Checking the moisture content of a piece of lumber

From unwanted wood expansion and contraction to rusty tools and equipment, there's no question about it: humidity is not a woodworker's best friend.

Let's take a look at what the experts have to say about dealing with woodshop humidity, how to protect your tools, and the how to judge the effects of moisture on the lumber you use.

Decreasing Humidity and Protecting Your Tools

It's not always easy to predict or control the humidity in your woodshop, but ignoring the moisture in the air can have serious consequences. It doesn't take long in damp air for corrosion to appear on your expensive woodworking tools. What can you do to protect your investment? Below, expert woodworkers Michael Dresdner and Rob Johnstone offer a few words of advice.

I am in the process of setting up a woodworking shop in my basement. The basement is dry, but can get humid on occasion. Should I be concerned about my woodworking equipment rusting? If so, what can I do to prevent it?

Rob Johnstone: There are several products on the market that protect against rust. It pretty easy to prevent surface rust on the exposed areas of your equipment, but the hidden (or more accurately enclosed) areas are a bit trickier. It would not hurt one bit to invest in a de-humidifier: an ounce of prevention is worth a pound of cure.

Michael Dresdner: Congratulations, and welcome to the club! You are about to embark on a long and very satisfying career of perpetually yearning to buy new tools, and trying valiantly to shoehorn them into your shop.

There are two things you can do to keep them from rusting, and I would suggest doing both. First, coat all the exposed (unpainted) surfaces with either paste wax, or one of the many rust preventative coatings (Boeshield, Slipit, etc.), all of which work even better than wax. Second, invest in a good dehumidifier and run it during the wet season. (See, I told you this would lead to buying new tools!)

A dehumidifier is the most direct solution for a damp shop. Still, bringing the humidity down doesn't eliminate the need for tool maintenance. Corrosion still takes place in "normal" ranges of relative humidity, although at a slower pace. Along with the effects of moisture, dust and grit routinely produced in a woodshop get into working parts and cause wear, and buildups of resin on blades and tool surfaces causes increased friction and poor tool performance.

The small amount of time and money involved in taking care of your tools and protecting them from the elements and excessive wear is a very worthy investment.

Dealing With the Effects of Moisture on Wood

Wood can dramatically change shape and size along with changes in its moisture content. That alone isn't an insurmountable obstacle for the woodworker. More, it's an invitation to read up on the physical realities that make dealing with changes wood moisture content a basic aspect of the craft, and to equip yourself with one of the most fundamental of all woodworking tools: a moisture meter.

There are numerous books and other sources of information explaining the effects of moisture on wood, drying techniques, and how to deal with ever-changing relative humidity. Understanding Wood, by Bruce Hoadley, is held by many to be the best, and covers moisture content thoroughly, along with just everything else you'd ever need to know about the physical properties of wood. We're sure that all serious woodworkers have a copy of it - or a similar book - in their library, and are experts on the effects of wood moisture content. For anyone new to woodworking, here are a few of the basics:

The moisture content of a piece of wood is the ratio of the weight of the water in it to the weight of the wood fibers if they were completely dry. Usable lumber is produced by reducing the moisture content of green wood (sometimes well over 200 percent) to 12 - 18 percent by air drying, or to 6 - 10 percent by kiln drying. The purpose of doing this is to bring the wood into equilibrium with the relative humidity of the climate in which it is intended to be used. If a piece of wood is in perfect equilibrium with relative humidity of the air surrounding it, it will neither absorb moisture from the air, nor will the air absorb moisture from it. For the woodworker, equilibrium is a good thing, because it means that the wood will stay (for practical purposes) the same size and shape.

In reality, of course, there are changes in humidity, and they can be dramatic, especially in climates where heating greatly reduces the relative humidity indoors during the winter months. "Equilibrium moisture content" is really an ideal. The best you can do is to build with wood that has a moisture content suited to meet future changes in humidity, and to make allowances for variations in moisture content in the way your design and build your projects. To begin taking moisture content into account, you'll need an accurate reading of the moisture content of the stock you use. And for that you'll need a moisture meter.

There are two basic types of moisture meters to choose from: Pin-Style Moisture Meters and Pinless Moisture Meters. A pin-style meter has two probes, or "pins" that are pressed into the wood; the meter measures moisture content by measuring the resistance of the wood as the conductor of an electrical current from one pin to the other. A pinless moisture meter uses radio waves to create an electro-magnetic field penetrating the wood. The meter measures the reaction of the radio waves to the moisture in the wood.

Adding either one to your tool collection, along with a reliable wood book, is an excellent way to begin sidestepping the frustrations that go along with ignoring moisture content: miter joints that pull apart, unexpected cupping or twisting, loose mortise and tenon joints, and a host of other surprise difficulties that can ruin a woodworker's day.