Woods with piths that are least likely to crack?

[Chris Gunsolley]

This question is not about how to include a pith in a bowl. It is about the wood species that may handle that the best. Initially, one may assume that it is a simple matter of wood stability. Let's consider whether wood stability doesn't necessarily reflect the crack resistance of its pith...

I've been wondering lately about making some bowls with the pith included on the side walls of the bowls, as seen in the second picture down on this page:
http://www.customwooddesign.com/turninggreenwood-1.html

This has me questioning, which types of woods would have piths that are least likely to crack?

Do any of you have experience with including piths in bowls, and, if so, which types of woods seem to handle the treatment the best?

The answer to these questions does not appear to be a simple matter of the stability of the wood. For example, Oak is rated as very stable, but does have a tendency to crack. Pine is rated as not very stable, and the pith seems to stay intact. So, what it comes down to, I presume, is our experience. (Unless there is a rating on the stability of the pith, specifically--I wish.)

One way I investigate the tendency of the pith to crack is to inspect my cut-offs that were made due to end grain checking as opposed to cracks at the pith, and also to simply inspect the piths of the sitting logs. On average, the softer hardwoods, such as poplar, and even softer woods, such as pine, seem to have the most stable piths. However, I've got walnut bowls, for example, that have piths in them that haven't moved a bit. And, I've got woods that are rated in between the two in terms of stability that crack more than walnut. So I'm hesitant to conclude that the answer is simply "the softest hardwoods." The stability ratings are accounting for general movement of the wood, as opposed to the how intact the pith remains when incorporated into a bowl and allowed to dry... Perhaps you could even have some woods that will warp a lot but whose piths will tend not to crack and warp right along with the rest of the wood? What do you think?

If you've had some success with incorporating the piths into your turnings with particular species, please share.

[Bob Bouis]

Most hardwood around here either starts with cracks in the pith or develops them IMMEDIATELY after cutting. Sometimes you can literally see them cracking immediately after cross cutting with the chainsaw. Sometimes they're fairly small, other times they spread out half way to the bark or further.

I don't have any insights as to particular woods, but my advice for avoiding pith cracking would be: small logs and very thin walls. Obviously the bowls would have to be once-turned from green wood. You'll still get some minor star-type cracking but with any luck it won't spread far.

[Steve Peterson]

Wood cracks around the pith when there is a difference between shrinkage tangentially vs radially.

Here is a table found by searching for "wood t/r chart" at http://www.hypersurf.com/~charlie2/Turning/WoodShrinkage/T_R_RatioGraphNew.gif It lists southern magnolia with a 1.2 ratio (5.4% radial and 6.6% tangential shrinkage).

The other risk is that drying needs to be slowed down to prevent the edges from cracking before the interior has a chance to dry.

Steve

[Chris Gunsolley]

Most hardwood around here either starts with cracks in the pith or develops them IMMEDIATELY after cutting. Sometimes you can literally see them cracking immediately after cross cutting with the chainsaw. Sometimes they're fairly small, other times they spread out half way to the bark or further.

I don't have any insights as to particular woods, but my advice for avoiding pith cracking would be: small logs and very thin walls. Obviously the bowls would have to be once-turned from green wood. You'll still get some minor star-type cracking but with any luck it won't spread far.

Thanks for the reply, Bob.

After cutting logs, I see those star-like cracks pretty much every time. If I had just turned a bowl with the pith in it, and it had those star-like cracks, I'd probably saturate the pith area and all of its cracks in thin CA glue, both on the outside and inside of the bowl, then sand it smooth later. I hadn't considered your insight about small logs before, but that makes sense, because the transformation on a smaller piece of wood would be less drastic than on a larger piece.

[Chris Gunsolley]

Wood cracks around the pith when there is a difference between shrinkage tangentially vs radially.

Here is a table found by searching for "wood t/r chart" at http://www.hypersurf.com/~charlie2/Turning/WoodShrinkage/T_R_RatioGraphNew.gif It lists southern magnolia with a 1.2 ratio (5.4% radial and 6.6% tangential shrinkage).

The other risk is that drying needs to be slowed down to prevent the edges from cracking before the interior has a chance to dry.

Steve

Steve, that's EXACTLY what I was hoping to find! Great information, and it looks like you've provided me the measure I was wishing for. Thank you!

Perhaps for those of us that don't mind warpage, but want to avoid cracks, tangential over radial shrinkage (as opposed to stability) is the more important measure. Perhaps stability reflects magnitude of general warpage, while tangential/radial shrinkage reflects the tendency to crack?

In other words, you could have woods that warp a lot but are relatively crack resistant, and others that don't warp as much but do tend to crack. In the former case, you'd have a wood that's not very stable, but has low t/r shrinkage, and in the latter, you'd have a wood that's relatively stable, but has a high t/r shrinkage. If my understanding is correct...If I think a warped bowl looks cool but I don't want it to crack, I could turn it out of a wood that has a low t/r shrinkage yet at the same time has a low stability rating...

[Fred Belknap]

Chris I don't have experience with a lot of species but I do a lot of black walnut bowls out of limbs that are 6 to 10 inches and have good luck keeping them from cracking. The pith is in the walls and sometimes I get a crack and sometimes there is a small pith hole in the wing. Walnut is high on the tangen/radial stability chart. I usually twice turn them.

[Dennis Ford]

A couple of issues with the original post: oak is not considered a stable wood, pine is more stable than oak.
Woods that are difficult to split (like elm) or soft (cypress) are more likely to survive without cracks, how much they move while drying is only one factor.
Getting good results with pith in turned work is not just about wood selection, consistent thickness and careful drying is very important.
I have poor results when the pith is centered in the bottom of end grain turnings but rarely have an issue when the pith is in the side of cross grain hollow forms. If the pith is through the max diameter, warping will be noticeable but the shape allows movement without stressing the wood enough to crack.

[John K Jordan]

After cutting logs, I see those star-like cracks pretty much every time. If I had just turned a bowl with the pith in it, and it had those star-like cracks...

A difference between trees of the same species can make a difference too. I've had black cherry split at the pith quickly. Then I've had black cherry from another tree simply refuse to crack at the pith even after several years sitting outside. Must be a difference it the way it grew. Or maybe it's parents gave it better genes.

One of the most stable species that grows here is eastern red cedar. To balance that, flowering dogwood will often split from the pith to the outside of the log. Some species such as with interlocked grain (spiral grain that alternates direction with each ring) is resistant to splitting.

As mentioned, turning thin can help. Also, some people boil the wood blanks or roughed out turnings. This evidently softens the lignum and prevents many cracks as well as improve drying. A wood/wood products dealer from South America once described that they boiled all wood. He said some tropical species he deals with are quick to crack and split unless boiled. They used 55 gal drums over a wood fire.

Expert Stephen Russell has an article about boiling: http://www.woodturningvideosplus.com/boiling-green-wood.html
His web site is excellent to browse, especially his Tips sections.

JKJ

[Olaf Vogel]

I turn almost only green wood and try to keep the pith in, whenever possible.
Like others, I've not experimented to see which is least likely to crack.
I just turn what I get from the hydro crews and we how it goes.

Mostly thats Thats maple and the results there are fairly good.
Poplar worked only when very thin.
Birch and cedar are no problem
hazelnut was a real challenge. But these were also some the biggest pieces I've tried yet.

Side grain with thin walls works well. End grain bowls are tougher. I try to keep the foot small and thin. So as it dries, the foot tends to bow outwards. Of course it's still rough then, so will be finished flush when dry. Some of These quite large. On suspect pieces, I've poured in Minwax Wood Hardener, in the same was as CA described above. CA is likely better, but unaffordable if the area is 6" in dia.

Slow drying is critical.
But for small pieces, I've had great results with putting them in the microwave. I guess it's similar to boiling.

[Chris Gunsolley]

I turn almost only green wood and try to keep the pith in, whenever possible.
Like others, I've not experimented to see which is least likely to crack.
I just turn what I get from the hydro crews and we how it goes.

Mostly thats Thats maple and the results there are fairly good.
Poplar worked only when very thin.
Birch and cedar are no problem
hazelnut was a real challenge. But these were also some the biggest pieces I've tried yet.

Side grain with thin walls works well. End grain bowls are tougher. I try to keep the foot small and thin. So as it dries, the foot tends to bow outwards. Of course it's still rough then, so will be finished flush when dry. Some of These quite large. On suspect pieces, I've poured in Minwax Wood Hardener, in the same was as CA described above. CA is likely better, but unaffordable if the area is 6" in dia.

Slow drying is critical.
But for small pieces, I've had great results with putting them in the microwave. I guess it's similar to boiling.

Great information! I've got some Paper Birch, Red Birch, and Cedar, so I'll have to incorporate the pith into some of the bowls from them. I notice that Birch has a low t/r shrinkage ratio on the chart that Steve provided.

You said placing the pith on the side grain (with thin walls) works well. Do you ever bias the pith toward the top (rim) or bottom (base) of the bowl? Or, do you just center it? I'm just wondering if one location would work better than the other (on the side grain).

[Reed Gray]

Most trees have a split on the pith before they are cut. Only resistant pieces I have seen are smaller branches, 4 inch diameter max. If I turned one that had the pith in the center, I would turn to final thickness, and then soak it in a vat of finish...

robo hippy

[Leo Van Der Loo]

/snip//snip/

This has me questioning, which types of woods would have piths that are least likely to crack?

Do any of you have experience with including piths in bowls, and, if so, which types of woods seem to handle the treatment the best?

The answer to these questions does not appear to be a simple matter of the stability of the wood. For example, Oak is rated as very stable, but does have a tendency to crack. Pine is rated as not very stable, and the pith seems to stay intact. So, what it comes down to, I presume, is our experience. (Unless there is a rating on the stability of the pith, specifically--I wish.)

/snip//snip/

If you've had some success with incorporating the piths into your turnings with particular species, please share.

Chris I thought we had addressed this once already, but maybe not.

The answer is not as simple as naming the woods that won’t likely split, it is much more complicated as to my experience.

Species of wood, shapes of the turned piece, time of year the tree is cut, how lang it already sat, the treating of the suspected areas with CA, speed, or rather the slowness of drying of the wood, turned to finish thin or thicker for later returning.

Now throw the variability of each piece of wood into it, with things like tensioning caused by a nearby limbs that grew close by the blank, or a curved tree growing along the outside of a bush, twisted grain and other reasons that I probably forgot to mention, and you can understand it is not straight forward.

I have at least a couple dozen pictures with piths in the wood because of knots or the actual pith of the tree going through it, so I choose 8 pictures of different woods that all have pith in them and have no splitting.

I have for years already used CA effectively on knots that are in the blank so they do not split, but still not a 100% success rate, but close. (soak thin CA into the knot and surrounding wood and the pith area of the wood)

Starting with freshly cut logs that have NO splits and turning it in one go then placing it in a brown paper bag that is placed in a cool place.

Cutting a blank into a thin walled piece so it can warp under the tension of the drying and shrinking wood is another manner for less splits anywhere in the wood.

346621 346622 346623 346624 346625 346626 346628 346627

Willow Aspen Catalpa Elm are a couple you could try your pith-in turning endeavour on, they are almost bullet proof :)

[William Bachtel]

I have had good results with Black Walnut, both in bowls and crotch cutting boards.

[Steve Peterson]

Perhaps for those of us that don't mind warpage, but want to avoid cracks, tangential over radial shrinkage (as opposed to stability) is the more important measure. Perhaps stability reflects magnitude of general warpage, while tangential/radial shrinkage reflects the tendency to crack?

In other words, you could have woods that warp a lot but are relatively crack resistant, and others that don't warp as much but do tend to crack. In the former case, you'd have a wood that's not very stable, but has low t/r shrinkage, and in the latter, you'd have a wood that's relatively stable, but has a high t/r shrinkage. If my understanding is correct...If I think a warped bowl looks cool but I don't want it to crack, I could turn it out of a wood that has a low t/r shrinkage yet at the same time has a low stability rating...

That's a very good point. The t/r ratio is only one factor that helps reduce stress in 2 directions. Crack resistance seems like it would also contribute just as much. I wonder if there is a table for this?

Combine crack resistance with t/r ratios and that seems like it would highlight some very good species.

Steve

[Steve Peterson]

I was at a department store last night and they had a few cutting boards made from wood cookies about 1" thick and 12" in diameter with the bark on. There were no cracks anywhere including at the pith. The label said acacia and I think they were made in either Thailand or Malaysia.

wood-database.com doesn't have a direct entry for acacia, but black wattle led me to several acacia sub-species. Koa (acacia koa) has a t/r ratio of 1.1 and is a similar color to the cutting boards I saw. Some of the other acacia sub-species have t/r ratios as high as 2.8. Interesting.

Steve

[john taliaferro]

walnut and elm , i had a picture on my phone of cherry that cracked and persimmon is worse yet .

[Perry Hilbert Jr]

According to wood data, paulownia rarely cracks or checks. It grows like weeds, some five to ten feet a year. I have two that came up wild in a fence row. A few miles away, a guy planted a Grove of them and the seeds blow all over the place. I plan to cut one this winter to see how it turns.