Setting Pitch in Douglas Fir
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Temperature and time for setting pitch. February 28, 2004
Question
I have heard conflicting reports about how to set the pitch in Doug fir. Can someone clear this up?
Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
What have you heard? Setting the pitch is the same for all resin-containing species. Heat the wood to at least 160 F (180 F is often better) for at least 24 hours when the wood is dry.
From the original questioner:
One person told me that heating the wood to a given temperature, say 140 degrees for 24 hours, will set the pitch up to that temperature. That is to say, if the wood was exposed to a higher temperature out in the world, the pitch would begin to run again at that higher temperature. Another person, from a large kiln drying operation, told me that they get the wood up to 170. He said this does not always work, that some boards will continue to bleed pitch after this treatment. I am hoping that there is some definitive document on this that someone can point me to.
As far as the temp that sets the pitch, I am not a chemist, but could you explain in layman's terms what is going on here? Have tests been done to determine at what temperature this transformation takes place?
From Gene Wengert, forum technical advisor:
Pitch contains many (1000s) of chemicals. Some are liquid at room temperature, some are hard. (I believe the chemist would say "solid.")
At 120 F, for example, some of these chemicals would turn to vapor (that is the nice smell around a kiln), some of the hard ones would become liquid, and some stay hard. Basically, we would say that those chemicals in the pitch that have a boiling point under 120 F will be evaporated at 120 F. The chemicals that have low boiling points are the ones that are soft and liquid at room temperature. When the wood is cooled, some of the chemicals that were liquid at room temperature would have been evaporated, but not all.
Now let's jump to 160 F. We evaporate a lot more of the pitch. When the wood is cooled, the pitch chemicals that remain are almost 100% hard at room temperature... a few might be liquid, but would not flow very fast at all - they are thick. But give the wood a little heat (sunlight, near a fireplace, etc.), and these "almost hard" chemicals could soften and flow a bit. They can actually flow through paint films and plastic.
So, we learned that at 180 F we evaporate the chemicals that would be soft at room temperature and even a little hotter. You can still see pitch in pitch pockets, but it will not flow.
Questions?
There is a little, but not much, in the DRY KILN OPERATOR'S MANUAL. It is not as extensive as what I just wrote!
From contributor T:
I have a Nyle L50 kiln unit. I have never got my kiln temperature above 120F. Do I understand that this unit is capable of higher temps to set pitch? I could add auxiliary heat if unit is not capable of higher temps. Is 24 hours at a high temp sufficient?
From Gene Wengert, forum technical advisor:
Ask NYLE about how they would suggest you set the pitch. I think that Don Lewis answered this question recently. Twenty-four hours at 180 F is considered long enough for setting the pitch.
From the original questioner:
I have been looking for someone with a kiln near Santa Cruz, CA who can achieve a temperature of 180F. The best I can do (so far) is a fellow with a kiln that will go up to 160 F. I have approximately 3000 board feet 8/4 VGDF with which I am having a pitch bleeding problem. It was dried by a small time operation at a low temperature about two years ago. It is about 12 to 15% moisture content right now. What schedule would you recommend for the kiln available to me? I thought I might take a few pieces and try and duplicate the schedule in my home oven as a test. Do you think this test would tell me much?
From Gene Wengert, forum technical advisor:
How is your oven heated? The emission from the wood is flammable, in addition to smelling good. I do not think you should heat it in a confined space. With a gas oven, it is likely to be very risky.
You can make you own heater, I would think. If the person that can go to 160 F is DH, then he can go hotter if the DH unit is shut off and safe auxiliary heat is used.
From the original questioner:
The kiln owner tells me that it is a hot water kiln and that 160F is the max he is comfortable with. Any ideas on how I should go about assessing what this will do for me? Can I make up for the lower temp, in at least some measure, by increasing the duration at that temp? What schedule do you recommend?
From contributor D:
I dry about 20,000 feet of DF a month and have done so for going on four years now with a shop built 4x4' wood burning furnace with a lot of fans, plenty of plenum space and regular rotation of carts in the kiln. We occasionally get over 150 degrees, but it is my experience that what is needed beyond temperature is time. This whole process seems to be about giving the liquids in the wood time to migrate from the center to the surface which process the heat augments. As for setting the pitch, some do and some don't, in my experience. Unlike pine, which crystallizes nicely, DF is quite slow to cooperate. I have had the kiln up over 170 on a few occasions when someone was over-firing it and I don't think it makes a difference. I have yet to see DF actually make hard, powdery crystal sap like pine. It gets quite stiff and goes white on occasion but it is not really a crystal at all. I certainly yield to Dr. Wengert's expertise in this, as I cut my sawyer teeth on his literature put out by Wood-Mizer, but this is my experience.
As for the comment about 'some small operator' or the like, I would contend that we little guys do a better job. Just go visit some mammoth kiln some time and tell me they actually care about the wood... not.
From Gene Wengert, forum technical advisor:
The pitch that is set looks just like the pitch that is not set... it is something that cannot be judged by eye. The pitch can be partially set as the temperatures increase. Many use 150 F or 160 F; I like 180 F as it goes faster and has a bigger safety margin.
From contributor D:
I re-read the initial response by the Dr. and I yield! I worry about burning the thing down, though, as we have already had a ceiling fire, which by dint of having hoses ready, fire extinguishers (many) ready and a 500 gal tank and many buckets and willing hands, we got put out. This happened on a hot summer afternoon with a green metal roof and combustion happened about 18 inches from the triple wall chimney.
We have now gone to a ten inch heavy steel pipe set in solid masonry and block! Another problem with getting real hot is killing fans. I am using poultry barn fans which hold up quite well but I have killed a dozen less hearty models. I guess hotter is better... can't argue that!
Dr. Wengert, this thread brings me to my big question of the year: We are getting serious about a new plant, which will necessitate new kiln(s). I am thinking about multiples of 10,000 feet. I know you don't want to recommend a brand, but would you steer me towards a variety or brand type, i.e. Nyle (who I am leaning toward), that is capable of reaching 180 degrees on a cold day? I have read that Nyles don't have the high temp ability. I would value your opinion highly.
From Gene Wengert, forum technical advisor:
Regarding your ceiling fire, wood dust will ignite at fairly low temperatures - well under the 450 F that is commonly used for solid, large pieces. As with all fires, you can limit them if you keep the kiln doors closed until the fire hoses are turned on and delivery water. As soon as the door opens, the fire will flase with the oxygen that comes into the kiln. The water hose will control this well. I always suggest that you frequently discuss kiln fires with your local fire department personnel. Make sure they know that there must be water delivery before the doors are opened.
Regarding fan motors, if the fans are inside the kiln, class H motors will provide the durability needed. The heat can also melt lubrication in the bearings, so be careful to use the correct lube. If your motors cannot stand the heat, then mount them outside and use a shaft with bearings running into the kiln.
Nyle does have kilns that can reach 180 F (and maybe even hotter... I seem to recall that 220 F is the peak). Their conventional kilns run around 150 F. When choosing a kiln, the first choice is the heating system - hot air or steam, and then the source of energy - natural gas, wood waste, electric, etc. Then choose a building construction type - wood, masonry or aluminum. Then choose a size. Then choose a loading system - track or forklift. Finally, choose a control system. I urge you to consider a kiln similar to those around you rather than getting a kiln that is not common to your part of the country. Why? Because your neighbors can provide helpful advice, operators will be easy to find, repairs and maintenance are easier, etc. I guess I am a little conservative and so feel more comfortable knowing I have a standard design and there are helpful folks nearby.
From the original questioner:
I am still curious to hear what you recommend for a schedule, given the limitations of the kiln available to me. I would like the wood to end up at about 8 to 10% moisture content, 12% is okay too. Do you need more info about the kiln?
Regarding setting the pitch, I am still trying to understand the chemistry. Do I have this right: we are not talking about polymerization, but rather driving out volatiles, and this will take place faster at 180F than 160F, but if the wood is kept in the kiln longer at 160 the end result will be the same?
From Gene Wengert, forum technical advisor:
Indeed, we are driving off the volatiles. A longer time at 160 F is close to a shorter time at 180, but 180 potentially will drive off a few more volatiles.
Schedule? For VG D-F? At what starting MC and ending MC? I would just look it up in the kiln schedule book, using a lower temperature, as required by the kiln.
From the original questioner:
This full sawn 8/4 VGDF is at 12 to 15% now and I would like to finish at between 8 and 12%. Again, the primary objective is setting the pitch and the top temperature I can go is 160F.
