are accounted for by the differences in drying rates achievable in each. In other cases, for example, black spruce, the log size and proportion of sapwood present precludes the presence of boards containing pure sapwood. In white spruce, however, the log sizes are often large and the proportion of sapwood is greater. Therefore, there will be many pieces that are purely (or predominantly) sapwood as well as pieces that are pure heartwood. There is a lot more water to remove from a pure sapwood piece than from a pure heartwood one.
Implications for Drying
As shown above, sapwood can contain up to eight times more water than heartwood. Regardless, it is normally more difficult to remove water from heartwood than sapwood. This is due to permeability differences between the two wood types. We can see this by examining some drying curves of lumber pieces that we dried several years ago as part of a research project on pre-sorting. In Figure 2, we can observe drying curves that were developed from lumber pieces containing either mainly heartwood or sapwood. We can see an initial MC level from 140 to 190% MC for the sapwood pieces compared to less than 50% MC for the heartwood pieces. After 30 hours of drying, both groups of pieces show similar moisture content. As mentioned above, it is more difficult to remove water from heartwood than sapwood. From the beginning, to almost the end of drying, the sapwood pieces in these tests dried faster (4% to 10% MC drop/hour) than the heartwood pieces (1.75% MC drop/ hour). This drying rate difference of five times or more faster, clearly demonstrates that sapwood cell structure is significantly different from heartwood. In the standing tree, sapwood is used to store and transport sap (water) from the roots to the needles and therefore needs to be permeable to allow the movement of fluid. On the other hand, the role of heartwood is to provide structural support for the tree. As the tree ages, the heartwood undergoes changes that make it less permeable. This includes the deposition of organic deposits (extractives) within intercellular pathways. This is the underlying reason for the different drying behaviour of heartwood versus sapwood.
As mentioned earlier, white spruce is often processed with fir in sawmills. Drying these species together poses a problem, as balsam/amabilis fir will typically take two to three times as long to dry. Figure 3 shows the same white spruce pieces but with the addition of a typical piece of balsam fir. The initial MC of the balsam fir piece is approximately the same as that of two of the three spruce sapwood pieces. It can be observed that the drying rate of the balsam fir piece is very similar to that of the heartwood spruce pieces even though the initial moisture content of the fir was over 140% MC. It clearly demonstrates that initial MC is not the only factor to consider when predicting drying time of specific lumber pieces. In this case, the abnormally high initial moisture content of fir heartwood combined with a slow drying rate is the reason for the longer drying time.
Let’s take a closer look at the spruce drying curves in Figure 2. In this example, we can see that if the drying decisions were based on heartwood pieces, the kiln should be stopped at around 21 hours in order to achieve a final MC of 19% or less. At that time, however, the sapwood pieces would be way over the limit. Table 1 shows the MC evolution of the five pieces at two-hour time intervals. It shows how critical the kiln shutdown is on sapwood pieces. An early kiln shutdown by only a few hours will result in a very high final MC in sapwood pieces that can then cause problems for the planer and kiln operators. This often results in overreaction on subsequent charges and over-drying is the result.
Summer Versus Winter
Some lumber grading inspectors and mill quality control staff have reported more problems with kiln “wets” in sapwood spruce pieces during the winter. They often attribute this to more difficulty in thawing the frozen sapwood. It is evident that these pieces require a lot more energy to thaw due to the extra amount of water present. But one must consider the laws of thermodynamics and what happens when these pieces are exposed to a warm, humid environment at the early stages of drying. Condensation will occur on any piece of wood where the surface temperature is below the dew point. Some pieces (especially heavy, wet sapwood pieces) will not be able to keep up with the kiln heating rate because they need extra energy and time to thaw and heat up. As a result, they will remain below the dew point for a period of time. During that time period, they will develop condensation on the surface, which not only delays the start of drying, but also adds moisture to the wood. They will get an extra energy boost from the condensation, but the result is a longer overall heat-up time for the load. During this heat-up period, the lower MC pieces of spruce will start to dry whereas the higher MC sapwood pieces will be delayed. The result is more variability between the two extremes than would exist under summer operating conditions.
Now let’s consider what happens in the summer. In summer, natural air drying is happening between the sawmill outfeed and kiln infeed. Even a few days of storage in the “green yard” can result in significant drying, especially of sapwood pieces. The fast drying rates for sapwood and differences in drying rate between sapwood and heartwood that happen in the kiln will also happen in the air drying yard. The difference in drying rates between sapwood and heartwood pieces is often enough to allow these very different pieces to be dried in the same kiln load.
Pre-sorting Dilemma!
Mills that have pre-sorting capabilities (visual, weight-based, species-based, etc.) have a decision to make with white spruce sapwood pieces in winter. Leave them with spruce or not? As we demonstrated, theses pieces need a little extra time to dry so drying them with normal heartwood pieces means accepting more kiln wets or extending the kiln run for a couple of hours with the risk of over-drying the rest of the load. Removing them from the rest of the spruce means that we then need to deal with them with another product. For mills that are already sorting balsam fir into two or three categories, they could consider mixing heavy sapwood spruce pieces with one of their fir groups. For example sapwood spruce could be dried in winter with the fastest fir category.
All of the above goes to show that there are no hard and fast rules for presorting. Even when we conduct detailed analysis (using our OASiS software) of pre-sorting options for mills, it is always based on a snapshot view of the log supply and time of year. Pre-sorting is a dynamic process that requires constant monitoring to make sure the strategy being employed today is the best for the material being processed. It is not uncommon to have to change our sorting or drying strategies between summer and winter. This article has covered just one aspect of material properties in SPF that affect pre-sorting decisions.
