Q:
I have recently noticed that our incoming lumber is about 4
percent MC. Our kiln operator says it is higher and that he
could not dry the wood that dry. What do you think?
A:It would be impossible for lumber to be dried under 3.3
percent MC at the surface in most dry kiln operations. In fact,
many could not achieve under 3.8 percent MC. The core MC would
be 1.5 percent MC higher, unless drying was continued for weeks
and weeks.
So, this means that 4.5 percent MC would be the absolute
lowest average MC value you would see. Further, at the end of
drying, equalization and conditioning are done in the kiln.
These processes would add about 1 percent MC to the lumber.
Therefore, it would be nearly impossible to see any piece under
5.0 percent MC average, and most of the drier pieces would be a
little higher (5.5 to 6.0 percent MC). I suspect that you are
using a pinless-type moisture meter (the pin type could not
measure under 6.5 percent MC), so my suggestion is that you
send it back to the manufacturer for recalibration. You might
also buy a calibration standard for your meter so you can
quickly verify proper, calibrated operation yourself at any
time.
Q:
I am having a little problem with quartersawn red oak pieces
that I am edge gluing into panels. I rip the individual strips
for the panel from regular flatsawn lumber, mainly 8/4
thickness stock. I then turn them 90 degrees, giving me
2-inch-wide strips with a quartersawn face. I use the
quartersawn face (the surface that I just ripped) as the face
of the panels. When I sand the panels after gluing, I notice
that often the ray fleck is loose and it is easy to catch a
finishing rag on the tips of the flecks. We might have a lot of
problems in one load and then none for a long time. What is
this?
A:When drying flatsawn lumber, especially 8/4, once in a
while surface checking will develop and then this checking goes
deeper into the interior of the lumber. It is then called
interior checking, bottleneck checking or honeycomb. All
surface checks and interior checks will follow the edge of a
ray. If we rip a piece of lumber with a small amount of
interior checking, we will notice the exact situation you
describe. So, you have honeycomb lumber, from time to time.
Work with your drying people or lumber supplier to avoid this
problem.
Q:
We are loading our predryers with red oak that has been
sitting on sticks three to six days before loading in the
predryer. We are noticing some surface checks on the top layers
as we are loading the predryers. We are going to predryers with
moisture readings from mid 50 percent MC to low 70 percent MC.
The first 10 days in the predryer we are pulling 2 percent MC
per day, and after that 1 percent MC per day. In samples the
checks are closing. When we pull a section to go to the kilns
we are ranging from 18 percent to 25 percent MC. We start the
kiln out at 110 on the dry bulb and 90 on the wet bulb. Lumber
is in kilns four to five days.
When the lumber is entering the plant and being surfaced
and machined we are noticing some checks that have been
driven deep and closed on the top, and some honeycomb also.
Do you have any advice that might help?
A:There are three problems with the way you are currently
handling this lumber. First, it is critical to avoid air drying
in lumber before it goes into the predryer. Why? To avoid
surface checking, which originates during the first few days of
the drying process (above 50 percent MC). The fact that you
notice some checking is indeed a red flag that tells me you are
air drying much too rapidly; your air drying is out of
control.
Second, the green MC of red oak is 75 percent MC or higher.
If some of your oak is going into the predryer at the "mid-50
percent MCs" then I suspect you are air drying the oak longer
than six days, or else, during the six days of air drying, the
lumber is drying extremely rapidly. After having been exposed
to such dry conditions in air drying, the humid predryer 70
percent RH will cause the surface to swell slightly, closing
the checks, but also driving them deeper. Although I never
suggest air drying before predrying, if you must do this, then
make sure your predryer is running at the same RH as the
outside conditions. (Of course, this is hard to do in a
predryer without affecting the rest of the lumber adversely.
So, the bottom line is do not air dry.)
Third, if the wettest piece of lumber going in to the kiln
is 25 percent MC and you dry it in five days, you are drying
way too rapidly. I would expect six days plus perhaps another
half day or more for equalizing and stress relief. Also, the
starting kiln conditions should be just slightly drier than the
predryer; if the predryer is 70 percent RH, then start the kiln
at 65 percent RH. Your starting conditions are too dry and will
drive small checks deeper into the wood.
Being quite blunt, the person in charge of drying needs to
attend an advanced drying class ASAP to learn some of these
fundamental principles, as well as some of the more subtle
principles and procedures.
Q:
We have a new customer for our oak furniture panels that
insists that our panels be evaluated with a boil and drying
test. We need to boil a section of the panels in water for 30
minutes and then dry them in an oven at 200 F for 24 hours. The
glue joints must stay intact. We find that we are getting too
many failures. Can you help us out?
A:The boil-dry glue line test is a specific test used to
evaluate adhesives, but it would be inappropriate for
evaluating interior adhesives that we commonly use for wood.
Why? The PVA adhesives (and even the UF adhesives to some
degree) are not designed to stand up to high temperatures or
long-term exposure to water, especially when the temperature is
212F. The glues soften and lose their strength. There are glues
that will pass the test, but they are more expensive and most
will leave a colored glue line. In short, unless you plan to
have the furniture made from your panels exposed to boiling
water and hot ovens, this test is ridiculous, far from
reality.
Q:
Does improperly dried pine lumber cause sandpaper to gum
up?
A:One of the reasons for gumming of sandpaper (some people
might call it loading the sandpaper) is that the resin in the
lumber is very soft. When heat is generated by the sandpaper
rubbing on the lumber, this heat melts the resin (or at least
some of it). The resin is picked up by the belt, the belt then
cools as it loops around, and with cooling, the resin
hardens.
In drying of pine and other resinous softwoods, it would be
essential to run the temperature up to 170 or 180 F (even 190 F
sometimes) for about 24 hours. This heat will evaporate much of
the resin that would be liquid at room temperature and at some
of the temperatures encountered during machining. This process
is called "setting the pitch." The resin left in the lumber
will soften and melt at high temperatures however, So, even if
the pitch is set, when the sanding operation generates
excessive heat (for example, slow feed, worn paper, excessive
stock removal), then this excessive heat will soften and melt
the resin and load the belts.