Hey people,

just yesterday, I've turned a small bowl, I sanded it til 400 grid, and used danish oil and friction pollish.

I was verry pleaseed with this, it was very smooth,
This morning I found a little crack in the wall of the bowl.

I've heard that there are people that put they're projects in a microwave to prevent cracks.

Is this true?
Do I need a rough turn and put it in the microwave and then finish the bow?

I also had this with other projects, like lidded boxes and mushrooms.

when I've finished my project, it's fine and later I see cracks.


Is this something that you guys say: "well that's the thing about wood, to bad, but it happens " ?

I consider myself as a patience man when it comes to woodturning, but some people say that it can take years to completely dry the wood so you don't have cracks. I don't have thát level of  patience

how do you guys handle this?

Ed Weber wrote on Jun 16^th, 2017 at 9:09am:


It's hard for me to recognize what type of wood I have, but I think I have some Beech, Oak, and Apple and Birch, But I havent payed attention what type cracks the most

Ed Weber wrote on Jun 16^th, 2017 at 9:09am:


no sorry not really, A part of my wood collection is from the woods/forrest, so probably lots of moisture.
Well some of the wood is very dry and that doesn't crack.

Ed Weber wrote on Jun 16^th, 2017 at 9:09am:


I think 1 maybe 2 millimeter width

and the length Varies between 2 to 80 millimeters

Ed Weber wrote on Jun 16^th, 2017 at 9:09am:


I don't think that there is a big difference between my work shed and my house

Ralph Fahringer wrote on Jun 16^th, 2017 at 1:37pm:


Ehhmmm I don't need a microscope to see a crack of 2 mm in width
I take a look at mij piece of wood, I see the patterns of the wood, and BAM! one straight vertical line, that's a crack. Ain't that hard to see.

We don't deal witch inch and feet here in Holland.
Uncle Google says:
2 millimeters =
0.0787401575 inches

I have no clue what that means, but if I take a look at my measuring tape, 2 little stripes means 2 mm, not a big deal here

Don't know your background so I'll include some background. 

The growth center of the tree (the pith) is hollow and considered somewhat unstable.  Cut down a living tree, slice off a 1" thick disk from the log, and almost immediately a few short cracks will appear radiating out from the pith.  If the pith, or even wood within 15-20 mm ( just under 1")  is included in a bowl cracks are likely.  Hence the general rule to stay 25 mm or 1" away from the pith when roughing out bowl blanks.

As green wood dries, it shrinks more along the growth rings (tangentially) than across the growth rings (radially).  So when a green tree is cut, the exposed ends begin showing shrinkage cracks almost immediately.

Cut down a tree that's been standing but dead for a couple years and you might not notice any cracks in the end or under the bark, but turn into a bowl and cracks often will show up as the inside is hollowed out because they were already present, just not noticeable.

Turn a piece of green wood (was alive when cut) and often the spinning wood will sling sap on you - there is a lot of water in the wood, depending on the species of tree.  As the wood dries, even on the lathe while the bowl is being turned, that differential shrinkage mentioned above is taking place.

Turn the outside of a bowl  in a short period of time and the shrinkage may not be noticeable - the center of the bowl is still present and the wood  can't bend.  But turn the outside, leave the lump uncovered while you have a long lunch, and when you come back there may be some new shrinkage cracks.  I keep a few plastic grocery store bags in the back pockets of my turning smock and cover the work if I am interrupted for even a few minutes.

After finishing the outside of a green wood bowl, hollow the first 25mm of the inside of the bowl, taking the wall down to say 6mm or 1/4".  Now hollow the next 25mm of the inside of the bowl, then check the top of the wall - likely you will find the top of the rim is no longer perfectly round - the differential shrinkage has already started to bend or warp the wood.

Finish turning a green wood bowl and leave on a table for a couple days.  Likely the rapid drying created more stress than the wood could accommodate by bending, and the bowl has cracks.

As soon as I finish turning a green wood bowl I record the weight on the tenon and place the bowl in a paper grocery bag on the concrete floor so it will dry slowly.  Each week I note the weight and when the weekly loss is less than 2% I move the bowl in the bag up onto a table and continue weighting weekly.  When the weekly loss is again less than 2% I remove the bowl from the bag and continue weighing weekly.  When the weekly loss is again under 2% I consider the bowl dry enough to sand and apply finish.

If the wood has a reputation for cracking as it dries, I'll put the just turned bowl in a double paper bag to slow down the rate of drying.  Still occasionally have a crack, but not very often.

A thicker wall is more likely to crack because it's harder for the wood to bend and relieve the differential shrinkage stress.

Hope this helps.

Don Stephan wrote on Jun 16^th, 2017 at 6:19pm:


I don't have much background about wood behavior.
I will/need to read this a few times, to understand.
Thank you Don for your explanation

robert baccus wrote on Jun 18^th, 2017 at 9:56pm:



ok I will take a look at that,
But what about microwave?
some people say that you kan put your bowl in a microwave if it fits of course.
Is that a good idea?

Bob_Macgregor wrote on Jun 21^st, 2017 at 8:10am:


Yeah I've read that it's not a good idea to microwave the bowl at full trotle all at once.
you need to do that a few short times, when you microwave it when theres is no moist anymore then you get burn marks.

I don't have experience with that, it's what I've heard

One more thing to think about is if the pith is in the wood and it will crack around the pith