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Author Topic: black mold growth in subfreezing firewood kiln.  (Read 729 times)

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Offline swmn

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black mold growth in subfreezing firewood kiln.
« on: May 06, 2020, 10:14:32 PM »
I have read back through the last seven pages or so, took me back to +/- 2013.   I think I need "more" airflow based on previous discussion here, not sure how much "more."

I have been running passive solar firewood kilns ever since I started building winter of 2015/16.  

Briefly, I have 2x_ pressure treated floor framing up on cinderblocks, then a double layer of thick plastic, with CDX not treated plywood for the floor, then SPF 2x4 framing, PVC poles and 6mil clear plastic sheeting secured with binder clips from Staples-Max. All my fasteners are well sunk since my wood stove has a catalytic converter in it.

I season eight cords annually, each floor assembly is either 8 or 6 feet long, I have two groups of three floors set up as two drying units, the third drying unit is two eight foot floors.

Here is an empty unit with no membrane on it and no cord wood in it, floor units are 2 eight footers and 1 six footer.

***img empty***


Drying schedule is to keep the dry bulb temp inside the kilns below 95dF (degrees Fahrenheit) until the load is well and truly below FSP, evidenced by the stacks moving around good, visibly shrinking.  From there (usually mid May) I close up the ventilation to about one cantaloupe sized opening in the membrane for each cord of wood enclosed.  Peak dry bulb temps in the height of summer will hit mid 140s dF in late afternoon measured with a BBQ thermometer at the top of the unit.  I look for 150dF every year and haven't seen it yet.  I haven't bothered with a wetbulb thermometer or humidity sensor because they have been working so well.  I have been complacent with my operational note taking lately.  I do ordinarily open up the vents a little bit in mid August so the finished MC going into freeze up in mid Sept comes in 12-16%MC, the sweet spot for my woodstove.  The year I didn't do that I had eight cords at 8% MC and one of the engineers at Blaze King on speed dial.  My local EMC is 11%.  

Results have been nothing less than spectacular, until January 2020.  Until then, every stick I took out of there could have graced the cover of Family Circle Magazine or Architectural Digest.  I only handle spruce since my wife is allergic to birch, and it has been just beautiful stuff.  One year I got into some really big trunks and did a cord of "chihuahua spruce" (not a speck of bark in the entire cord) that went for $950 through a charity auction at my church.  The winning bidder said he would have gone to 1200 if the other bidders hadn't dropped out.

So first part of Winter 19/20 I burnt everything out of the empty unit pictured above.  Uniform complacent excellence, not a care in the world.

A friend of mine bought a new house with a wood stove in it, closed Dec 2019.  I offered him two of my cords of spruce thinking I could spare them.  My wife and I took a truckload of awesome splits over to them the weekend before Christmas out of their two cords so they could run their stove while they were unpacking all those boxes.

I was in and out of the kiln with Nathan's two cords in it helping them load their truck when they came over every couple weeks, and I saw black mold growing on their wood in January 2020.  Mind you it was -20dF outdoor ambient and the splits were at 12-16% MC.  

To measure MC in firewood my habit is to bring a split indoors for 48-72 hours or so to warm up to the +55dF ambient my garage is heated to, then split the split in half, measure the MC on what was the middle of the inside of the split, and then add 1% to the MC I find since my meter is calibrated at +70dF.  I use one of those $35 MiC meters with the pins on the end, runs on a 9 volt battery.

At +90 dF ambient and +148dF in the kiln (dry bulbs) in high summer I have 58 degrees (F) of solar gain.  This with 21 hours of direct sunlight daily.  
With five hours of direct (dim) sunlight in January at -20dF ambient I was measuring between 0 and +10dF inside the kiln at sundown.

I do not understand how I had black mold growing on wood at 12-16%MC at +10dF.  

***Jan/Feb mold growth on Nathan's wood***


It gets worse.  After I emptied the good kiln pictured empty at the beginning of this post, and committed to selling two other cords, I opened the kiln with my remaining three cords in it and found this.



I was working with the atmospheric science and microbiology departments at my local University, but COVID hit before I could get anyone on site.

My plan is to leave the empty kilns open to direct sunlight for a few weeks each, spray them down thoroughly with Concrobium (tm), let that dry off in more direct sunlight, replace all the membrane this year and ask for help.  I am truly open to any reasonable affordable suggestions you care to make.  

Thanks in advance.

Offline swmn

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Re: black mold growth in subfreezing firewood kiln.
« Reply #1 on: May 06, 2020, 10:16:46 PM »
To check my in process MC I have little cubbies made of scrap 2x6 and thick plywood scattered around so I can pull a split out of the bottom of the shady side of the kilns to check the MC without having to move a bunch of wood around.

Offline DiamondLane

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Re: black mold growth in subfreezing firewood kiln.
« Reply #2 on: October 24, 2021, 03:42:01 PM »
Black mold spores spread on shoes, wind, insects, rodents, lots of varieties, cold tolerant too, (glacier melts have revesled lots of ancient types ,and vold living fungals, bacterials, virals and parasites), Hydrated lime powder is what I use to kill black mold in walls, windows ,damp ,sap sugars on wood seep to ends ,wood sap amino acids are food for many species, dust with Bulidetd hydrated lime or sulpher lime, spritz antibactetial dishsoap and water ,add vinegar, dust baking soda, treat floor and outside entry paths walked on rodent access ,fresh wood brings with whatever spores are on it from nature and wildlife.

Online doc henderson

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Re: black mold growth in subfreezing firewood kiln.
« Reply #3 on: October 24, 2021, 04:31:38 PM »
black mold does not cause much in the way of human disease.  throw it in the wood stove.  the black stain you see is harmless.
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Offline GeneWengert-WoodDoc

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Re: black mold growth in subfreezing firewood kiln.
« Reply #4 on: November 02, 2021, 09:34:37 AM »
Lime sulfur or lime mixed with copper sulfate is indeed a fungicide.  I am not aware that hydrated lime alone is a fungicide that is effective on wood.  It does raise the pH and so perhaps making the wood more basic can provide some control.

The mold on wood is using dust, dirt, and other organics in the air that land on a wood surface for their food.  This is why poisoning the wood itself does not control new mold or mildew fungithey do not use the wood as a food source.

When possible, eliminating water can control mold and mildew.

Hydrated lime is very caustic, so avoid skin contact, eye contact, etc.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline jimbarry

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Re: black mold growth in subfreezing firewood kiln.
« Reply #5 on: November 02, 2021, 11:42:17 AM »
Like @doc henderson said, it's not really an issue. Now if you got fungi growing, that's a different thing. 

I suppose the operation of a solar kiln is different that an electric one. I kiln dry a fair bit of firewood using a Nyle L200, stacked wood like you show. 


Generally speaking, I find the best results are with around 250 ft/min (3mph airflow through the stacks). 


Make every effort to have the air flow through the stack. Air will flow the path of least resistence so plug up any gaps.


You need to ensure good air flow and quick removal of air borne moisture. 

Do you measure relative humidity percentage?


Those pins on your meter, do you have them aligned with the grain of the wood? More accurate reading that way.

Jim - Livin' the rural life in Nova Scotia.
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