Wood slice cracked when shellac applied
#11
  
The quick question is, how can I prevent my wood slice from cracking when applying shellac?

Here's the longer version. 
I acquired from lovely wood slices from a Cedar tree. They were quite thick - about 6" or so. Much thicker than I could use.  They were about 20" in diameter and I didn't have access to a saw that could cut thinner slices. So I cut the wood in half (resulting in two 6" thick half moons). Then I took one of the half moons and created slices out of that (since it would now fit under the band saw).
Then I took 2 of the slices which were the same thickness and after some cleanup glued them together.  
I can't figure out how to insert an image so here's the URL for the image(s)

https://photos.app.goo.gl/vocLlrpKRvG9QKM43
https://photos.app.goo.gl/sBy9BNXVMQCqiXJy2

After waiting for the glue to dry, I sanded the piece down and then applied shellac to one side and the edges.
Within less than 1 minute I started to hear cracking sounds. I looked at the piece and it had visibly swelled such that it was no longer flat, it was doming itself up. Then there was one huge crack sound and the center had split.  Sad 
It did not split along the glue joint but almost perpendicular.
Here are some URLs for pictures after the split
https://photos.app.goo.gl/kG1LaBK77o3GBvO63
https://photos.app.goo.gl/lZLZxRacqOaWTVZN2


And one showing the thickness of the wood (about 1/2")
https://photos.app.goo.gl/EoqlRQjM6kaRJzpw1

So... clearly the wood absorbed the shellac very quickly, causing the wood to swell resulting in the crack.  The question is, how should I have finished this piece? What technique could I use to prevent this?  I like shellac since it doesn't darken the grain too much. I found that polyuruthane made it so dark that the grain was hard to see.  Maybe I should have 2 people apply shellac to both sides simultaneouly? That sounds risky.

Note: I am considering putting a float coat of resin over top of the wood but before I do that, I have to seal it with something or the resin will seep into the wood resulting in tons of bubbles (and perhaps more cracking as well).

Many thanks in advance for ideas. I love this wood and really want to create something out of it!





[Image: vocLlrpKRvG9QKM43]


Attached Files Image(s)
   
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#12
  Re: Wood slice cracked when shellac applied by deb27 (The quick question i...)
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
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#13
  Re: Wood slice cracked when shellac applied by deb27 (The quick question i...)
Thanks Phil.
In fact I was planning to fill the crack with a contrasting epoxy color. I.e. rather than try to hide it, make it part of the design.  Nice to know that idea wasn't crazy.
But I'd still like to know if there are any techniques that would reduce this possibility in the future.  I've got several more chunks of this wood left to play with. Smile
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#14
  Re: Wood slice cracked when shellac applied by deb27 (The quick question i...)
Not sure I have an answer either, but I would give it another shot with some time added between steps.  It seems like you might not have given the slices enough time to finish moving before gluing them together.  I'm not sure about how much cedar moves, but I would guess that the slab had some moving it wanted to do when you cut it half and them some more moving when you cut it into slices.  I have no idea how long to wait.  I would probably even flatten them after they have warped and then wait again before gluing to see what happens.

Good luck,

Steve
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#15
  Re: RE: Wood slice cracked when shellac applied by deb27 (Thanks Phil. In fac...)
(12-07-2017, 04:29 PM)deb27 Wrote: Thanks Phil.
In fact I was planning to fill the crack with a contrasting epoxy color. I.e. rather than try to hide it, make it part of the design.  Nice to know that idea wasn't crazy.
But I'd still like to know if there are any techniques that would reduce this possibility in the future.  I've got several more chunks of this wood left to play with. Smile

Phil speaks truth m'lady!

More technically, wood is both hygroscopic and anisotropic.

Look those terms up and add them to your woodworking bible to be religiously obeyed in any of your future constructs.

Warm Regards!

Beachcomber Bob
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#16
  Re: Wood slice cracked when shellac applied by deb27 (The quick question i...)
Forgot -- shellac (from shell of the lac bug/beetle and mixed with alcohol) is an old time and very good sealer (since the 1800s). It does not soak into the wood but is generally a surface barrier. Because of the alcohol it dries very quickly.
A laid back southeast Florida beach bum and volunteer bikini assessor.

Wink
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#17
  Re: Wood slice cracked when shellac applied by deb27 (The quick question i...)
(12-07-2017, 02:42 PM)deb27 Wrote: The quick question is, how can I prevent my wood slice from cracking when applying shellac?
...
So... clearly the wood absorbed the shellac very quickly, causing the wood to swell resulting in the crack.  The question is, how should I have finished this piece? What technique could I use to prevent this?

I am not able to see images, but from your description
I assume wood is very dry (otherwise coating ends would prevent cracking not causing it).

Since you are open to ideas, here is mine: try to apply shellac
not on whole flat face but only on sections, in clock terms:
1st coat (12--1 oclock and 6--7), 2nd coat (3--4 and 9--10) and so on, each time you cover 1/6 of the possible area.
Wait until dry, for example 30min between coats.
It could probably also help to do it on both sides at the time.
Also do not try to put as much as wood can soak -- limit the coat to few strokes.

Does this make sense?
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#18
  Re: Wood slice cracked when shellac applied by deb27 (The quick question i...)
Deb
Goggle stopping wood slices from cracking using denatured alcohol it works most the time
There is a commercial product wood turners use I believe it is called peg
I usually have better luck drying soft woods than hard woods
Also experiment with cutting a centre hole in the slices then when it is dry fill it with a Dowell or a plug cut from the same log
The split was not caused buy the shellac but from the wood drying as has been stated
Drying rounds is iffy at best good luck
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#19
  Re: Wood slice cracked when shellac applied by deb27 (The quick question i...)
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.
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#20
  Re: RE: Wood slice cracked when shellac applied by omark ([quote='deb27' pid='...)
(12-07-2017, 07:10 PM)omark Wrote: I am not able to see images, but from your description
I assume wood is very dry (otherwise coating ends would prevent cracking not causing it).

Since you are open to ideas, here is mine: try to apply shellac
not on whole flat face but only on sections, in clock terms:
1st coat (12--1 oclock and 6--7), 2nd coat (3--4 and 9--10) and so on, each time you cover 1/6 of the possible area.
Wait until dry, for example 30min between coats.
It could probably also help to do it on both sides at the time.
Also do not try to put as much as wood can soak -- limit the coat to few strokes.

Does this make sense?

That makes sense. And it's a really interesting idea. Simple enough to try on my next attempt. Wish I knew how to attach images.  The URLs are from google photos.
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