Kiln Temp for setting pitch?

[Bigshooter]

I just talked with a company that make fir doors. He said one crucial part of drying Douglas Fir is to get the kiln up to 185 degrees for 24 hrs to set the pitch. This will stop the pitch from bleeding. Is this true? I thought pitch was almost impossible to stop.

[Ianab]

Heat treating like that should pretty much eliminate any pitch bleeding problems.

What happens is the heat evaporates the lighter compounds in the pitch and drives them out of the wood. The stuff that's left is the heavier compounds, with a higher melting point.  These now wont melt and run, at any sensible temperature, like the mix of light and heavy pitch tends to. 

Then higher temp the wood is kilned too, the hotter the wood has to get later before the resins can liquefy again.

[xlogger]

If I dry some pine and sterilize it at about 140° would the pitch be set there?

[Don P]

It will be "set" to 140°. There is a big caveat in all this though.  An analogy might help think about it. Water boils at 212° so all the water will boil out of a kettle on the stove at 212. If the kettle is on the stove for 5 minutes at 212 will it boil dry? Sure if there is just a teaspoon full of water in it but if it is full it'll take longer. The same is true with setting pitch. If the wood is very pitchy or has pitch pockets it will take much longer at the pitch setting temp to cook out those volatiles and set the pitch to that temperature. If the sun sees the wood 140° is probably not going to be enough.

[GeneWengert-WoodDoc]

Don P is indeed correct, so that is why we use 24 hours and also why we prefer 180 F or hotter. 

Analogy continued:  If the water is on the stove and the burner is at 220 F, the water will boil slowly, but at 300 F it will boil quickly.

So, 140 F will not remove pitch with low boiling points (meaning pitch that will ooze out at room temperature or so) very quickly and will not evaporate all the "soft" pitch even if done for several days.

[YellowHammer]

I've had pretty good luck with 160° which is as high as I want to go in my Nyle kilns.  Upon inspection, the remaining resin in and on the boards is a very hard crystal like structure.  I haven't seen any drips or runs subsequent to this temp.  Higher is better, I assume, but this works for me.

[Bigshooter]

So my next question to this is what kind of Kiln do I need to build for this 185 degree pitch setting? I have a building that I am going to convert to a kiln. It is stick framed with a tin roof and a concrete floor. The dimensions are 12' wide, 20' long and a 10' ceiling. 185 is pretty hot for a wood building in my opinion. Should I just build something else ? Convert a sea can?

[ScottCC]

Thanks everyone for the info also, never knew this was possible to be almost able to stop pitch issues in logs.  As far as heat in a wood building, it would pay to continuously insulate interior and that would keep highest of heat away from wood structure for short high heat times.

[GeneWengert-WoodDoc]

If all you want todo is set the pitch, a wood kiln with insulation will work fine.  The issue with a regular kiln is that high humidities will lead to decay.  Note that the roof in a wood kiln is tricky, as rain and snow can cause decay unless you have a second roof over the top.  In the long run, a reefer might be a good idea if the insulation will withstand the heat.

[Bigshooter]

That's what I'm looking at now is a sea can. Simple to setup, made from steel, so easy to alter.

[scsmith42]

Like Yellowhammer I too set the pitch in pine at 160, but have noticed that it seems to harden about 15 degrees or so lower than the highest temperature that it sees in the kiln.

I have also noticed that the lumber will still leave a lot of sap residue on my millwork equipment (jointer, planer and wide belt sander). 

I presume that this is because the surface temp during the millwork process is higher than the hardened temperature of the sap.

[K-Guy]

scsmith said:
"Like Yellowhammer I too set the pitch in pine at 160, but have noticed that it seems to harden about 15 degrees or so lower than the highest temperature that it sees in the kiln.

I have also noticed that the lumber will still leave a lot of sap residue on my millwork equipment (jointer, planer and wide belt sander). 

I presume that this is because the surface temp during the millwork process is higher than the hardened temperature of the sap."

That is correct scsmith. Friction will raise it very quickly over 160f.

[caveman]

We recently ran a small load of longleaf pine in the hot box to finish drying and set the pitch.  The MC dropped to acceptable levels but after planning, we noticed some of the boards were weeping pitch after just sitting in the shop for a few days.  The temperature here has not gotten above the 80's since finishing in the hot box.

Should we have left the boards in the box longer?  They were in for 2-3 days with temps reaching over 160°F.

 

 Should we just plan on putting them back into the box and running them for a few more days at 160°?  IN the top photo, it looks like the sap is cooked out on the far end just about as much as the end with the fan and the lights. We may need to add another fan to circulate the air more completely but it seemed to me that the single, 3 volt fan moved the air adequately for such a small box.

I appreciate the advice and guidance from my experienced and knowledgeable Forestry Forum friends.

[GeneWengert-WoodDoc]

The standard recommendation is 180 F for rapid evaporation of pitch that will be liquid at room temperature.  This is for up to 1-1/2" thickness.  The time is at least 24 hours.  In your case, the wood is thicker, you have a cooler temperature, and the box does not have a vent to remove the evaporated pitch.  How thick is your wood?  If 3" thick, three days at 180 F, so maybe four days at 160 F.  But, I do not know about venting.

The time starts when you reach 160 F.  

I do not know if having a lot of evaporated pitch in a small, tight box, creates a fire hazard or explosion risk, especially with electric heat or lights.  Venting might reduce the risk.

[Don P]

That looks like serious fatwood too, it's going to take more time at temp to volatilize all the 160° and lighter compounds than less resinous wood. How long, not a clue, those are some occluded cells there.

[caveman]

We will run it again and see what happens.  Thank you for the input.  If it goes up in smoke, it is far enough away from other things that the loss will be the hot box and its contents and maybe the kiln.