Cracked bowl - teach me

[OneStaple]

Ok, I asked questions a little over a year ago about turning a large bowl green for a salad bowl. The wood I had saved at the time turned punky, so I had to scrap it. But LOML still wanted a salad bowl. A few weeks ago, a neighbor was cutting down a (silver?) maple tree, so I grabbed a piece.

This past Saturday, I chainsawed out a blank and rough turned a bowl. Good grief green wood sprays water! I had a line clear across my 2 car garage including on the ceiling.

I left a center post for support on re-chucking. Dimensions are just over 15" diameter and 1" thick. I tried to do a continuous curve and not a flat bottom. I left it in its own shavings for about 48 hours, then just put it uncovered on the floor of the unheated garage to fully dry, following advice I'd read here. Now, 4 days after turning, a decent crack has appeared in the rim closest to where the heart would be. Ugh!

I still have the other half of the log, so I'd like to try again. Any advice on what to do differently? Or was this just part of the "you always lose a few"?

Thanks,
Tyler

[EdW]

It looks like the pith was still in the bowl blank, which is a common area for cracking to emanate from. When you prepare the logs for bowl blanks, cut them so the pith is not in the blank. If the other half of the log still has the pith present, turn the rim of the bowl to a point where the pith is no longer there.

[Rodneywt1180b]

I don't turn a lot of bowls but 48 hours in it's shavings seems like too short of a time to me.  Keep it in the shavings longer.  Slower drying is better.
Rodney

[Dennis Puskar]

Coat the end grain areas with Anchor Seal or Parafin to prevent your bowl from drying out to fast.

[MichaelMouse]

Couple things.  Heart checks are normally there in the log.  Always look for them before you make the halving cut.  Sometimes it helps to wipe the end of the wood with some liquid and see if it gets sucked into existing heart checks. Cherry one of the biggest offenders.  Soft maple normally not, but there are tons of species of soft maple, so might be in your particular ornamental.  

Second, the curve that redirects some of the drying stress down when you do the bottom also helps up top.  You're almost vertical in that area, so the risk of a split increases.  Increases as well with the HUGE growth rings.  The early (softer white) wood shrinks more than the latewood as it dries faster.  The split is in the area of maximum mechanical stress, and doesn't show too much spread on the rim at the time you took the photos. 

Soft maple mildews ugly so fast I would never pack in wet shavings.  Unheated storage, down low, given favorable material and no hot air duct blowing down on it (DAMHIKT) should have been all you needed.  If you want to buffer the drying, use dry shavings, change after two or three days. Will help prevent the outside developing V shaped breaks.  If yours hasn't a pronounced V wider out, nothing in yet,  it's heart check.

To answer your final question, it's been so long since I lost one to a surprise crack, I've forgotten. With care cutting, shaping, and curing, should be no problem.

[NCPaladin]

First this is only what I would do and may not work for your situation.
Drying:  I would coat the end grain (figure 8) area with Anchorseal or similar product.  Then place in a closed paper grocery bag with no shavings.  An alternative would be to wrap the entire outside with stretch film (a 1,000 ft roll 6" wide is about $12).
Especially as green as it is I would want to stop air movement on the exterior.
Base:  It appears you used a tenon and left a larger area for the base and that is fine.  But, you now have a very thick base.  It appears you have enough wood for a recess instead of a tenon.  Some turners (Bill Grumbine) suggest turing the base thinner since it is side grain and loses moisture slower.  So with sides 1" the base may be turned to 3/4".  I also do not see a dimple in the center of the tenon.  I suggest after turning the tenon to bring the tailstock back up to mark the center for when you remount it.
Interior pillar:  I understand leaving a pillar if you initially turn with a pin chuck.  If you are grabbing this with normal jaws I do not see an area for the top of the jaws to seat against.  It is also cross grain and you will have a lot of unbalanced weight hanging off.  I think there is a high probability that the pillar will shear across the grain.  I would finish the interior without the pillar.
Remount:  With the dimple in the bottom you can use a simple friction chuck to re-true the tenon when dry.
Other:  It appears you did slightly round the top edges which helps prevent cracking.  You have gone this far so I would remount with the tenon and turn the top edge down to the bottom of the crack (1-1.5"?).  You will still have a nice bowl.
As wet as it was I would have left it a bit thicker given the size; maybe 1.25-1.5".

[OneStaple]

Sorry all for the long delay in responding to this thread.  I got sidetracked on some other stuff and didn't have a chance to respond.  Thank you for the thoughts though!

It looks like the pith was still in the bowl blank, which is a common area for cracking to emanate from.

I double checked the bowl, and there's no pith in the bowl blank.  The picture shows a darker line that looks like pith, but it's just discoloration.  I don't think the pith would have been very far from the edge of the bowl though.

I don't turn a lot of bowls but 48 hours in it's shavings seems like too short of a time to me.  Keep it in the shavings longer.  Slower drying is better.

The reason I took it out is because it was looking like it was starting to mildew/mold, which I really want to avoid.  Just some early signs, nothing super obvious.

Heart checks are normally there in the log.  Always look for them before you make the halving cut.  Sometimes it helps to wipe the end of the wood with some liquid and see if it gets sucked into existing heart checks.

I don't believe there were any pre-existing heart checks in the blanks.  The log was freshly cut and about 6" longer than the blanks on each end before I cut it into blanks.  The log showed some very minor signs of checking on the ends, but not much.  And I was turning the blank within an hour or two of cutting it out.

Second, the curve that redirects some of the drying stress down when you do the bottom also helps up top.  You're almost vertical in that area, so the risk of a split increases.

So aim for less vertical sides?

It appears you used a tenon and left a larger area for the base and that is fine.  But, you now have a very thick base.  It appears you have enough wood for a recess instead of a tenon.

Yes, I left a larger area for the base, but I also cut away wood inside that ring.  Except for right on the ring and the tenon, the base is probably 3/4" to 1" thick.

I also do not see a dimple in the center of the tenon.  I suggest after turning the tenon to bring the tailstock back up to mark the center for when you remount it.
Interior pillar:  I understand leaving a pillar if you initially turn with a pin chuck.  If you are grabbing this with normal jaws I do not see an area for the top of the jaws to seat against.  It is also cross grain and you will have a lot of unbalanced weight hanging off.  I think there is a high probability that the pillar will shear across the grain.  I would finish the interior without the pillar.
Remount:  With the dimple in the bottom you can use a simple friction chuck to re-true the tenon when dry.

I turned the outside using a face plate, then turned it around and used a chuck (power jaws on a supernova 2) to turn the inside.  The pillar was supported by a basic point live center (you can see the dimple if you look closely).  My thought was to just remount it in the chuck with the support of the live center and clean up both the inside and outside in that orientation.  I know it's not ideal for turning the outside again.  Is this a horrible idea?  And should I use a different technique completely?  I have pin jaws for the supernova 2 as an option.

I'm including a few more pictures of the state of the bowl now.  You can see that the initial crack has grown all the way to the bottom of the bowl and is visible on the inside.  The pillar also has multiple cracks, and there are other smaller radial cracks visible on the outside.

So I have another blank that I'd like to have a shot at (ends painted heavily to prevent checking).  I liked the overall shape of the first bowl, and, while I don't need the extra wood, the second blank is deeper so I can do more with a pin chuck or something else.  These are things that I'll change/do for this attempt:
-Cut further from the pith
-Make the bowl sides a little less steep.
-Perhaps use a mortise instead of tenon on the bottom.  I've only ever done tenons, but the mortise should allow the base to be a bit thinner.  I'd still like to create a small ring a little outside the mortise as the base.
-Put a dimple in the center of the mortise so I can do a friction hold to true up the mortise.  Then clean up both the inside and outside with the base held in the chuck.
-Try to keep the sides around 1.25" thick and a little thinner toward the base.
-Remove the pillar?
-Slow down drying by painting endgrain (or the entire thing) and putting it in a paper bag.  Still in the unheated garage with no draft.  MichaelMouse, i know you said just naked on the floor should be adequate, but if slowing down drying using these methods will increase my chance of success, I'm willing to wait the extra time.

Any other thoughts or suggestions on my approach?

Thanks,
Tyler

P.S. Pictures to come in my next post because they're on a different device.

[OneStaple]

Pictures!

Tyler

[MichaelMouse]

I'm still going with a heart check primary. Note the configuration of the crack down low.  A stress crack would look like that on the rim itself.  As I said, they do require close inspection.  That piece was a bit broad in the bottom for best survival, anyway, as the shape of the crack low demonstrates.  Making the sides less steep, and the bottom less broad would make a world of difference.  Has for me, anyway, and backed up by Hoadley.  Look at the bottom.  Five inches of continuous wood contracting can put some real stress on the piece, as the wide gap shows.  Not much to contract on the rim, and not much movement of moisture to keep it plastic.  That's where end coatings have effect, they keep the surface from starting a drying check that mechanical stress expands.

Bottom thickness and pillar make no difference.  Pillar is so handy to recenter, and holding both ends, much safer, that it ought to be used all the time.  Back in the bodgers' woods, always was.  They used points, I use a 1" pin chuck, and hundreds of production people used screws like the woodworm.  All good.


Here's a look at what broad bottoms, where there's a width of wood to contract, do for you.




Shrink is 1/8 in 2, so a half inch in 8 uninterrupted wood.  As soon as the wood meets air on the other side of the thickness, everything's rewritten. See the top which is 1/2" wider along the grain than across.

About as vertical as I'd go, unless I had elm, aspen or birch , woods with interlocked grain.