Crack Prevention and Repair for a Large Wood Turning

      Here's a long, thoughtful technical discussion of humidity conditions, wood cracking, prevention, and fixes, focused around the case of a very large wood turning that has developed hairline cracks. January 20, 2011

I recently made a large (20.125"x22") birch turning for a local woodworking business. They're fairly large and produce a lot of architectural wood products, so I assumed they had a temperature and humidity controlled shop. They didn't. They unpacked the turning and set it in a hot, dry room, where it cracked within 24 hours. What would be the best way to fix this piece? They're fine with a little bit of putty showing. I was thinking I'd just let the part set in our shop for a few days to reabsorb moisture from the air and then use a syringe to fill in the cracks with glue, then putty over the top.

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Forum Responses
(Architectural Woodworking Forum)
From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Your shop should be about the same dryness the customer will have. If your shop is wetter, the cracks will reopen. In fact, the cracks will reopen any time the wood is exposed to low RH conditions. Bottom line, for most exposures, is to avoid repair if the ambient RH is much of 32% RH. For larger cracks, a spline is possibly best, if the appearance is okay. Minimize putty, as it will squeeze out when the humidity goes up, and may not go back in when the RH drops again. (Most exposures will be around 30% RH in the wintertime and 50% RH in the summertime.) A finish will slow the change, but no finish prevents the annual changes.

From the original questioner:
We keep our shop at 65 degrees and about 35-40% RH, so all of our material doesn't split in the winter. Where they stored the piece was about 28% RH and significantly warmer. If I used splines to fill the gaps, wouldn't they act as wedges when the humidity goes back up, either at the installation point or when the weather changes, and split the piece somewhere else?

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
My opinion is that the shop should be 32 to 35% RH, as this is closer to the wintertime values for a home or office. You may stop splitting in your shop, but it will then occur at the customer's low RH facility. A value of 28% RH is very common in homes and offices.

Note that heat makes moisture move faster, but does not contribute to checking, as wood does not shrink or swell with changes in temperature. Wood only moves with changes in moisture content and MC changes only with RH.

The splines will compress if the piece gains moisture and swells. Do not make the splines too tight.

From contributor E:
It's interesting that these appear to be stress cracks at the corners of the square hole. What goes through this hole, and what keeps the wood from shrinking around the object and cracking?

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
A sharp corner is a point at which the stress will concentrate. If the hole were round, it would not have been so severe, and we might have seen the outside crack.

From contributor K:
I am not sure what the nature of the piece is. I suspect a newel post cap or something. It looks like you did a nice job of turning a sphere. I assume it is solid, which does not help your situation.

If you had turned out the inside, or even constructed it where there was some hollow, this would allow the loss of MC to allow some deformation, rather than the surface splitting due to the lag time from the middle being behind the surface in lost MC.

I am finding CA glue to be useful for dealing with small checks in some parts. It is thin enough for capillary action to carry it all of the way to the ends of small checks. It also seems to have pretty good bridging capacity, and when it is sanded down after totally filling a split, it just looks like a dark line in the wood. Of course that works better in dark woods than in light woods like this.

However, I like to pack sanding dust into the check if it is a light wood, which helps to blend. If you want to try this, I would start by applying it in the bottom corner as it is shown in your photo, until it starts to run out of the bottom of the crack. You can wipe it off down there, but then quickly start trying to smash the sanding dust powder into the widest crack while it is still wet.

Then start wetting it again from above in the corner, then spray the accelerator along the line with filler, to keep it in. This causes it to kick fast enough to self dam, so you can totally fill the split. If you see a gap, rotate it up, then fill, and spray accelerator instantly to keep it from running out.

At this point, you can start sanding and scraping any overflow with pretty fine paper on a pad. You will quickly be able to see your final results, and I will assure that it will be more stable than putty or most other filler.

I have turned a lot of large hollow green wood spherical vessels, which I managed to dry without checks, or used this to deal with some that did. This seems to work best for me and has not come back to bite me. If you plan to start over, I would suggest that you try another glue-up method.

From contributor I:
I agree with Gene that if you bring the product back to your shop and make repairs, it will adjust back to your environment and then get stressed again when you bring it back. One point to consider - where was the part stored in the customer's facility? Over the years I have seen numerous problems caused by storage near a heating unit, usually from the ceiling blowing hot air on the products. This has caused cupping, checking, shrinkage, splitting of glued products, etc. Sometimes this occurs when the stock is going through production and a cart is left near a heat source for a short while. I saw a load of 10/4 red oak treads ruined this way a few years ago. This can happen even though the shop on average has good humidity control.

From contributor J:
I can hardly see these end grain cracks and you're not showing a profile picture (which is where it really matters). Hairline cracks are usually no problem as long as they stay hairline. Drive a chisel into the crack to determine the extent of the problem. If the whole thing starts to split easily, you may have to replace it.

Sometimes you can split the post on a crack and then re-glue it together using band or strap clamps. After that you can put it back on the lathe for re-sanding. Sometimes a wax putty stick is the best solution for solid woodwork that refuses to stop moving. This is a simple fix after the finish is applied.

I guess you should just fix it the best you can and send it back. You can always replace it if you have to, but not until you've tried the putty (that's why they make that stuff). As a general rule, hollow architectural wood turnings like columns and lamp posts require an interior and exterior sealer prior to shipping. The best glue-up methods for large diameter turnings are typically stave or box construction. (I prefer the hollow box if possible, which has only four pieces to worry about.)

From the original questioner:
Here is a side shot of the piece. You really can't see the crack in the picture. It's only the top that split and the cracks are very narrow. I would have glued it up differently if I could have, but I didn't see another way. Staving would have been near impossible with the size.

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From contributor O:
That is an awesome turning. I can't tell without some scale, but if that's 3/8 sheathing it's on, it must be 18" diameter. Wow!

I am really curious as to why the cracking is in the center. I would expect it more at the outside diameter. I'll guess that since the segment construction was okay, it is going to get a dark or opaque finish, so I'd just use CA and some cleanup, and move on.

From contributor D:
Large architectural turnings like the one pictured need to be segmented and stacked as separate pieces. This is the way they have been done historically. The round hole is also crucial. In a perfect world, everyone can control RH and we would all have the same, but it is practically impossible to make those demands with any expectation of full compliance. Over time, it is more likely that some rare swing in the RH will occur, dooming any marginal woodwork. Is this a viable way to practice the craft?

Even if you explain, in writing, and customer understands and agrees, then it cracks, you still are faced with either making it right (even though you get paid, how will you do it?) or walking away - either "solution" makes for a less than happy customer.

Large column bases (Ionic, Doric, etc) are typically flat segmented and brick laid and turned. Breaks at the flats allow for some differential movement after turning and in service. Turing hollow is also a good strategy, as noted above. Just as wide mitered parts (think picture frames 8" wide) are made of 2-3-4 pieces of molding stacked on rabbets, the same needs to be done with large turnings.

From contributor L:
As noted, the problem is loss of moisture and the associated shrinkage. As such, is it possible the customer would allow you to treat the item with something to inhibit moisture gain and loss?

Presuming they want something that looks raw, you might be able to use a hardening oil. Linseed oil would likely darken too much. Tung oil, though not as bad, may also darken more than tolerable. Thinned walnut oil may work. Then there are also satin based waterborne finishes you could try. After curing, you could knock off any remaining shine. Of course, you should experiment on some scrap to test results.

From contributor N:
The others are right about the change in humidity when you delivered the bowl to your customer, but you cannot keep a wooden bowl all the time in a moisture controlled atmosphere. Climate changes from region to region and from summer to winter and from day to day.

But on your bowl, you made some major mistakes in how you glued the wooden sections together. You caused tension, which will cause cracking. I learned 10 years ago from an old wood turner in Norway that he keeps the finished wooden bowls for 3 or 4 weeks in a mixed pile of sort of moisturized sawdust and hay. It prevents fast drying and gives the bowl time to dry smoothly. And if you follow the old fashioned gluing rituals of tension-free wood gluing and give time to the bowl to get rid of tension smoothly, you will never have cracks!

You can't solve the problem with putty - the cracks will always move. Dip the bowl into a bucket of water for 15 minutes once you are finished with wood turning, then let it rest in your haystack and after 3 weeks, sand it. That's what the Norwegians have done for hundreds of years, and it works just great without failure.

From contributor N:
Sorry - I thought it was a bowl from upside down. Still, it's a big block and you didn't follow the tension free gluing rule. You can prevent future cracking.

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Although finishes of any sort may slow down the moisture change, there is no practical finish that will stop moisture movement. As mentioned, the hole is critical for relieving the stress and it should be round. If the moisture change is slow, then the issue is not as great as with a fast moisture change.

In most homes and offices in the USA, the interior environment varies from 6% EMC (30% RH) in the wintertime to 9% EMC (50% RH) in the summertime. (The dry SW USA including Denver and the moist coastal regions do sometimes vary from these values.) For these conditions, as shrinkage is more of a problem than swelling, the wood should be no higher than 7.0% MC, and 6.5% MC is even better. Unfortunately, wood that you buy is often "6 to 8% MC," which means sometimes 9%, and the core may be even wetter. Therefore, a moisture meter costing $200 or more is essential to get a reliable estimate of the wood's MC before you use it. If it is too wet, store it at 25% RH (in a plastic walled room with a home dehumidifier). In any case, store any wood you buy and the wood you are working on (especially over the weekend) at 32% RH.

From contributor T:
If the turning is to be painted, fill the cracks with CA adhesive, medium viscosity. With all turnings, sanding end grain is a worrisome labor. To get a nice smooth finish you tend to use too fine paper and create heat buildup. On a big piece, use slow RPM with coarse paper and do not allow heat to build up in any area. Move about the work areas. Heat will cause fine end grain cracks in seconds in 6% - 8% MC material. Fill it, sand it and seal it. The square hollow too. Let it stand in the shop for a few days. May luck be with you - redos are a triple headache. Also put the turning in a plastic bag and keep it in a cool corner away from blowing heaters.

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