Wood Floor Mystery #13: These Black Spots Aren't Mold ... But What Are They, Then? Part 2: The Answer!

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1 3 23 Photo 2 New Blackspots Like Ink

Part 1 of my Mystery 13 with this photo ...

Photo 1 Original Mist

... stirred up a little floor pro chatter online, for which I am extremely appreciative.

One example:

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Well, now we’re talkin’. Literally! Unlike most of my black spot blog posts, yes, there WERE scraper shavings in this case, BUT…as my floor mysteries go, there was more here than meets the eye.

This floor inspection seemingly started as simple black stains caused by file shavings from repairs done by the flooring contractor during the finishing process. There were normal metal filing stains, but there was also much greater black stain damage: stains that were much larger and looked like permanent ink:

1 3 23 Photo 2 New Blackspots Like Ink

In Part 1 of this post we learned about what black mold needs, and in Part 2 we are learning what black rust needs:

1) Moisture
2) Iron (metal)
3) Limited oxygen
4) Tannic acid

The floor contractor sanding and finishing this project argued the wood was common grade and the black stains were there before the finishing. The installer did not agree. Some folks online even said black stains were common in flooring purchased in this geographic area. What?!

The sales slip stated the grade of the flooring as “Select & better,” and I found the material was of excellent grading and milling.

If the stains were in the lumber, why would the flooring contractor install it? That would be asking for trouble, and this crew knew their stuff. Why would any qualified installer neglect to cull out a few black-stained boards? The answer: They wouldn’t. The stains were not in the wood when it was installed.

My thought was the stains came in after the installation and were not apparent until after the floor had been finished. Knowing these devilish black spots, I didn’t think the contractor finishing the floor saw them, either.

There was one area where filings caused staining: There had been an errant top-staple someone attempted to pound deeper into the floor rather than removing and replacing the board. They had filled the scar with wood filler and scraped the area. This is where the fine, peppery metal filings were found, and they had turned black.

So I knew all the ingredients for black staining were present: red oak, tannic acid, wood fiber, water-based finish, and metal. The puzzling thing was that there were those much larger stains.

I left the inspection recommending they remove and replace any stained boards, then either spot-finish the repairs and follow up with a full abrade/recoat of the entire floor, or refinish the entire floor after repairs. It was for those involved to work out.  

I was certain the stains appeared during the finish process, but how?!

Upon returning home, I thought about every previous inspection with black stains, and it struck me … the last one I wrote about was caused by installing metal racking in closets. I checked my phone for the photos I had taken at this inspection and there, off the kitchen, in a laundry room with ceramic tile, I spotted a good amount of metal wire shelving. I hadn’t bother to look in that room because there was no hardwood.

Could someone have been cutting metal shelving after the floor installation and before the finish process? In my photos I saw there were floor registers and vents for HVAC. I had opened a few just to see what the underlayment was, but then it struck me: I hadn’t put my hand in to see how dirty the vents might have been. I typically do that if the floor finish is dirty or rough.

I called the homeowner (let’s call her Mrs. B). I found out she was a retired science teacher. She wasn’t the pain in the butt (as had been reported)—she just didn’t buy the “it was bad flooring” explanation, because it didn’t add up. I asked her if she wouldn’t mind looking into the venting and letting me know if they were clean or dirty. She said no problem, she would get back to me.

I then went back to my photos and learned that 100% of the stains were in the main living area only.

What happened next was remarkable. Mrs. B said not only were all the vents filthy, they were full of sawdust and granules that looked like … metal. She offered to drive to my office and share with me what she had found. Remember, she’s a science teacher.

Mrs. B and her husband arrived with a dozen pristine little Ziplock plastic bags, each sealed and labeled with the rooms of her home:

1 3 23 Photo 4 Sample Baggied

I had asked her to bring a piece of flooring left over from the installation, and she did. She has also used a brand-new microfiber and cloth for each area she swabbed.

They were excited to see what I could find, thanked me and left me to my samples and inspection tools.

Here is the testing I did:

I sanded the piece of red oak flooring with the grain using 80-grit paper. I segmented the board into 11 equal blocks using a black Sharpie. I then opened the baggies one at a time, removed the microfiber cloth, cut a section out of the middle and placed each strip on one of my eleven segments, labeling each by room and number.

I then spray-misted the entire sample board with white vinegar (acidic moisture to stimulate the tannic acid and iron reaction), covered it all with a thin plastic wrap (to simulate the film of a urethane floor finish and reduce oxygen supply), covered that with a towel and let it sit overnight:

1 3 23 Photo 5 Swab Testing Covered

The next day, black stains exactly matching those in Mrs. B’s home were everywhere:

1 3 23 Photo 6 Blackstained Test Board

Well, not “everywhere.” When I looked at the notations on my sample board, the stains appeared only in the kitchen area; none in the bedrooms, entry, closets, stairway … exclusively in only the main open-concept living room/kitchen/dining room.

The debris from each microfiber swab contained drywall dust, sawdust, and metal fragments, which I used to create an exact match for the black staining in the floor. I also was informed the closet racking was, in fact, staged and cut in the living room area prior to installation.

They did eventually replace some boards and refinish the floor, and the matter was resolved.

Taking it one step further, during my testing I observed how badly stained my sample material was and wondered if anything would remove the damage I had made. Oxalic acid can reverse the staining that happens with metal and tannic acid, so I went to the local woodworkers supply shop and picked some up.

The video shows what happened when I used warm water and oxalic acid on the metal staining:

Props to the Sharpie people, because Sharpie ink beats black rust staining, hands-down. FYI: The only stain more stubborn than both was … pet urine. Oxalic acid didn’t touch that! 

Whether the metallic particles were moved around via the sanding process, contaminated the job site between coats, or were distributed by the HVAC system as the floor finish was drying remains unanswered. The builder and contractor refused to discuss the matter with their distributor or with me, and they would not bring in another inspector. Regardless, the material and stain tests were a perfect match providing perfectly replicated results. Case cracked.

I would like to report this is my final blog on black spots, but alas, things get even weirder in Mystery 14! Although we are seeing spots once again, I’ll cut you all some slack and tell you up front there is NO metal shelving, scraper filings, or metal of any type involved this time. But, since our readers are paying such close attention, I’m sure you will all have plenty to post about! 

I once heard the story of a pastor that gave the very same sermon week after week until finally a parishioner asked him if he was aware he was giving the same sermon every Sunday. The pastor replied, “I’m glad someone finally noticed. I mean every word of what I’ve been preaching, and I’m gonna keep repeating myself until it finally sinks in with you people!”

The last thing I want is to become the WFB “Black Spots” one-hit-wonder. But I get so many texts, emails, and phone calls about “mold in my floor” that I feel compelled to keep repeating my sermon as we sort it all out.

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