Wood Cracking

[Harold Rohde]

I have had an issue while turning some cherry limbs. My nephew gave me some pieces of cherry tree that he had cut up, and although not real fresh they are by no means dried. I put one on my lathe that was about 6” diameter and turned it into a candlestick, and then turned another into a smooth cylinder and then had to stop turning for the evening. I returned to the shop a few days later and the wood had split down the entire length. I understand the mechanism of the wood drying out and shrinking from the outside, thus causing the crack.
The question I have is do I have to turn until I am done with my project once I start, or is there a way to be able to stop at the end of a day and not have the wood cracking?
I had thought of coating it with anchorseal or maybe wrapping it in plastic wrap to try to slow moisture loss.
Any suggestions would be greatly appreciated.

 

[Don Stephan]

You don't say what part of the 6" diameter you used. The center of the growth rings, called the pith and not necessarily the geometric center, is likely to crack - common practice is to avoid using the pith and a 1" radius cylinder around the pith,

As green wood dries it shrinks I think around one-third more along the growth rings (tangential) than across the growth rings (radial). That differential shrinkage creates stress in the drying wood, and when the stress is greater than the force holding wood fibers together one or more cracks form.

If the wood can accomodate the stress by moving it might do that rather than crack. For example, stand a section of branch or log (a cylinder) upright on end on a band saw table and make a cut through the pith to make two half cylinders. Initially those two surfaces through the pith are flat. Give the half cylinders a couple months bark side down to dry slowly, perhaps in paper bags, away from moving air. Put the two cut surfaces together and they no longer meet perfectly because each has shrunk slightly along the growth rings.

Bruce Hoadley's first book UNDERSTANDING WOOD can require multiple reads but is an excellent source of information on wood, moisture content, drying, and so much more.

 

[John K Jordan]

As Don mentioned, the pith and the surrounding juvenile wood is often unstable. Unfortunately, Black/Wild Cherry from some trees can be notoriously unstable. When using a limb it make have a relatively large proportion of juvenile wood, and if the limb grew at some angle off vertical, you may also have some reaction wood adding to the stress.

What I do with such pieces is seal the ends with Anchorseal immediately after cutting, then take it to the bandsaw as soon as possible and cut down the middle. Then, since I like to turn spindles, I'd probably cut each half down the middle again leaving the pith in the corners where it usually can't do too much damage. Then square up the blank and put it up until completely dry, then put it on the lathe. A small piece like that might dry fairly quickly. This, of course, leaves you with a small blank but possibly a useful one.

I've processed and dried hundreds of blanks from log and branch sections, including the tricky cherry (and tricky holly, oak, dogwood, privet, persimmon, ...)

If you do leave some partially turned piece of wet wood on the lathe overnight, it might be helpful to wrap it in plastic to slow the drying. Some people spritz it with water first. But with a piece containing the pith, you might well face the same problem the next day.

I second the recommendation of R. Bruce Hoadley's book. Hoadley is both a wood technologist AND a craftsman - unlike many technical books, his writings explain things in ways practical to us. Also, learn the T/R ratio which describes the manor and extent of shrinkage for various species. (The online Wood Database is a good source of these numbers)

JKJ

 

[Dean Center]

Best practice is, Yes, once you start a green to finished piece, keep going until you're done. There are methods that may keep things intact for a short time, but gittin er dun all at once would be most reliable. With a fruit wood branch, you were doomed.

 

[Brian Gustin]

Agree with the rest - a 6 inch fruit wood green whole log, you want to do it all in one sitting , getting your project turned as thin as possible and let 'er warp (if it can't warp it is gonna crack) Fruitwoods in my experience are notorious for rapidly cracking - Apple and cherry that I have had (I even got it within minutes after it was cut down) started cracking in a matter of hours if it was not promptly dealt with (sealing end grain can help extend "working time") by splitting them down the pith and then sealing end grain - smaller branches I left as long as I could the ends will crack and split, but once cut off, you can trim them back to where you have uncracked wood (but then it'll start cracking sooner or later too, hence turn it all in one sitting) I did have the idea that you could dismount the piece from the lathe and drop it in a bucket of water if you have to "park it" for a time but never tried it yet....

 

[Darryl Fective]

Wood shrinks as it dries. It's almost nothing lengthwise, most is around the circumference of the growth rings. The shrinkage of each ring is proportional to its size, so larger rings are trying to shrink more than smaller rings. The center pith really has no diameter, so it doesn't shrink, so all the rings around it are building tension, getting tighter around the center. It all holds until the weakest point if a ring lets go, and like an instant chain reaction, they all let go in the same area, leaving a radial split.

Cutting or splitting your logs/blanks thru the pith will allow the rings to shrink without tearing it apart.

 

[Don Wattenhofer]

It is possible to turn with the pith and it will introduce a whole new way to view the grain.
The form must take into account all of the factors involved and often that means thin enough that the wood can distort rather then crack.
It has already been mentioned but it bears repeating read Hoadley's Understanding Wood.

The goblet form above is black cherry that measures about 7" diameter X 8" high. The walls are 1/16th inch or less thick of the and the base is also under cut such that the pith bulges up instead of cracking. The piece was turned inside first, refined with a burred shear scrap then the outside in stages using a light to help gauge the wall thickness and all in one session.
The cremation urn is cherry that measures 10" diameter with the base measuring 2" diameter with the pith included. The important thing to prevent cracking especially with cherry is to make the walls uniform thickness all the way down to the base. The urn was rough turned then put in a brown paper bag to dry and after about 5 months when the weight had stabilized it was finish turned.

 

[Randy Heinemann]

I used to just turn until I couldn 't regardless of whether I was done or not. Sometimes that's OK, but, moer often, cracks appear where there weren't any. This seems to happen unless the wood is completely (and I mean dried to the environment moisture level). Even then, internal stresses in the wood may, in fact, cause some cracks. I now don't start a piece unless I can finish turning it and put it somewhere to dry for awhile (like I have a pail with replenishable beads which dries out smaller items. If I follow these rules, everything turns out well. Even wood that seems dry can crack partially turned and left on the lathe. It all depends on the part of the tree it's from what type of wood, and just how much moisture might still be in the piece. I've left a piece on the lathe overnight and, even though it I felt it was sufficiently dry, it's out of balance so I sometimes need to retrue it. It's wood. It always moves, even when dry.

 

[Marc Banka]

I don’t always turn to final in one sitting, in fact it’s rare that I don’t stop. It’s all fine as long as you wrap the piece with a plastic bag. As long as that’s done, the piece will be exactly as it was because the moisture content stays the same.

Some wood will mold quickly, wet sweet maple is the worst. But almost all others are fine, even if left for a week or more. For overnight I leave it on the lathe and wrap the piece and chuck. For longer I take it off and seal it well.

Sadly,,that won’t help the problems of turning something solid containing the pith.

 

[Darryn Achall]

I turn green wood all the time over multiple days. At the end of each session, I back off the tail stock if present, spritz the wood with water, cover with a plastic bag, and put a velcro strip around the chuck end to keep the moisture sealed in. This works well during the turning process. Drying the wood without cracks after turning is a whole different subject that has extensive posts on this site.

 

[Will Armstrong]

Lots of great wisdom above.
I have had occasional trouble with mold when leaving it wrapped for more than about 2 days.

 

[John K Jordan]

Lots of great wisdom above.
I have had occasional trouble with mold when leaving it wrapped for more than about 2 days.

Yes, and then fungus! I try not to leave it for more than overnight. Note that another proven way is just submerge the entire blank/log/bowl in water - a preservation method sometimes called "ponding." Will stay in good shape indefinitely as long as the water is replaced with clean. Another method is to put it in a freezer (I wrap in plastic first.)

I wrote about this before: I once wraped a wet bowl blank in plastic for "just" overnight then forgot about it for a LONG time. (I'm not very smart.) When I found it, it was covered with a thick layer of interesting fungi. Cut it open and found the most wonderful spalting, rich green and pink color, nice zone lines. I made a few things small from it like this egg.


I've since tried to duplicate that mistake - have one experiment in progress now.

JKJ

 

[R Henrickson]

Yes, and then fungus! I try not to leave it for more than overnight. Note that another proven way is just submerge the entire blank/log/bowl in water - a preservation method sometimes called "ponding." Will stay in good shape indefinitely as long as the water is replaced with clean. Another method is to put it in a freezer (I wrap in plastic first.)

Ponding was used for storing potential masts for sailing ships -- entire trees.
I took a mini-hat class with Chris Ramsey long ago. At the end of class he gave each student another hat blank -- very nice curly maple. When I got home, I decided to put it in water for a while in a small plastic tote until I wanted to turn it. A year or so later I came across the forgotten tote --- the water was soupy brown and the blank slimy. Cleaned it up and changed the water. Another year or two later I found it again. Water was brown, blank had a thick coat of brown slime. Scraped the slime off and put it on the lathe. One cut and the surface was clean, and the wood had no staining whatever. Turned beautifully, although it threw a remarkable amount of spray in the process. It also bent very easily with really nice curl to the brim.

 

[Dave Hulett]

I have had an issue while turning some cherry limbs. My nephew gave me some pieces of cherry tree that he had cut up, and although not real fresh they are by no means dried. I put one on my lathe that was about 6” diameter and turned it into a candlestick, and then turned another into a smooth cylinder and then had to stop turning for the evening. I returned to the shop a few days later and the wood had split down the entire length. I understand the mechanism of the wood drying out and shrinking from the outside, thus causing the crack.
The question I have is do I have to turn until I am done with my project once I start, or is there a way to be able to stop at the end of a day and not have the wood cracking?
I had thought of coating it with anchorseal or maybe wrapping it in plastic wrap to try to slow moisture loss.
Any suggestions would be greatly appreciated.

If I can't finish a green wood piece before end of day I leave it on the chuck and wrap it with plastic shrink wrap. Now that's assuming I'm back on it the next day. If you go too long you can have mold spots, but as long as you don't go too long (ie: a week or more)) those can be turned off. (wear a respirator though)

 

[Doug Freeman]

For any green wood, and that can be logs that have been stored for a year or more with the ends sealed, the work needs to be covered anytime you leave it for more than 5-10 min. Don’t go off to lunch w/o covering it. Spritzing with water is not a bad idea. I use a plastic trash bage wrapped around the chuck and fully encompassing the work.

If being hollowed, I spritz the OD with water, then wrap with stretch wrap.

If I need to leave a wet wood project for days or longer I put in a plastic bag. Just depends on a lot of things whether it molds, but generally the mold can easily be removed with a little shear scraping.

 

[John K Jordan]

If being hollowed, I spritz the OD with water, then wrap with stretch wrap.
...Just depends on a lot of things whether it molds,...

Depends on temperature, humidity, presence of spores in the air, etc. But spores are everywhere, inside and outide, and can survive in a dormant state until the find the right moist, tasty surface.

If experienced mold on wet wood in the shop in the past, try spritzing with a a little household bleach in the water, less than 50%, before wrapping.

The late, great John Jordan, the famous one who only turned wet wood said
- keep a suitable log at hand, off the ground, in the shade,
- go outside with a marker and a chainsaw,
- cut 6" of the checked end off the log,
- evaluate the log, mark and cut a blank,
- bring the blank into the shop, mount on the lathe,
- turn it immediately to finished or rough-turned size.

This might be more challenging for those who have to wait until they get home from work to start.
Wait, what is this "work" thing?

 

[James Tung]

Still working.