Does Sealing Wood Help Prevent Warping?

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Even though the warp drives in Star Trek lore were merely make-believe and came from Ray Bradbury’s imagination, engineers and scientists today appear to be close to a warp drive engine.  Imagine that.  (pun intended)

A warped sense of humor can be both funny and scary, depending on the context.  A warp on a weave refers to all of the threads running in one direction, as opposed to the weft, which refers to the threads that move over and under the warp.  We bet that was something you didn’t know, and weft is probably a new word for you.

In woodworking, though, warp refers to something we don’t like to find.  It refers to a distortion in the shape of the wood we are working with on our projects.  We don’t like warp or warping.  While sometimes a warp can be cured, it’s extra work we don’t want to have to take on, and it can mean a change in our project plan is warranted.

What Is Warping in Woodworking?

A distortion in shape is the simple answer.  But, warping can take on a number of forms and be caused by a number of factors, most of which can be prevented.

Basically, it’s a deformity in the wood that is caused when the moisture within the board dries unevenly. The moisture content of the wood evaporates/dries in a number of ways depending on the mill where the wood was cut from the tree logs. 

Lumber is cut lengthwise (of course) from the logs in long sections after the slab has been removed.  The slab is the outer part of the log that includes the bark; after the first slab is removed, the log is turned 25%, and another slab is removed.  This is repeated until the log has been “squared,” if you will, and then the breaking down begins.  The slabs are mulched, and the fibers will be used in the making of paper.

Cuts of 2″ and less are referred to as boards; cuts thereafter up to 5″ are referred to as dimensions, and cuts above 5″ are referred to as timber.  In the home woodworking shop, we’re usually dealing with boards.

After the log is broken down into these constituent parts, the boards and dimensions will be dried in some way.  The primary drying methods include:

 Air Drying Wood

Exactly what it sounds like, air drying is simply letting air dry the wood.  Duh.  The wood is stacked and separated so that the air can circulate all around the boards and dimensions.  It takes longer than other methods of drying, and the wood will be covered in some way to protect it from the environment that would introduce even more moisture into it.  Air drying with proper ventilation will reduce moisture content in the wood to about 20%.

Kiln Drying Wood

Air is also used in this method, but the area is enclosed and sealed, and the air is heated before being circulated around the wood.  With temperatures in the 110-degree to 180-degree range being circulated around the wood, the drying is much faster.  This method of drying will reduce the moisture content of the wood to about 15%.

Kiln-dried wood, the result of which is a lower moisture content, will be used for interior projects like molding, doors, and flooring because the risk of further shrinkage has been minimized by the kiln.

Vacuum Drying Wood

This is the least-used and most expensive way to dry wood, least-used because it is the most expensive.  The wood is placed in a sealed room, and the atmospheric pressure in the room or chamber is lowered.  This lowers the temperature at which the moisture in the wood would boil, and thus the temperature needed to introduce it into that environment is lower to help the moisture evaporate.  It is a particularly effective method of drying wood and dries it much faster than the other methods.  Again, though, it is also the most expensive, and fewer mills choose this method over the others.

With an even drying of wood from these methods, it is less likely that some parts of the wood will dry faster than others.  It is this discrepancy of drying that causes the various forms of warping.  The moisture level is fairly even, and so the wood retains its cut shape.  It will shrink, of course, since some of its content (humidity level) is reduced, but it is far more likely to shrink evenly and thus retain shape.

Why Does Wood Distort and Change Shape?

As we have already stated, it is the uneven drying of the wood that leads to warping and distortion.  When one part of the wood dries faster than another and thus shrinks faster, it turns in some way that causes a change in the shape of the wood.  We’ve also said, and we woodworkers know, that warped wood is bad.  Warped wood interferes with our projects and our work, as we have to take steps to cure the warped wood before we can continue with the work.  

In the processing of wood from tree to mill to boards and dimensions, the primary part of that processing is the breaking down of the logs.  You must remember, though, that trees take in water through their roots, and the water is distributed throughout the tree during its growth.  That moisture causes the wood fibers to expand.  During the drying process, those wood fibers contract as moisture leaves them. 

As long as the drying process is even, the wood will retain its shape.  When it’s uneven, distortion occurs.

By drying, we’re not suggesting the wood dries completely.  Rather, we are talking about the moisture content reaching the same humidity level as the environment surrounding it.  That Equilibrium Moisture Content is the equal moisture content of the wood with the relative humidity level in the air around the wood.  

Conversely, when the moisture content of the wood is lower than the environment around it, the wood will absorb that moisture until the EMC is reached.  This will cause the wood to expand, and if some of the wood absorbs more humidity faster than other parts, this, too, will lead to distortion.

As we have written many times before, water (moisture content) is the enemy of wood.

Distortions in Wood From Warping

Wood distorts and warps in a number of ways and forms.  Types of wood warping include

The Wood Will Bow

Bowing is easily envisioned if we say simply that the ends curl up lengthwise.  This is to distinguish the warp from distorting across the face of the wood.

The Wood Will Kink

One end of the wood will turn and change the straight line edge to edge of the wood.  Envision a 2 x 4 where only one end of it bends from a straight line.

The Wood Will Twist

Turning the four corners of the board such that neither face of it is no longer on the same plane.  Envision holding it in your hands and twisting each end in the opposite direction.

The Wood Will Cup

The sides of the board will curve in toward each other, creating a concave face on one side and a convex face on the other.

The Wood Will Crook

Instead of the sides curving, when wood crooks, it is the ends that curve, creating a concave edge and a convex edge.  Think of a smiley face for this one.

This is not to suggest that wood will warp in only one way, too.  Sometimes there will be a serious enough moisture content problem in the wood that it will distort in more than one way among these five.  That will probably mean the wood will be set aside and saved for some other use, as it might well not be fixable.

Can All Wood Types Warp?

We’d have to say that almost any type of wood will warp under extreme circumstances.  However, some woods are better at resisting warp than others, all mostly hardwoods. 

The less porous a wood is, the less likely it will absorb moisture, and its moisture content after drying will remain fairly constant.

Drying, by the way, and reaching EMC level, is another way of saying curing.  Cured wood is wood that has been dried sufficiently that its moisture content is consistent with the relative humidity level of the environment surrounding it. 

Ways of Preventing Warping in Wood

Wood Planks Drying

When wood is properly dried and then stored in a way that allows air to circulate well all around it, the EMC will be maintained.  It might absorb a bit of moisture, it might lose some, but as long as that absorption rate is not extreme in either direction, wood should hold its shape.

The edge ends of wood are the easiest entry point for moisture, and you may be able to prevent warping by sealing the edges.  However, as a general statement, sealing wood will not prevent wood from warping completely.  The edges are your best bet, but the sides and surfaces will still accept moisture.

The best ways to prevent warping of wood in your home shop or prevent warping from being a problem you need to deal with are:

Inspect the boards before you buy or accept them.  Check those 2 x 4s and dimensional lumber before they get into the shop. If you miss that and bring them home, you’re stuck with them and will have to find a way to unwarp them, if that is a word.

Store the boards properly.  Keep them dry and away from where they will get wet.  Inside storage protected from rain is one way; maintaining a relatively dry storage environment will also help.

We’ve written about dealing with warping in wood, including how to store wood to prevent warping and how to fix warped kitchen cabinet doors.  There are ways to fix warping, but it does require some effort on your part and will slow down your project.  Storing it properly will lessen that need, so at least read the first piece.

Although this video doesn’t discuss all 5 types of warping, it does cover 4.  It’s only 5 minutes, too, and will give you an idea of causes and cures. 

Buy right, store right, and you will lessen the possibility of wood warping.  That’s the message in this article today.

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