By Dave McRae and Peter Garrahan FPInnovations
As lumber drying specialists, we at FPInnovations are constantly asked for the best method to set pitch in Douglas-fir and pine species. While we always give the same general advice, we think it is worthwhile to dedicate this article to addressing a few of the questions and typical scenarios we run into.
By Dave McRae and Peter Garrahan FPInnovations
Pitch bleeding or resin exudation is a problem typically found in softwood species and, from within North America, mostly restricted to the various pine species and Douglas-fir. The reason the problem is more prevalent in these species is that they have a higher resin content and they are used in many appearance grade applications. Since the resins are naturally present in a semi-liquid state in the wood they can remain mobile and move to the surface under certain exposure conditions – especially when the surface is heated. The compounds present in the resin can interact with wood finishes, and therefore any pitch making it to the surface will be visible regardless of whether or not the wood is unfinished, painted, or coated with a clear finish. The solution is to expose the wood to conditions that will cause many of the more volatile components to evaporate and change the remaining material from a semi-liquid to a fixed (crystalline) state within the wood.
Door and window manufacturers seem to lead the charge for seeking ways to deal with the annoying and costly problem of pitch running in their product. For example, one door manufacturer had many call backs from his customers in California because of pitch bleeding through his product, while another had pitch running on his office door. He told us he also had call backs throughout Canada and the United States.
Millwork producers are not the only ones concerned about this problem. Companies that dry lumber and supply their product to remanufacturers are also looking for advice on the best way to set the pitch.
Solutions – What is the Right Temperature?
In general terms, the solution to pitch bleeding is to expose the wood to elevated temperatures for a given period of time. The question then becomes what temperature and time is sufficient? In the world of kiln drying, the advice one hears is that achieving a certain temperature for 24 hours will set the pitch.
Gene Wengert, A.K.A. the “Wood Doctor” and one of the premier drying experts in the world has a spot on his internet site called “the Sawing and Drying Forum”. He suggests running 180°F for 24 hours when the wood is dry – in other words, during the later stages in the drying schedule.
A study conducted at FPInnovations a number of years ago suggested that for red pine (a particularly resinous pine) that final temperatures up to 200°F may be required to achieve
a successful setting of the pitch. Many others maintain that temperatures of 160 to 180°F are adequate for setting pitch.
Our experience has shown that you can run 170°F for at least two hours in the final step and not have a problem with pitch running. The key is to ensure that the coldest piece of wood in the kiln reaches 170°F for two hours. Doing this at the end of the kiln schedule should not be a problem. However, care must be taken, as applying higher temperatures or running longer than necessary has the potential to over-dry wood and cause excessive degrade, which negatively affects the bottom line of
In some species, such as white pine, preserving the light colour of the wood is important for the end user. In these cases, running a slightly lower temperature, i.e., 160°F, for a longer period of time will achieve a similar result.
All of the above are correct and the particular solution for your mill will depend on the material you are drying and the degree of sensitivity of the final product (or customer) to issues with resin bleeding. The temperatures mentioned above are guidelines, but the proof is in the product. If you are still seeing problems with resin exudation, you are not running hot enough or long enough to achieve the level of pitch setting required.
Why is the pitch setting solution different for everyone? The answer to this is that every product has different exposure conditions. One of the rules of thumb on pitch setting is that the maximum temperature in the kiln must exceed the maximum temperature that the product will achieve in service or other stages of manufacturing. In that respect, think of a dark stained or painted wood siding on the south side of a building on a hot, sunny, summer day. Wood in these sorts of exposures could easily reach 120 to 140°F. Final steps in manufacturing can also raise the wood temperature beyond normal ambient conditions. Many mills use abrasive planing systems to do a final surfacing on glued up panels. Abrasive planers raise the wood temperature due to the friction created. Curing lines on finishing systems often use infra-red heating systems to cure the finish. Either of these situations could cause pitch that is not completely crystallized to become mobile and accumulate at the surface.
Implementation and Pitfalls
So, with all this advice on the internet and from many credible experts, why are many people still having problems setting pitch while others are not? It is because some people have not yet discovered the solution that works for them or they have kilns that are incapable of achieving the required temperature set points.
An important consideration in setting pitch is that it is the wood temperature and not the air temperature that is critical in achieving a successful treatment. This is one reason why setting the pitch at the end of the drying cycle is more effective. At the end of the drying cycle, the wood temperature will be at, or very close to, the dry-bulb temperature of the kiln. Earlier in the drying cycle, while there is still significant moisture evaporating, the wood temperature will be closer to the wet-bulb temperature.
There are many internet sites and articles in leading “wood related” magazines that say that kiln drying logs or lumber will set pitch. However, simply kiln drying logs or lumber will not set the pitch successfully. Setting the pitch is purely a function of achieving the necessary temperatures.
One company that was visited recently had this same “age old” problem with setting pitch in Douglas-fir. They had attended an FPInnovations seminar we conducted a few years
earlier and heard our suggestion on how to set pitch. They employed it and it worked – most of the time. They still found that 15 to 20% of their production had pitch setting problems. When we got to their site, we found that they had a number of identical kilns and they ran similar kiln schedules on each to set the pitch in Douglas-fir. After spending some time going over the schedules in each kiln, it was discovered that one kiln was unable to reach the temperature set points. Going back over a year’s worth of data showed that this kiln had always struggled to reach temperatures above 160°F while the other kilns had no problem reaching the required set points. The original solution for setting pitch was not the problem; the culprit was that particular kiln. Remember the important point here is not what your schedule or set-point on your controller are saying, but what actual conditions are achieved in the kiln.
For some, the solution to ensuring that the pitch will be set is to have their products custom dried in facilities that have good drying practices and where the kilns have the ability to achieve at least 180°F. Unfortunately, many millwork owners and some kiln operators still believe that you simply need to kiln dry and your problems will vanish into thin air.
How to Ensure that Pitch Setting Happens
If you rely on custom drying and setting pitch is crucial for you and your customers, then you should have that facility verify and prove what temperatures they can achieve.
If you are drying your own material, verify that your control system is reading the temperature in the kiln accurately; that you are achieving the temperatures suggested for setting pitch; and ensure that you have an adequate and balanced airflow throughout your kiln.
When businesses evaluate new equipment, they often want to know if the “drying technology” will work on setting pitch. The technology or drying method is often not the real issue – achieving the necessary temperatures to set pitch with the technology available is the issue. There are some technologies that are not capable of reaching temperatures above 150°F. For example, some dehumidification and vacuum drying systems are not designed to achieve the temperatures mentioned earlier. So if setting pitch in Douglas-fir or pine is important to you, we would suggest avoiding those particular technologies or specifying versions that can achieve the required temperature.
One way to test the effectiveness of your pitch setting procedures is to take several small samples of the finished product and expose them to elevated temperatures in an oven. If your pitch setting was conducted at 180°F, then setting the oven to 170°F is recommended. Once the samples have been exposed long enough to achieve the operating temperature of the oven, remove them and feel the resinous areas. If they still fell sticky or resin is left on your finger, the pitch setting has not been successful and you either need a longer treatment time or a higher temperature (or both).
In summary, it’s really no mystery at all. The temperatures required to set pitch in Douglas-fir or pine are somewhere between 160°F and 180°F and we recommend achieving those temperatures at the last stage in the drying process. In our opinion, this is a safe guard that not only sets the pitch, but also helps maintain the grade of the material while in the kiln.
The authors would like to thank Don Friesen, co-owner of Coldstream Lumber (Corporate Partnership) in Vernon, B.C. for his input and advice in the preparation of this article.
Dave McRae (firstname.lastname@example.org) is an industrial advisor with FPInnovations based in Ladysmith, B.C. Peter Garrahan (email@example.com) is a drying expert with FPInnovations based in Ottawa, Ont.