Controlling Mildew in Air-Drying Lumber

Surface mildew in a stack of boards being air-dried should respond to some dehumidification in the space. April 15, 2012

I recently landed a nice score in Maine - two American chestnut logs. A customer called me about two American chestnut tress that he recently felled due to construction on his property. I doubted his claim, thinking they were probably Chinese chestnut so I told him to have a local forester confirm the species. To my surprise he confirmed they were American chestnut. I made the 1200 mile round trip, acquired the logs, and milled 42 slabs worth of American chestnut. The slabs are all 8/4 and range from 10" to 26" and are all 10' long.

So with that being said here is my question. I have these all neatly stacked and stickered and they are being air dried now for approximately 30 days or so. I have fans moving air across them in the part of the shop where they are stacked. I just noticed that I have a decent amount of mildew now forming on them. What do u think the cause may be and how do I fix it? I also have approximately 80 other slabs stacked and stickered on the first floor of my shop, none with the same problem.

Forum Responses
(Sawing and Drying Forum)
From contributor X:
Congrats on the score. I'm sure no authority on it but I've read in more than one place that no native American chestnutís survived the blight, and that any chestnut trees today have been crossed to some degree with the CC which had no susceptibility to the blight. I would like to discover I'm wrong.

The only thing I can think of as to why one group of stacks is mildewing and the others are not is that the conditions are different. If it's mildew, the conditions can appear to be similar but even just a hair too little airflow, too much moisture, etc. can push it into conditions suitable for mildew so you have to study the two conditions very closely and modify the environment somehow.

Are you sure it's mildew? If it's not actually mildew is it possible that stack has been exposed to some spore or fungi that the other stacks haven't? Did you put on an older blade perhaps to finish a log that those boards came from? Most likely it is mildew and the airflow is inadequate or something of the sort.

From contributor B:
Moisture is always the first thought, maybe it is because the sap is high due to having been cut in the summer? It might be a good idea to increase you sticker height to two or more inches. Nice grab, by the way. How did he know it is American and not Chinese?

From contributor T:
I don't know you location but it's been extra humid and damp here in Tennessee. The fans alone can be just blowing the dampness around plus the humidity existing. The minimum I'd do is put dehumidifiers on them, but the best would be order a set of D/H kiln plans.

From Gene Wengert, forum technical advisor:
If it is mold, then it will only be a surface issue. Note that the sap is the same year round. The sap does not go to the roots in the wintertime. If it is mold, then you are drying too slowly. Either the RH is too high or the air flow is poor (or a little of both). With your fans, we can probably discount the poor air flow. So, it is likely then that all the moisture in the AC (or a cross, as it seems unlikely that anyone would be allowed to cut a true AC tree) has increased the moisture in your shop to a high level. Possible fix: Open the windows, etc. so you have the same conditions as outside.

From the original questioner:
I am thinking it had to be a cross of some type but here is what I was told from the customer. The forester came and explained to him that the tree was showing signs of being blighted, the top of the tree appeared to be dying. He expected that the rest of the tree was probably to follow. There were two trees and one was 84" around at the base and the other 64" around at the base. Once he cut the bases to ten foot sections it appeared to have four clean ten foot sections with no rot. So, after plenty of email and pictures I decided to make the trip. I figured even if it were a cross (which it probably is) it would still be worth it. In the beginning of my discussions with the customer I told him if they were actually American chestnut he definitely should not be cutting them down, that was when he sent me the pics of them already down. As far as my mildew problem I will try and increase airflow and put a dehumidifier in there and hopefully this doesnít persist. What kind of damage does this pose to the slabs if the problem were not corrected?

From contributor T:
Unless you are setting up a kiln (which I would), it's just a D/H running in your shop bringing the humidity/moisture down isnít going to dry it too fast. If you set up in a "box"/as a kiln, then you need to monitor the heat the humidifier sometimes creates.

Drying too fast isn't the usual with this set-up, youíre not adding excessive heat as the larger kilns do on purpose under controlled environment to speed the process but still needs to be watched.

This is a simple solution and better then ruining AC lumber. The AC could be set outside under a shed and be safer than in an enclosed shop with no fresh air exchange limiting the moisture reduction and increasing the mold.

From the original questioner:
As always the advice from you guys is very valuable. I set up a DH and added a couple more fans and am confident this should do the trick.