Managing Mold while Drying Cookies with Pentacryl

      In a specialized air-drying situation, a woodworker puzzles over how to deal with surface fungus growth. April 20, 2011

I have two large cross-sections ("cookies") of Siberian elm. I have given both many applications of Pentacryl to prevent cracking as they dry. Per instructions from Preservation Solutions, I have boxed both slabs up and stored them in my cool, dry basement to dry slowly.

The problem is that the slabs were in my warm, humid garage, often covered in plastic, while I applied numerous coats of Pentacryl and waited for it to soak in. During that process, a mold or fungus began to take hold. I have sprayed the wood with more and less diluted bleach to kill the growth - enough to lighten the wood surface visibly. Today, I checked the slabs again after a week or so and the fungus is back.

See the white fluff on the images below. Does anyone have any suggestions how to get rid of the fungus/mold? I'm hesitant to keep using bleach for fear that I'll damage the wood too deeply to retain its nice color. Will an alcohol application work? Are there any other, non-bleach products?

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Forum Responses
(Sawing and Drying Forum)
From contributor X:
Plane them to the extent the mildew has penetrated, treat with fungicide/mildewcide and reseal. The mildew fungii may return since it's probably penetrated the cells deeper than you can plane and treat, but it will slow it. If you can keep them in a cool dry place it may not return. Seal them very well though so they don't dry too rapidly.

You may have to repeat this process more than once. Once they are dry, if there are stains you don't want, you can test a small area with Kleen Strip WB-19 two part bleach. Test both sap and heart to make sure it doesn't bleach out the color you want to keep. Every species reacts differently to bleaching. Most heartwood will fade out but not all. It's worth a shot.

From Gene Wengert, forum technical advisor:
Even though you can kill the surface mold, mold on the interior or that comes later is not affected. The bleach does not have a long term effect.

From the original questioner:
Thanks for the advice and info. The good folks at Preservation Solutions (who make Pentacryl) have suggested Lysol as a bleach alternative. I'm actually fine with periodically treating the surface fungus or mold (whatever this fluffy growth is), but my concern is whether there's internal damage going on that will weaken or discolor the interior wood.

I've seen products like Tim-Bor or BoraCare, but they seem more targeted at the construction world than the furniture world. Do I need to be concerned about fungus or mold possibly growing within the wood, or just the surface stuff?

Sealing the wood is not the recommended process for Pentacryl. I'm told that once the wood is well soaked-through with the solution, it should be boxed up in cardboard and set to slowly dry in a cool place, with a relative humidity of 40-60%.

From Gene Wengert, forum technical advisor:
If the conditions are suitable for mold growth on the surface, it is nearly 100% that conditions are suitable for sap stain (also called blue stain) fungi on the inside.

From contributor X:
The bleach is not intended to kill the mold or spores or whatever other organisms are in the wood. The bleach is used to diminish and in some cases completely remove the stain once the wood is dry and the condition is remedied. The effect of the bleach is permanent as long as it is not put in an environment where mildew/fungus whatever can again thrive in the wood. This is probably what you are saying.

I've been using the WB-19 for going on six years and I can vouch that the pieces I've used it on have remained the same. This ERC beam was felled in a dry riverbed where the sulfur content was very high. The sap on these trees was quite yellow. I apologize for not having a before pic for this but you've all seen the yellowish looking sap in some ERC I'm sure. ERC is not a species that you want to bleach the heart as it will fade dramatically, but the sap responds very well in my experience.

This picture is about three years old and the sap is every bit as white today as in this picture. One of our sons carefully applied the bleach to the sap only. You can see it is still a bit blotchy with light yellow but not even close to the pre-bleach color which was unacceptable to the whole family. I had to get it white or log and process more trees. We have numerous ERC bar tops and flitches throughout our home that came from this stand of trees and they are all still just as white as the day they were bleached.

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From contributor X:
On the other hand, Flame Boxelder is one of those rare species where both the sap and heart respond favorably to bleaching. The two cookies below were vacuum dried by Den Socling at PCS Specialties. I dragged my feet and didn't get them to him before the mildew and spores attacked them. Once I got them back of course, all that was killed but I now had to deal with the consequences of my procrastination.

The cookie on the left was about two inches from the one on the right in the log. IOW there was another cookie between them but I selected these two to work first. The one on the right has been processed and has the first coat of shellac on it. But it looked the same as the one on the left prior to bleaching. The shellac gave the sap a little amber back, but you can clearly see how well the bleach has hidden most of the stains. This bleaching effect will not fade unless the cookie starts to mildew again or fungus etc. gains another toehold.

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From contributor X:
Not that it's relevant to our discussion, but looking at all the cookies again just now I think these two cookies were more like 6" from each other in the log.

From the original questioner:
Once a piece of wood is bleached, and has been lightened by that process, can you bring back any natural color through oiling? What does bleached wood with oil added look like - shiny white wood? Or at that point are you basically trying to color-match the original wood color by using additives like dyes/oils/stains to whitened wood? That sounds like it'd be a difficult task.

From contributor X:
Each species will react differently, and pieces within the species can also vary. Color matching has never been a goal of mine when I bleach wood. I use it only when I want to achieve some different than what the wood is offering such as in the case of whitening the ERC sap, or hiding stains in FBE, pine, etc.

Like everything in woodworking, not all techniques can be used in every situation, but everything has its place. Bleaching works for me in certain situations so I use it. Some others do also but still others will turn their nose up at it (not inferring to anyone here just speaking in general). I just thought I'd mention it because I know most woodworkers aren't aware of the product and potential benefits of using it. I don't like to use biscuits or pocket screws, but I can understand why others think I am crazy to avoid using such technologies. If you do ever try it, make sure you have a defined goal that you're trying to achieve with it and as stated, test both sap and heart in an inconspicuous area.

From contributor B:
The fuzzy white stuff is not a surface problem, it's merely a part of a life form (probably fungi) that is in the wood and growing out, like any plant or fungus will if it can. Bleaching will kill what it reaches, but will not kill what it doesn't; hence the return of it once stored again. Fungi and molds just love warm, wet environments with little to no air flow, and it seems the process you are using requires such an environment. To be blunt about it, it sounds like you are creating the problem by following instructions.

I have not worked with elm, but I have a lot of maple and it just loves to grow that same looking fuzz after a while of spalting. It degrades perfectly good wood into a papery substance if given time. I think if it were mine I would find a way to hinder such a favorable environment to fungus growth, starting with opening the wood to a more open air supply.

From Gene Wengert, forum technical advisor:
Bleach (ordinary household) will indeed kill fungi. It can be used to stop the growth. However, its effects are not long-lasting so the fungi will come back if conditions are suitable - warm, wet, oxygen, and food. Wood preservatives or dips used with green lumber to prevent fungal and insect activity poison the food and last for weeks to centuries. Note that poisons can be inherent in the wood (cedar or white oak for example) can be added (mainly pressure treating).

The second effect of bleach is that it changes the wood color (bleaches it). This effect was illustrated by Contributor X in his posting. Note that such bleaching will minimize the grain appearance so it must be used carefully. Further, bleached wood should usually be neutralized or well rinsed.

From Gene Wengert, forum technical advisor:
In case there is any doubt or confusion, the white fuzzy stuff is a mold fungi. It is using some sugars in the sap for its food as well as any microorganisms, dust or other surface contaminants. The water it needs comes from the wood. This is why dry wood will not stain, decay, mold or support other fungal activity. Of course, warmth (over 50 F) and oxygen comes from the environment. The term dry rot is a misnomer, as the wood must be wet for the rot to occur. I repeat, the fuzz is fungal and it is called mold. Mold can come in many colors. These wood molds do not grow deep into the wood but are surface molds. Other fungi, such as the blue stain fungi and rot fungi, can grow inside wood.

From the original questioner:
Gene, thanks for the info. So just to be clear, you mentioned: "These wood molds do not grow deep into the wood but are surface molds. Other fungi, such as the blue stain fungi and rot fungi, can grow inside wood. "

Based on your earlier comments, it sounds like this white fuzz mold is a surface-only issue, but the fact that it's there indicates a very high likelihood of other, discoloring molds within the wood. Is that about right?

Do I have treatment options to stop or remove any interior molds that may be present? Keeping in mind that this is a big end-grain cookie that will crack very easily, a treatment that doesn't require rapid drying would be preferred, but it sounds like that may be a pipedream.

If the options are between (a) throwing this thing in a kiln or otherwise rapidly drying it and therefore almost certainly causing it to crack, or (b) treating the surface mold and keeping it in a cool/dry place to let it continue to dry slowly, and then seeing what I end up with when I get to planing and surfacing the piece, I guess I'd have to opt for (b) and hope for the best and deal with the issues when I see what they are. Is there a practical, dependable third option? Thanks again for everyone's time and expertise.

From Gene Wengert, forum technical advisor:
We know that very slow drying will minimizing the risk of cracking, but it is still likely to crack. Apparently this Pentacryl will reduce or stop shrinkage. So, the question is how fast can you go with this chemical? You are correct that the mold does not cause discolorations, but it does indicate that conditions might be good for other fungal activities that include stain.

From contributor X:
I was not happy with Pentacryl. I had better results with denatured alcohol bath. Submerge for several days - remove for a day, back in for several days and out for a day to two as you continue. Repeat this until you get the MC below 20% and then seal with wax. This method didn't work for me for dense woods but worked better than Pentacryl in porous woods no thicker than 4" for decreasing the tendency to crack. Alcohol comes in various grades and it so happens that the brand Lowes carries in the gallon is Crown, and it is a good high grade product.

Iím not knocking Pentacryl - it may work in some situations but I didn't tinker with it for long. The best way is if you can get it vacuum dried. That will definitely minimize cracking and stains. I don't think any of my cookies would have cracked had I moved them to Den in a timely manner.

From the original questioner:
I'm curious how vacuum drying is any different than regular drying, aside from likely being faster. My understanding of how Pentacryl is supposed to work is by reinforcing the structure of the wood cells, so that when the moisture is removed the cells don't collapse and shrink (as much). Mechanically, that makes sense to me, though I don't know what ingredients achieve that feat, or really if it works yet. I guess we'll see how it does for me in a couple months. Certainly a cookie this size is going to be a tough test. Even a few percentage of shrinkage will result in clearly visible cracks at this radius.

From Gene Wengert, forum technical advisor:
I am not aware that vacuum drying reduces shrinkage of wood. The alcohol technique does.

From contributor X:
I've not had dimensioned lumber dried in the vacuum, but the cookies seem to do a lot better when dried fast like that. I know they shrink. That's what wood does end grain not excepted.

I don't know if having only between 1.5 to 4" of long grain is making the difference, but that's the only thing I can think of. The radial or tangential shrinkage isn't changed just because there is less longitudinal, and longitudinal is the least of them anyway. I can't explain the scientific reasons why cookies seem to have less destruction (to my observations) being dried in a vacuum. Some still crack but not nearly as many or as bad. That's been my experience.

From contributor L:
Contributor X - how many cookies have you dried in a vacuum kiln? About what percent of them came out of the vac kiln with no cracks? Have you ever dried pecan cookies?

From contributor X:
I've had eight dried - all Acer Negundo. I know that isn't many to claim empirical data, but I have dried hundreds others various other ways using about every process you can imagine. The vast majority of my experience is with Acer Negundo so that's an important factor. I've dried small cookies and large cookies. Thin cookies and thick cookies and the one thing I've noticed is that there's only three methods that are noticeably effective to varying degrees, compared to just sealing and air drying.

You have to have some kind of benchmark for comparison, and seal and air dry is what I use for comparison. Most turners like to get them green anyway, so I just seal and stand them upright on plywood pallets, in a lean-to I have to keep them out of the sun, with several inches of space between each. My avg. humidity is between 70 - 80% so that helps. I couldn't do this in Yuma AZ they'd all crack wide open early and often.

Out of the eight cookies I've had dried in the vacuum kiln, only two cracked that were not already cracked, and the cracks weren't as bad as they would have been IMO if just sealed and dried, based on my experience. Iíve never transported any of the cookies to the kiln as fast as I should have. For various reasons I have allowed other things to take priority and time flies around here pretty fast.

I only send cookies that really have something going for them. My business model dictates that sealing green and stacking them is most profitable because that's what most of my market demands. Next time I cut some that are exquisite, I have vowed to stop the circus and get them to Pennsylvania immediately. They have emphasized the need for me to do this and next time I will. Out of the last five, three did not crack at all and two of those were quite thick. One was a 24" diameter cookie 4" thick and it did not crack. The pith barely showed any checking. That's not common at all.

The other two methods that I have also noticed significant success with are the alcohol method I mentioned, and for cookies small enough for the microwave, I have combined the alcohol bath with microwave heating and periods of stress relief between. Don't ask me to explain this in detail it's more like voodoo and flying by the seat of your pants. I have success with it because I wrecked about a dozen before I got the knack for it. Works great for pen blanks and other small blanks as well, but the smaller end grain cookies respond very well to this method.

If given the choice between making a living drying pecan cookies and beating my head against a concrete wall I'd have to give it some serious thought before deciding. Try the alcohol bath method. It's bound to help, but pecan is difficult to dry under the best of circumstances. There's no silver bullet, but I am confident that getting them dried in the vacuum decreases checking significantly, and I haven't even given the vacuum a chance to show its true potential IMO.

From the original questioner:
The denatured alcohol bath approach would be one to try. I still don't know how I would submerge a 49" x 39" x 3" thick cookie in an alcohol bath. I'd need to first construct an alcohol-proof basin of some kind. And wouldn't you need a lid to prevent the alcohol from rapidly evaporating from the bath?

In any case, a basin like that would have help with the Pentacryl, too. Submerging for a few days and then immediate storing would probably have significantly reduced the likelihood of mold taking hold, as opposed to leaving them warm and wet under plastic in my Chicago garage while I applied multiple coats of solution.

From contributor B:
To the original questioner: I missed on your original post that you had covered the wood in plastic; while I don't know the instructions on Pentacryl, I would think that since plastic holds air movement to nil (giving molds/fungi a haven to live) you would move to wrapping the piece(s) in thick paper, like a brown grocery bag(s), that will slowly wick the moisture through.

From the original questioner:
During the application/soaking phase, the instructions are to either submerge the piece for a day or two depending on thickness, or if you can't submerge, brush on thick coats until the Pentacryl no longer gets absorbed, covering the piece in plastic to prevent evaporation between coats. I put on one coat a day, to both sides of both pieces, for about two weeks, with an occasional missed day here and there. I used about two gallons of Pentacryl in the process. During the two weeks, though, I have two big hunks of wet wood, in a wet solution, in a hot and humid garage. So no wonder mold grew.

For drying, you're right. The prescribed method is to rest the slabs on edge, inside a cardboard box, in a cool dry area, to minimize airflow. I custom built a crate out of 2x4's and then sealed the whole thing in corrugated cardboard, and placed it in my basement. Of course, the box does get opened periodically so that I can get to the mold to treat it, but then it gets sealed up again to rest. Theoretically it will take three months of drying per inch of wood thickness.

From contributor O:
A little background on the vacuum drying: the last cookies from Contributor X were dried with 3-1/4" hard maple squares heading to a baseball bat manufacturer. The charge was dried in nine days. The heating ramp that gets these billets dry in eight or nine days is pretty steep. The box elder is pretty porous. The heat provides the energy needed to boil the water under vacuum. The porosity allows the water out of the wood uniformly. Pecan cookies would be a different story.

From the original questioner:
Just a quick follow-up on the cookie-drying: Since moving them into my cool, dry basement, the mold is a non-factor. It's stopped spreading and looks to have dried up. There is white powdery residue in places, but it wipes clean with a damp rag. I did try treating half the mold with a diluted tea tree oil spray I concocted, based on the reports of tea tree oil as nature's best antiseptic. The treated areas were cleaned of mold and had zero re-growth, but that's probably as much because of the cool dry air as anything else. After talking with the good folks at Preservation Solutions I switched from having the cookies held inside a large box to cutting the cardboard and taping it directly to the surfaces of the cookies. The edges are still exposed.

Everything seems to be going fine for now. No cracking yet, but it's only been a few weeks. I'm going to get a moisture meter to track the drying progress. Stay tuned.

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