Wood slice cracked when shellac applied

[Alan S]

Gluing the two halves together caused the crack.  As was mentioned above, wood responds expands on soaking up moisture, and shrinks as it's lost.  This happens more around the outside edge (tangentially) than across the middle (radially).  That means as a whole disk dries out, its circumference shrinks too much, and a crack appears from the outside edge inward.  If you cut it in half, let it dry some, then straighten the edges and glue it back together, what happens next depends on whether it loses or gains water.  If it gains water (or alcohol, the part of the shellac that cracked yours) the outside edge expands more than the middle can keep up with, so the middle cracks while the edges stay together.

If I am correct, just letting it dry out more will first close up the crack, then as it dries more it will crack from the edge.  If you fill the crack now, it will crack sooner.

To get it to stop moving, you need to fill the wood with something else, to prevent water getting in and out.  It would be best to do that while the wood is cut in half, then straighten the edges and glue it back together only after it has been stabilized.  Because your pieces are endgrain only 1/2" thick, they will soak up things much faster than usual.  If you read Hoadley's "Understanding Wood", he describes trying to prevent such log cross sections from splitting.  Soaking them in PEG (polyethylene glcyol) was about the only thing that worked, but it left a wet waxy surface.  More recently a version of PEG that is chemically modified to harden into plastic has been used, called Pentacryl.  It is probably your best bet.

[MichaelMouse]

The short answer - you probably can't.  

Long answer - Wood shrinks more tangentially (along the rings) than radially (across the rings) as it drys so a section like that will get smaller faster in circumference than in diameter.  Something has to give to accommodate that difference so the section splits.  Finishes can slow the drying, but I don't know of anything reasonable that will prevent it.  The shellac may have been the trigger, but I don't think it was the ultimate cause of the split.  You may have to live with the split.  

This is a good reference for any woodworker.  https://www.fpl.fs.fed.us/documnts/fplgt...gtr113.htm  Chapter 3, specifically deals with the moisture/shrinkage problem.  Is your "cedar" what they call eastern red?  If so, it has one of the driest heartwoods out there (Table 3-3).   Problem is a mechanical one, as illustrated in Figure 3-3.  The early wood has more space and less strength, therefore contracts easier than the latewood.  Draw mental lines across the early wood between two annual rings, and you'll see that a 10% shrink is less across the shorter secant than the longer, resulting in greater mechanical stress as the wood contracts.  This is the origin of distortions, and when there is enough mechanical stress to start a check, can lead to a full split. 

You can use the same trick woodturners use when they taper a wet blank to minimize mechanical stress,  cutting not directly across, but at an angle across the log.  This will reduce/redirect the mechanical stress of contraction somewhat,  and increase your chances of survival.  You can also help your case by using a bulking agent like PEG, but unless you're a Moulthrop, it is devil to get a finish to stick to the waxy hygroscopic surface left after soaking.  Other hygroscopic surface treatments like starch will minimize chances of that first small surface check that grows.  Better method is to put newsprint, or cardboard over both surfaces, changing periodically to stay ahead of mildew, as the paper {made of wood!) buffers the RH.  I put legs under a number of "natural" tabletops for an old woodsman friend who did exactly that.

Don't bother reading about "drying" wood with alcohols.  They're bunk.  After all, you mix the shellac resin with what?

As to moisture barriers against reuptake in seasoned wood, shellac is about the best, and it also gives a bit of a barrier against resin seep from within, which is why it is used as a seal coat.

[Steve N]

Hi Debbie-

Welcome to the forum!

The short answer - you probably can't.  

Long answer - Wood shrinks more tangentially (along the rings) than radially (across the rings) as it drys so a section like that will get smaller faster in circumference than in diameter.  Something has to give to accommodate that difference so the section splits.  Finishes can slow the drying, but I don't know of anything reasonable that will prevent it.  The shellac may have been the trigger, but I don't think it was the ultimate cause of the split.  You may have to live with the split.  I'd suggest filling the crack with something like a tinted epoxy that pleases your aesthetic senses.  Splits like this often run to the edge of the slab, but your slab is still intact all around, nicer in my opinion.  You could try to match the color of the wood, but the fill will still be pretty visible. I have heard of people creating faux grain with tints of different color, but I'm not artistic enough to pull that off and have it look good.  Another approach is to use a contrasting fill and "celebrate" the nature of wood.  I am working on a mesquite box.  Mesquite seems to really enjoy cracking.  The cliche approach for mesquite is to fill these cracks with a mix of powdered turquoise and epoxy.  I will do something like that, but perhaps with something other than turquoise - the jury is still out on that.

Phil

Phil's explanation about the movement of wood as it dries is correct. I would slightly disagree with Retiree that Shellac not penetrate. You mention that you put the Shellac on one side only. Not just Shellac, but anything you put on wood from a finish, to a veneer cover you want to do it to both sides. Usually you are working with wood that has been cut, and had a chance to dry, or be artificially dried in a kiln, so essentially a more stable product. It would allow you the time to coat both sides before the wood reabsorbed the liquid and started to change the moisture content of the first side of wood coated. With slabs they are just thin cross slices, and their characteristics will differ from a board which is a different set of circumstances. Your slab will dry quicker, much quicker because it is almost all end grain. As it has this very rapid drying it will also crack and split much quicker, due to the sudden shrinkage from the very quick loss of moisture. Again very different from a board.

Here I am going to go out on a limb, and suggest that the wood you had was cut some time before you "acquired it"?  If so a lot of the water loss was done already, and putting moisture on one side only caused a rapid change in what equilibrium that piece of wood had left, CRACK....

I had tried for many years to make a wooden sun dial from cut tree slabs, and I had one cracked, and or "exploded" slab after another. I was getting the slabs from my friends who were  cutting wood for their fireplaces. They had almost always cut it several days to a week or more before. Sometimes the slabs were almost destroyed before I ever saw them.

I am not a tree Doctor, nor do I suggest I could write a book on the works of a tree. I have however read Hoadly, who is the word on all things wood. Since then when something weird happens with wood I try to analyze what happened. First think like a tree. You get your nourishment from the ground, also you pull up water so you can have healthy leaves, needles. So in part you are like a huge drinking straw, and on whole you are very wet.

So some bad guy chops you down, and horrors, starts to slab you up with a chain saw. You are now a series of thin slabs, what happens???????

Well in just the first few hours a significant amount of your water leaks out, this will continue until you are very dry. So the further you are from getting cut down, the drier your cells will be. Make sense?

If left alone all slabs will start to crack due to the shrinkage Phil talked about. So if you are going to even begin to slow that down you need to do it immediately after the wood is cut. I've found the absolute best product to be Anchorseal, found right here. It is available in stores, or online. Amazon sells quarts and gallons, kinda pricey, but it works. Whoever you acquired the slabs from you would need to bring your anchorseal, and a brush with you, and accompany them as they felled the tree, and then cut your slabs. Coat it right then, and now I can get pretty stable slabs for making a wooden sun dial


I must warn you, even doing it like this, many slabs don't make it 6 months. So you cut 5 for every one you want



Like Phil said, welcome to the forum.

[overland]

I have a slice of cedar very much like the one you've shown, although it's thinner--about 2"--and the shape is a little more irregular. First, you're not going to be able to keep it from drying out without sealing it entirely with some impermeable material. I suggest you let it dry out and let it crack. That's what happened to mine. Then you can put some butterflies in on the back side across the crack or cracks. You won't remove the crack, but you can keep the slice from falling apart. I suppose you could also fill the cracks--I'd recommend a dark epoxy--but it might look just fine if you don't. The good news, I think, is that the cedar isn't going to shrink as much as some other woods. In any case, the piece I have looks nice, even with a crack.

[deb27]

Another thing you can try is cut a series of slices, and in each one make a straight cut from the bark to the pith, but in a different location around the face on each one. As the wood dries, the cut will open up like a missing piece of pie. Same crack, but you will get nice smooth edges on it. Once it's dry and stable, choose one slice to be the sacrificial piece. Cut slices of pie from that to match up with the missing wood in each of the other pieces, and epoxy that into place. Because the patch is from an adjacent slice, the grain and growth rings should match up pretty close,and you will barely notice the repair.

I'm not sure it would be possible to match the grain but on the other hand, contrasting grain might look quite lovely. But at this time, I don't have the skills for that.  Perhaps someday. Thanks for the interesting idea.

[TomFromStLouis]

I see this kind of question rather often. The frustration is real because the challenge is real; you are attempting to fool the Wood Shrinkage God and it simply is not a job for beginners. If this is eastern redcedar, the aromatic stuff, this proves my point since it is probably the one domestic wood that shrinks the least from green to dry. So cracking will happen, especially to woods that split easily. I have seen elm cookies (disks) not crack and I believe slow drying and the interlocking grain of elm were factors that helped. I've seen sycamore chunks hold together and guess what, sycamore has interlocking grain and is difficult to split.

Ianab's fix is the truly correct and best woodworker approach in my book. Cutting them in half, letting each half thoroughly dry, and then joining the halves back together also has good chances of working.

[Arlin Eastman]

Well it was not because of the shellac but it cracked at the pith line which almost 100% of a lot woods will do.

One thing you can do is if the log has dried for many years is cut off the ends until you find no more with cracks, but it will take years for a log to dry.

How long have you had that one?

Also I know that there are a lot of people including me that mix up epoxy and use powered metals or turquoise in the rock form and put them up and sick them into the cracks and then sand it to smooth it all out.  If you are going to use this on the table or counter make use you put felt on the bottom so it will not scratch it up.

Good luck and look forward to seeing more of what you do.

[deb27]

Well it was not because of the shellac but it cracked at the pith line which almost 100% of a lot woods will do.

One thing you can do is if the log has dried for many years is cut off the ends until you find no more with cracks, but it will take years for a log to dry.

How long have you had that one?

Also I know that there are a lot of people including me that mix up epoxy and use powered metals or turquoise in the rock form and put them up and sick them into the cracks and then sand it to smooth it all out.  If you are going to use this on the table or counter make use you put felt on the bottom so it will not scratch it up.

Good luck and look forward to seeing more of what you do.

I'm not sure how long the tree was cut down. A friend gave me the slices. I think he said he cut the tree 1-2 years ago. But I suspect he hadn't cut it up til recently. He gave me 4 large "slices" about 4-6" thick each.  I took one of them and cut it up to make the piece I'm talking about. The others are sitting on a shelf in the garage.  I can tell based on some interesting crystals appearing on the surface that the wood isn't dried out yet.
Some woodworking friends suggested that I put all the pieces in the attic to dry out (I live in Florida where the attic gets really hot...but also humid). I can check the weight before storing them and periodically after to see if they lose weight as they dry out.
Meanwhile, epoxy will be my friend. Thankfully I actually like the effect of filling cracks with colored epoxy.

[MichaelMouse]

I'm not sure how long the tree was cut down. A friend gave me the slices. I think he said he cut the tree 1-2 years ago. But I suspect he hadn't cut it up til recently. He gave me 4 large "slices" about 4-6" thick each.  I took one of them and cut it up to make the piece I'm talking about. The others are sitting on a shelf in the garage.  I can tell based on some interesting crystals appearing on the surface that the wood isn't dried out yet.
Some woodworking friends suggested that I put all the pieces in the attic to dry out (I live in Florida where the attic gets really hot...but also humid). I can check the weight before storing them and periodically after to see if they lose weight as they dry out.
Meanwhile, epoxy will be my friend. Thankfully I actually like the effect of filling cracks with colored epoxy.

The pictures are of a cross-section with heart checks, so the reply which describes the origin of radial checks is superfluous.  If you take a peek at the suggested chapter in the FPL handbook, you'll learn that it is relative humidity which affects drying rate and that is a percentage of moisture ambient compared to moisture possible for any given temperature.  If it's humid in your attic, but warmer than ambient air, there will be less relative humidity than what's in the garage in lower temperature air.  

Do you have air conditioning, and did you bring the slab into an air conditioned space?  If so, the RH was much lower than your attic, with the cooling coils removing water from the air.

As to the "crystals", they are probably resin seeping out. Which is why shellac is used on resinous woods to keep the other finish from being lifted.

[Arlin Eastman]

I'm not sure how long the tree was cut down. A friend gave me the slices. I think he said he cut the tree 1-2 years ago. But I suspect he hadn't cut it up til recently. He gave me 4 large "slices" about 4-6" thick each.  I took one of them and cut it up to make the piece I'm talking about. The others are sitting on a shelf in the garage.  I can tell based on some interesting crystals appearing on the surface that the wood isn't dried out yet.
Some woodworking friends suggested that I put all the pieces in the attic to dry out (I live in Florida where the attic gets really hot...but also humid). I can check the weight before storing them and periodically after to see if they lose weight as they dry out.
Meanwhile, epoxy will be my friend. Thankfully I actually like the effect of filling cracks with colored epoxy.

Please do not put them in the Attic


One thing I learned while turning and using powered brass or copper or nickel or aluminum is mix both of them together and then add the powder to it until it gets thick like play doo and then push it in the cracks and it will get hard within 15 minutes or less then just sand it down to the grit you wish like 220g to 400g