Moisture variance in red oak

      Basic drying procedures and a discussion of bacteria in red oak. February 13, 2001

Q.
I am looking for information on drying northern red oak. I am finding a large variation in the moisture content, from overdry to wet. The wood is from the same set of kilns and cut at the same sawmill. A consultant came years ago and said the only thing he could think of was a fungus.

Forum Responses
These thoughts come to mind:

1. Not going to equalizing step at the correct time. In other words, if you are going to equalizing when your lowest sample is 5%, how are you turning up with overdry?

2. Not equalizing long enough. This should take care of your wet pieces.

3. Instrument calibrations off (RH/temp sensing devices not giving you correct readings).

4. Air flow problems, i.e., fan motors out, fan motors not all going in the right direction, line shaft broken, reversing timer device malfunction?

5. Exiting MC data needs to be obtained from freshly cut pieces along with conventional oven tests. The sample data used from the beginning and throughout the charge does not suffice for accurate data towards the end of the charge.

6. Large variance in MC of lumber at start-up. Could be due to improper mixing of lumber from varying sources--green and predried, predried and shed dried, etc.



The above is a good "shopping list" and I would add that it is possible in the best drying operations to get oak lumber so that about 95% of the MCs are + or - 1% from the average. This takes time to get it so close--most have more variation. Less time = more variation. (Bad idea: They will often send as much variation in MC to the customer as the customer will accept without complaining.)

Gene Wengert, forum technical advisor



Red oak usually has bacterial infection. Which causes wet pockets. This is overcome by the above methods. It just takes more time.


I have to disagree with the statement that red oak "usually" has bacteria. Maybe 1 in 20 logs and then maybe 5% of the lumber in that log. Often the bacterial infection is in the lower grade, which is not kiln dried. It takes perhaps 75 years before a red oak tree will typically have bacterial infection in the merchantable wood.

Gene Wengert, forum technical advisor



From the original questioner:
Gene, when you refer to lower grade lumber, are you talking 2 and 3 common material? That is what we dry. Also, what does a bacterially infected oak log look like?


Indeed, No. 2 and No. 3 Common material.

Regarding bacterially infected wood, its identification is difficult. However, red oak that is over 90% MC, that smells terrible, and perhaps has some colors that are darker than normal would be highly suspect.

Also, whenever you shake (separation parallel to the rings), you can be 99% sure that it is bacterially infected. The problem is that shake is only in part of the bacterially infected wood.

There is a report out about detecting bacterial presence with acoustics, but the report has a flaw. The way they determined if there was bacteria present was visually (they divided it into none, light, medium and heavy infections) and then they confirmed that the acoustic readings were different in these pieces, meaning that the acoustics worked. The problem is that visual identification is nearly impossible--if it was easy then why use acoustics? In this research, the ID was done by a technician that has probably dried under 10,000 board feet of oak in his lifetime! The same group also "proved" that DH drying dried oak with higher quality than conventional--of course, the conventional kiln was a lab kiln which will have more severe conditions than in a commercial kiln, due to temperature drop and RH rise in a conventional kiln.

Gene Wengert, forum technical advisor



From the original questioner:
So how can we further prevent shake when we already end coat and shade dry (black cover)? I am looking at changing our kilns to a gentler schedule. Please give me some trouble shooting for this.


The shake is an indication that the wood in the tree has already been substantially weakened, so there is no cure in the lumber. As I have stated before, most people cannot afford to slow dry enough to prevent damage in bacterially infected wood. Even slow drying conditions will not give a satisfactory product in most cases. If you do dry it okay, there is always the issue of having the smell return. Plus, when machined, it develops excessive defects. So, eliminate it from the lumber stream--maybe sell it green or saw a RR tie. Always saw 4/4 if the log is suspect. Always dry lumber with the best, but not excessive, drying procedures.

Gene Wengert, forum technical advisor



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