Steaming, Drying, and Honeycomb in Walnut

Bacterial infection as well as steamer and kiln settings may have caused reported honeycombing at one end of thick Walnut boards. June 18, 2009

Question
About a month and a half ago we steamed some 8/4 walnut at 205 degrees with the core probe at 180 for 60 hours. We then took it out and put it on sticks within a few days and then within a week it was in the kiln. It was in the kiln on a maple schedule which had the EMC at 14 and temp at 100 for 14 days and then we started to lower the EMC and increase the temp. We then took it out after 32 days and dried to 6% (it went in at 32%). We graded it through our line and trimmed off the end and noticed a lot of what looks to be honeycomb. Is this possible as it clears up after six inches on the board? Does it have anything to do with the steaming? Only one end has been affected. I just don’t see it being dried too fast in our kiln. The moisture of the sample boards never dropped more than 1.8 % a day the whole kiln charge.

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
The maple schedule is just a bit too severe. It is also important that 8/4 and thicker walnut be adequately end coated, as honeycomb will often occur if it is not coated well. I do believe that the key in this case is that you notice it only on one end. This is characteristic of bacterially infected logs where the bacteria may have progressed from the roots only a few inches up the stem. Were the logs from a site that had grazing animals? The bacteria weaken the wood structure, so that honeycomb is likely when the schedule is at all accelerated, and sometimes even when the schedule is mild. So, I think that it is about 70% bacterially related, 10% for the fast schedule, and 20% for the lack of end coating.


From the original questioner:
Ok thank you so much. My other question would be this - in the steamer we have a depression of no more than five degrees when it hits our temp at 205 and 202 or else it will start to dry in there. When the temp is ramping up and for the first eight hours or so going from 50 degrees to 205 degrees and with the dry bulb and wet bulb having a depression of 10 or 11 do u believe this is too harsh for the wood? Is it too hot too fast? It is not end coated in there either or it will just wash off?

I feel it may have happened in there and once the temp settled at what we want the end checks closed up so tightly that once it when in kiln it just blew open. What do you think? Is it possible? I feel this way because I found some end checks that are so tight in some green walnut 8/4 that came out of steamer a while ago and am wondering if that is what it looks like and then blows open. Could it be only on one side of the board due to the 12 hour drive to our yard from the supplier’s yard and blowing only on the one side?


From Gene Wengert, forum technical advisor:
A steamer must use wet steam (100% RH) at all times and that is why we boil the steam through water in a trough. You must have a zero degree depression. You have some sort of problem if you have a 10 depression when heating. In a steamer, we heat only with steam spray and not with heat in pipes. Is this the problem? The steam should be only a few pounds of pressure and this means that any pressure reducing valves must be far away from the steamer too. Maybe this is the problem. Certainly, checks can easily develop into honeycomb.


From Gene Wengert, forum technical advisor:
Incidentally, I have never seen a steamer with a core temperature measurement system. I have also not seen anything written about using core temperatures. Where did you get this information? Who is the manufacturer of your steamer?

From the original questioner:
We do not start our steaming time until the core of the wood hits 180 and then it is in there for 60 hours. It is measured by a probe that we stick in one of the pieces of wood. We do use wet steam that comes out of evaporators but we also use heat to get it up to temp quickly. Would it be the drive here that might have caused it and then go into the steamer and the checks would close up so very tight and then when put in the kiln they blow open. Yes these piles were wax on the ends three times before it went in.

From Gene Wengert, forum technical advisor:
Do not use heat. That can check the wood and then the moisture will close any checks until drying begins. I am not sure by what you mean that the steam comes out of the evaporate and that it is therefore wet.

From contributor T:
When we first started kiln drying large volumes of walnut, we experienced exactly what you are describing. .5" in from one end and completely gone in 4" would be what appears to be honeycomb. There was quite the learning curve to dry the thick, 20" wide flitchsawn walnut planks.

We found that the honeycomb was always only on the butt end of the log. (The majority of our material is log run from veneer quality logs, 90%+ butts, so it was easily identified). Possibly from bacterial infection, possibly the way the grain flared at towards the base of the piece. I’m not sure if it was all bacterial infected. We have found it seemed to be more related to the pieces that had large flares or very wild grain at the butt of the tree. My thoughts would be that your initial emc was too low for walnut above FSP. We always start our 8/4 walnut at 16.5% (and with very valuable charges even higher). The temp starts between 95-100 degrees.

I don't think the steaming was the issue. We have found this honeycomb in both steamed and un-steamed walnut. Although longer air time on sticks between steamer and kiln would not be a bad idea. We are mainly drying un-edged material, but have had similar experiences with square edged. Thick walnut can be finicky to dry.

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