Newbie pith question

[Doghouse]

I keep reading that you should never have the pith in any bowl you turn.
I keep seeing people with natural edged bowls that have the pith in the center of the bowl and Sap wood on both ends.

What gives? Do they crack and that is just the way it is or is there a way to keep them from cracking?

 

[Mark Mandell]

What you are describing are end-grain bowls. The pith "rule" applies to them as well but with much less force than removing the pith from face grain turned pieces. I turn end grain pieces with the pith included, but I drill out the pith to about 1/2" before putting up to dry (I later plug the hole when the rough is dry and turn it off). That way, when the wood contracts, it's shrinking against air (the hole) and the piece is far less likely to crack or distort. Face grain cut-rim work you want (or I do) to have the pith out if at all possible, although I have successfully drilled & plugged small branch piths.

Mark

 

[Marc Phillips]

Well... it depends...

... on the wood of course...

On end grain turnings I see a lot of the better turners will leave the pith in, but they also take a great deal of care to oil and dry the turning slowly after it has been turned.

My friend DaveZ just finished an excellent end grain maple.... see it here:

http://www.woodworking.com/dcforum/DCForumID13/2758.html

 

[hockenbery]

No absolute rules

There are no absolutes in turning, although the laws of physics should only be violated with great care or lots of confidence.

In addtion to being prone to crack, two other considerations are that the surface around the pith is all engrain and the pith will be a structual weakness in a thin peice.

Usually the least attractive view of the wood is the endgrain. Look at Marc's link to a very nice bowl with a wonderful starbusrt pattern. Compare the top photo to the next two. Which do you like more? I think the endgrain around the pith is the least pretty portion of that piece. However to get that wonderful starburst effect the pith will be at its center. How often do furniture makers use the pith?

To be redundant the problem with the pith is that it is very prone to cracking. As wood dries, it shrinks across and around the growth rings. The simple way to look at this effect is that the growth rings try to straighten out. Very hard to do when the rings are connected so they like to split to accomodate the shrinkage.

I occasionally leave the pith in a piece and usually the results are nice.

Happy Turning,
Al

 

[Steve Worcester]

There are no absolutes in turning, although the laws of physics should only be violated with great care or lots of confidence.

I would agree (although not about the great care, more like with abandon).
A great case in point is the way Ellsworth tells you how to leave the pith in on a side grain hollow form. You just don't want it dead center for visual reasons.

With a natural edge bowl, you are less worried about the cracking, as long as it doesn't split. Slow drying, with either controlled environment or slowing it by using a finish to assist in sealing would both help.

 

[Doghouse]

Thanks for the information, lets see if I have absorbed it correctly.

1. If you drill out the pith and put in a plug, it should not crack.

2. End grain turnings are ok on the pith if it is thin.

3. Applying a finish (laquer or poly?) will slow the drying down enough where cracking should not be a problem.

pls correct me if I am wrong.

Thanks
John

 

[Mark Mandell]

Drilling Correction

John,

I drill the included pith out when the piece is wet. After it is completely dry, I ream the hole to a set diameter and THEN glue in the plug. This has helped to greatly reduce, But Not Always Eliminate, shrink cracking. Piece must be dry before the plug is put in.

Mark

 

[dkulze]

Oh, one more thing........ if pith is included, don't finish the inside of any vase until the outside has been fully soaked in finish. If the wood is really dry, it will soak it up like a sponge and crack due to expansion of the inside before the outside (a friend did this trying to waterproof a vase by pouring eurythane oil in and swirling it around. sounded like a rifle shot).

dietrich