Overview: One of the more popular ways of drying green wood roughouts is to place them into ordinary paper grocery bags. Through the years, this Plain Paper Bag drying method has been one of my favorite methods for drying roughouts with a minimum of effort and expense. I frequently use the paper bag method to dry my production bowls when I'm short on time. It's quick and easy while maintaining a high degree of success with most timbers.
However, as many of you know, I'm always experimenting with different protocols in an attempt to speed up the drying process, while maintaining the high success rate of the base drying protocol. In my continuing search for better, faster and cheaper ways for drying green wood efficiently using various protocols, I have developed a simple and efficient way to speed up drying green wood roughouts when using paper bags.
Why Paper Bags Work So Well
for Drying Green Wood Roughouts
Inserting green wood bowl
into paper bag for drying
The reason the paper bag drying protocol works so well is because the paper bag creates a self-regulating "micro-climate" inside the bag, producing a near ideal environment for drying green wood. As the moisture vapor is released from the wood, the paper bag acts as a barrier layer slowing its dissipation into the surrounding ambient atmosphere. As a result of this action, the paper bag effectively maintains a higher moisture level inside the bag, than the surrounding ambient air outside the bag.
This keeps the exterior fibers of the wood inside the bag moist, helping to prevent the rapid onset of steep moisture gradients from forming, which can lead to checking. Paper bag walls seem to offer just the right amount of moisture release, not too fast and not too slow -- without compromising the drying process and introducing excessive cracking or warping. It's a kind and gentle process that Mother Nature and the timber seems to favor in the journey towards equilibrium moisture content.
For full details on my paper bag drying protocol, read my article in the education library.
Drying Green Wood Roughouts:
Not content to leave well enough alone, I decided to experiment with ways to improve my paper bag drying protocol by speeding up the amount of time it takes a green wood blank to reach equilibrium moisture content, or what most people call dry or seasoned wood.
Equilibrium moisture content, or EMC is defined as the point at which the moisture level inside the blank has reached equilibrium with the moisture level in its ambient storage environment.
Bagged green wood bowls drying on wire racks
goal was to find a way to allow the water vapor inside the bag to
escape faster, without increasing the defect rate for roughouts dried in
paper bags using my standard paper bag drying protocol. My early
experiments with trying to puncture the bag with tiny holes did not
significantly reduce the drying time and it was a labor-intensive task.
Various experiments with other vapor permeable materials fashioned into
bags offered similar drying times and success rates, with no clear
advantage noted in the base protocol.
As I mulled over my options one day, I remembered that roughouts (drying in paper bags) located near the air-conditioning ducts in my primary drying room always dried faster than those that were farther away from any direct airflow. I reasoned that this was partly due to the fact that the forced air moving across the surface of the bag helped to remove the water vapor from the bag faster than the bags that received little airflow, effectively reducing the total drying time.
I knew this had consistently worked in the past, but I needed a way to duplicate the beneficial effects of the forced airflow on a larger scale, while maintaining the simplicity of the paper bag drying process. The easiest way I found to add forced airflow to my drying stacks was to use an ordinary oscillating fan.
Note: All of my primary timber drying is done in climate controlled drying rooms that are air-conditioned, or heated year round.
To test my new theory, I set up a test using a large stack of bowls and platters of various species, sizes and thicknesses placed in paper bags. Each bag was placed on wire racks for optimal airflow around the bag. To create the forced air, I used an ordinary 16" variable speed fan, positioned to sweep across the stacked bags. The fan was set to run (24 hrs a day) on high. I enabled the variable direction sweep function to minimize any concentrated drying winds. The idea was to sweep the air across the stacks, keeping the forced air moving in and around the stacks as evenly as possible.
This forced air moving across the surface of the bag, speeds the drying of the exterior surface of the bag. This allows the water vapor to make its way through the paper bag walls faster. This in turn, allows more water vapor to be released by the wood, which dries the wood faster than just letting it sit inside the bag in the ambient air. Of course, the wood will only release the free/bound water at a certain rate using a method like this, but my goal was to speed up the drying as much as possible, while maintaining a high overall success rate with as few drying defects as possible.
Over the next few years, I conducted extensive tests with hundreds of roughouts and solid wood blanks using my new protocols. Numerous permutations of the protocol were tested, including tweaking the velocity of the air from the fan, the position of the bags on the racks and the position of the pieces in the bags. As a result of this extensive testing, I developed two new forced air drying protocols (oscillating fan on high and stationary fan on high) that proved to be highly successful, offering significant reductions in overall drying time, without any increase in drying defects over the base protocol.
Drying Green Wood Roughouts: Results of the Testing
Success! Test pieces dried an average of 25% faster in the oscillating fan test group, versus the control group that were dried in paper bags with no forced air flow other than the occasional cycle from the air-conditioning or heater. Not wanting to rest on my laurels, I performed additional tests with a stationary fan (not oscillating) with the fan set to high and directed at the bags stacked on wire racks. In this subset, the pieces dried an average of 35% faster without degrade, versus the control group.
I've been using this fan forced drying protocol for many years now and it consistently reduces the overall drying time in paper bags 25% (oscillating fan forced air) - 35% (non-oscillating forced air), depending on the species and the placement of the bags on the drying racks. I've also used this fan forced drying protocol with roughouts that I have boiled and bagged, with similar reductions in overall drying time. Of course, green wood roughouts that have been boiled dry much faster than non-boiled green wood dried in paper bags anyway, but this new protocol still reduced the overall drying time in boiled and bagged test pieces.
Recently, I have been experimenting with using high velocity fans instead of standard house fans to produce the forced air circulation. Results thus far are excellent, but I have not completed enough of the test to publish the results at this time.
Drying Green Wood - Final Thoughts
If you're drying green wood roughouts in paper bags now, give my Fan Forced Paper Bag Drying Protocol a try. Be sure to allow full airflow around the bag and set up your fan in either oscillating, or stationary mode. I like to use wire racks for my primary drying room as the spaces in the racks help to insure a good and even air flow around the bags, even if you do not employ the fan forced protocol. The first fortnight or so after the piece is bagged is a prime time for mold to develop and you want to do everything possible to discourage mold growth. Good luck!
Safety Note: Always follow all manufacturers safety instructions before working with your lathe, or any of the tools or products you may use. If you are unsure about any operation, obtain competent professional instruction before proceeding. Use and wear all necessary safety devices during turning and observe safe woodturning practices to prevent accident or injury.
Steven D. Russell is a professional studio woodturner, teacher and writer. He has written numerous articles for international woodturning magazines, which have been published in more than 78 countries around the world. Steve has demonstrated in numerous cities across the United States. His studio, Eurowood Werks, specializes in bowls, platters and hollow forms with unique visual and tactile treatments.
Steve is also the current and founding President of the Lone Star Woodturners Association, Inc., an AAW member chapter. The LSWA is a 501(c)3 non-profit educational organization dedicated to teaching and demonstrating the art and craft of woodturning.
Steve is also a featured writer for the Guild of Master Craftsman's "Woodturning" magazine, published in London England. Woodturning magazine is the world's leading magazine for woodturners. Look for his articles covering technical topics, or project based articles in an upcoming issue.