It’s hard to think of anything other than a wood bowl crack as a wood bowl turner’s number one enemy. A wood bowl crack appearance can make grown men cry and draw attention away from an otherwise perfectly turned wood bowl.
To prevent and repair cracks in wood turned bowls, we need to understand better what is causing the cracks to form.
Put simply, change is what causes cracks.
More specifically, uneven change causes a wood bowl to crack. Anytime temperature, humidity, or internal moisture content changes, there’s a good chance there are internal structural changes that will take place inside the wood cells, fibers and grain.
It’s imperative as wood turners that we pay attention to these conditions and show our respect as much as possible. When we don’t, our beautiful bowls suffer the consequences.
Wood Bowl Crack Prevention
As they say, “An ounce of prevention keeps bowl cracks to a minimum,” or something like that. So what are we trying to prevent? Uneven, abrupt or excessive wood movement essentially is what we’re trying to prevent.
Wood movement occurs from a few major factors: internal stress, moisture content and temperature changes.
Let’s dive right in and start by covering ways to reduce all of these factors and greatly reduce the chance cracks will visit our bowls.
The length of green wood logs prepared for bowl blanks does make a difference. Even with the additional preventive measures, we will discuss below, there will be some checks and cracks on the ends of the rough-cut bowl blanks. Cutting logs longer than needed will provide excess waste material on the ends of the log that can be trimmed away later.
The length to add to a log will depend on the overall diameter of the log. In general, it’s good to add at least six inches to the log diameter to determine the cutting length.
For example, if a log is 10 inches in diameter, cut the lengths 16 inches long. The extra length leaves about three inches of wood that can crack and check on both ends.
When the time comes to turn the log, simply saw off an inch or so from one end. Look at the newly exposed cut area. Are there still cracks? If so, trim off another inch and check, again. Once no cracks appear, do the same trimming on the other end of the log. Now you have a crack-free log to begin turning.
Half the Trouble
When preparing green wood bowl blanks, it’s critical to get the full round logs split in half as soon as possible. This one step removes a great deal of tangential pressure caused by the inner growth rings and the fact that they are unevenly sized.
The inner rings at the pith of a log are under much more pressure than the outer rings. As the whole round log ages, the outer diameter of the log loses moisture faster than the interior, thereby causing increased tension.
Watch the Pith
Some people swear the pith CANNOT be in the bowl or the whole thing will blow apart. While this can be true, it’s also possible to leave the pith on the bowl walls, and everything turns out fine.
The particular wood species and how the bowl is created and dried later will greatly determine if the pith will or won’t crack and deform.
At a minimum, the pith will usually stretch and pull away from the turned shape of the bowl as it dries, making the bowl more elliptical than round.
If the pith is something with which you’d rather not take a chance, by all means, avoid it. Cut away any wood that is within the inner circle surrounding the pith. This distance may only be a quarter inch on a small log or several inches around the pith on a larger log.
Either way, I would encourage you to experiment with the pith being left in a bowl or two and try this with different types of wood just to see what happens. You might be surprised.
When a log has been cut to the proper length and split in half, that’s a great way to store green logs for a longer period. But before they get tucked away, it’s best to seal the end grain.
Think of the log as a straight linear bundle of long drinking straws. The ends of the straws correspond to the end grain of the log. The fibers run lengthwise through the log much like the drinking straws. And like those straws, the wood’s end grain has small openings allowing moisture to move in and out quickly and easily.
Because of this physical property, the ends of the log will lose moisture content much faster than any other part of the log. The best way to counter this is to seal the end grains with Anchorseal (check current price on Amazon).
Take an inexpensive chip brush (Amazon link) and slather a liberal coating of Anchorseal on each end, covering all the end grain. I also will put a small path along the flat cut edge near the ends to make sure all the end grains are sealed.
Wood storage plays a big factor in how it will later behave. If you’ve been following along, you understand we want “no sudden movements,” literally. So the storage area you select should continue along these lines.
Green wood storage is a tricky area, because what works for me might not work for you based on your local temperature, humidity, and climate.
So instead of giving some blanket answer, let’s think about what is needed. If the wood is green, wet or has any moisture, it needs to lose that moisture slowly. Or rather IT IS going to lose that moisture. It’s our job to slow that process and make it as gradual and even as possible.
In general, cool, not overly dry, dark (or at a minimum no direct sunlight) spaces will slow this drying process. While I can stack wood along a wall in a very open barn outside where I live in a humid area, that practice will create splintered bowl blanks in the harsh dryness along the Rockies. There, it might be necessary to have a humidified space to store wood, such as a cool basement.
Remember the basics: no direct sunlight, no harsh temperature or humidity changes and avoid excessive air movement. Simple enough, right? Ha!
Get It Done Quicker
Once a bowl blank is mounted to the lathe, it’s a good idea to complete the piece, exterior and interior, as soon as possible. I’ve seen people spend weeks working on the same piece, only to have massive cracks appear midway through the turning.
Why does this happen? The newly turned areas exposed to the elements change unevenly. Usually, the outside of the bowl is fine at first, but the added forces within the bowl’s still solid moist interior are under their original pressure and continue pushing and pulling. The newly exposed area is losing moisture content much quicker than the solid interior, and those internal forces usually manifest as cracks on the outside of the bowl.
It’s important to turn the wood bowl within a reasonable timeframe to prevent these internal forces from ruining a bowl. The bowl needs to be completed more quickly when there is more water content in the bowl blank. A tremendous amount of the wood’s internal pressures are removed when the interior of the bowl is turned away and removed.
If a bowl cannot be completed promptly, inside and out, consider wrapping it in plastic and possibly spritzing it with water first to slow the drying of the outside bowl walls.
Nice and Even
Unevenly turned bowl walls are another contributing factor for the production of bowl cracks. When an area of a wood turned bowl is thick in one area and thinner in another, the two areas dry and move differently. When those movements oppose one another, cracks can form.
Try to turn the bowl to a uniform wall thickness throughout. The base also needs to be turned to a similar thickness as the walls. A thick solid base under a thin-walled bowl will dry much slower than the side walls, usually causing issues.
Avoid the Heat
Once a bowl has been evenly turned, it’s time to sand. Be aware that sanding at too high of a speed can cause heat buildup. If either the lathe or a power sander, or both, are turning too quickly, they will produce excess friction and heat. Moisture is lost more quickly when the wood bowl surface is heated, which can cause cracks to form.
Heat-related cracking is especially true with thin-walled forms and bowls. Slow down the lathe and the power sander and only work for short times in a given area to reduce heat buildup.
An even better approach, although one that makes many people cringe, is to hand sand the final turned wooden bowl.
Once a bowl, which is made with green or moist wood, is complete and off the lathe, the drying process needs to be controlled. Again, the control that is needed is usually to slow the drying process. A great way to slow the drying time is to bag the finished wood bowl.
Before you sweep the floor, grab several handfuls of those moist shavings from the bowl that was just made. Place a shallow bed of shavings in a brown paper grocery bag, then add the new bowl. Fill around and inside the bowl also with those shavings and loosely close the bag.
Store the bagged bowl away in a cool dark place that’s not too dry or moist. Usually, a space indoors at similar conditions to where the final bowl may be displayed is ideal. After all, the wood needs to acclimate to the space where it will eventually live.
There it is! A fine wood bowl crack line appears along the edge of your beautiful bowl. What to do? Even with taking all the preventative measures covered, cracks can occur. If a wood bowl crack appears, don’t fear. There are several options for recovery available.
Small bowl cracks can easily be repaired and mitigated. This process is simple and will also help to reduce telltale stains around the crack.
To complete this task, you’ll need a good quality CA super thin glue, CA accelerator and a Clear Spray Lacquer (Amazon links check current prices). Don’t worry, the spray lacquer is only acting as a mask and will be sanded off later when the bowl is completed.
Lacquer is a film finish that sits on top of the wood and once sanded off; it will not limit any final finishes you choose to use. The purpose of first using lacquer is to prevent the CA from staining the wood immediately around the wood bowl crack.
This process is best done while the bowl is still on the lathe before final sanding and finishing. Here are the steps to correctly fill a small to medium wood bowl crack and prevent them from growing.
- Spray around the wood bowl crack with lacquer, then let dry a few minutes.
- Pack very fine sawdust into the crack.
- Using the fine tip provided, drip super thin CA into the crack.
- Rub fine sawdust over crack.
- Spray crack area with CA accelerator.
- Continue turning or finish sanding area smooth.
Celebrate the crack! It’s natural, after all, and nothing is perfect. Some cracks look amazing and can be left “as is,” adding unique character to a gorgeous wood turned bowl.
If you’d like to fill, instead of look through a crack, there are many decorative and colorful powders available to highlight the wood bowl crack area. There are even turquoise chips and metal powders that can be used for this step.
If the crack is small, the same steps can be used from above, substituting the colored filler for the sawdust.
Larger areas may require an epoxy mix to fill the entire area. A dam on one side of the bowl can be made by applying tape to cover the cracked area.
Mix the Two-Part, Five-Minute Epoxy (check Amazon for current price) according to the product instructions in a portion large enough for the given wood bowl crack area. While mixing the epoxy parts, add the color mix and then apply the mixture to the cavity.
Let the colored epoxy dry according to the epoxy manufacturer’s details. Again, this step is best done before the bowl is complete, so the repaired area can be further turned and sanded to the desired finish.
If the repair area is large or if you’re interested in creating new material to turn, try the Alumilite Casting Epoxy (check Amazon for current pricing). Alumilite can also be colorized and then sanded after it has dried.
Let’s face it, some cracks become way bigger than originally thought acceptable. But before you turn that bowl into a piece of fancy luxury firewood, consider reinforcing architecture.
Depending upon the bowl and your taste, many things can be done that both transform a “crack” into a “design decision” and make the piece more structurally sound at the same time. Not only can these structures be practical, but they can also be quite creative.
Stitch up a seam by drilling holes parallel to each other on opposite sides of the crack. Using string, leather or wire, stitch the crack up. It doesn’t need to be drawn tight or closed to have a supporting and striking visual effect.
Butterfly joints can be applied as well. Often used in furniture construction, butterfly, or bow tie, joints can be hand chiseled and inlaid into the bowl across the crack area. The dovetail shape of this inlay will hold the crack as is and not allow it to continue to grow, all the while appearing somewhat planned and deliberate.
There you have it, 12 tricks for preventing and repairing cracks. When making wood turned bowls, we need to remember we’re working with a beautiful natural resource: wood. It is not, nor was it ever intended to be, perfect. That is nature.
We can do as much as possible to avoid a wood bowl crack, and deal with them when they do appear, but after all is said and done, cracks happen. I guess it’s all in how we look at them that makes the difference.
How do you deal with wood bowl cracks? Leave me a comment below.
– For details of equipment mentioned in this article see my Recommended Equipment Guide.