Paper bag drying uses plain brown paper bags which create an ideal environment to dry green wood rough outs.
|Saturation vapor pressure is achieved when the ambient air holds the maximum amount of water vapor possible. When the existing water vapor is less than this maximum, the ambient air is capable of taking up more moisture. Relative humidity is the ratio of actual vapor pressure to the saturation vapor pressure, expressed as a percentage value.|
The temperature of the ambient air and the wood itself also have a significant impact on the rate of evaporation from the wood surface. Fresh cut logs stored in the direct sun will have surface evaporation rates higher than those will stored under cover or in full shade. Artificially induced heat, such as that in found in a drying kiln, can also increase the drying rate of green timber.
Air movement over and around the wood, as well as its velocity, also affect the evaporation rate of surface moisture. The velocity of air in direct contact with the surface of the wood will be slower, and will contain a higher vapor pressure, than the faster air moving just above the main airstreams. This characteristic is known as the “boundary layer effect.”
Supplemental heat, if any, also affects the movement of water in the wood and the amount of evaporation from its surface fibers. Logs or blanks stored in heated buildings will generally dry faster than those left in outdoor spaces. Obviously, wood dried in kilns will dry much faster than traditionally air-dried timbers as well.
The water in wood moves from zones with higher concentrations of moisture to zones with lower concentrations. For this movement to occur, the outer portion of the wood must be drier than the inner portion. The movement of water occurs in two separate, but distinct phases: movement from the interior to the surface, and evaporation of this water from the surface of the timber.
The fibers on the surface of most timbers reach moisture equilibrium with the ambient air very quickly after cutting. A moisture gradient is soon formed by the difference in moisture content between the inner and outer portions of the timber. Paper bag drying reduces the rate of surface moisture loss, as do wax emulsion sealers and similar methods. They also prevent the formation of steep moisture gradients and subsequent differential stresses that contribute to surface checking.
|When water on the surface of the wood evaporates, the moisture lowers on the surface. Moisture diffusion, in conjunction with capillary flow, causes the moisture contained on the interior of the timber to move towards the drier exterior surface of the wood.||
The paper bag drying method can also be used on solid wood blanks.
Bowls ready for the paper bag drying process.
|If the evaporation rate on the surface of the wood exceeds the rate that interior moisture can move to the surface fibers, a steep moisture gradient is formed. If this steep gradient continues, the outer surface fibers may drop below the fiber saturation point. Stresses may develop as the drier outer fibers tendency to shrink is resisted by the moister interior fibers. These stresses may cause a host of drying defects, including checking, warping and twisting, to name a few.|
The reason paper bag drying works so well for green wood roughouts is because paper bags create a self-regulating “microclimate” inside the bag that produces a near ideal drying environment. Paper bags offer several important benefits:
|Loading bowls, platters or other roughed out green wood pieces into the bags should be done in a specific and deliberate manner. The natural urge is to “nest” the bowls or platters to get the maximum amount of pieces in each bag. However, this is a recipe for mold and mildew to form, which may cause permanent staining on your pieces. You must allow free airflow inside the bag.||
Loading a roughed out bowl into the paper bag.
When using the paper bag drying method, the bags should be closed around rough outs.
|The optimum method of loading your paper bags is to place the pieces into the bag in such a way as to allow free airflow around the pieces inside the bag. This is easily accomplished by mixing different sizes of pieces in each bag and loading the pieces with alternating rims and tennons, or by simply mixing the pieces inside in a haphazard fashion.|
|The ends of the bags must be closed to prevent any excessive air or drafts from reaching the surface fibers. Once the bags are fully loaded and closed, you are ready to place them onto your drying rack. When you are stacking your loaded bags, do not tightly nest the bags together. Remember, you need to maintain airflow around the bags as well otherwise, you will allow mold and mildew to form.
One of the best ways to insure good airflow around the loaded bags is to use wire racks or shelving that has holes in the shelves. You can also simply stack them on the floor, as long as you stack them in such a way as to maintain good airflow.
Wire racks make great storage for your drying rough outs.
Date stampes are ideal for marking the bottom of your rough outs.
|Before you load your bowls or platters into the bags, add the date and species if you know it for sure. Once you get hundreds of pieces drying, you may forget how long a piece has been drying. Sometimes, it is difficult to recall the species later, if it’s one you do not work with regularly.|
|I tried numerous things to speed up writing this information on the bottom of the roughed out pieces. I settled on using a date stamp, the kind that is used in offices to stamp incoming mail. This allows easy updates for the date and the stamped ink is easy to see when the piece has dried.|
Date stamping a roughed out bowl prior to paper bag drying.
When paper bag drying, always write the species on the bottom of the rough out prior to bagging.
|For the species, I still hand write it on the bottom in a permanent marker. It is also useful to write the date and species on the outside of the bag. This makes it easier to locate specific dates or species later, if necessary, without opening any of the bags.|
Your bags are loaded, stacked correctly and drying. One question that everyone wants to know the answer to is how long will it take before the pieces are dry and ready to finish turn? That question is difficult to answer because there are so many variables in the drying process. Earlier in this article, I reviewed these variables, which can be easily summarized into these six main areas.
Since we all live in different parts of the country, with different weather and storage practices, it is impossible to give a standardized time for paper bag drying. However, there is a very easy way to tell when the piece is dry and ready to finish turn -- simply weigh the piece after you place it into the bag and record this weight on the outside of the bag.
|As the roughout begins to lose moisture, it will also lose weight. By periodically weighing the piece every two to three weeks, you can easily tell when it has reached EMC. You will find that the piece will start out at a certain weight and as you reweigh the piece, it will be less each time you weigh it until it starts to level off. When the piece has weighed the same for three times, when measured at least two to three weeks apart, it should have reached EMC.||
The spigot on the bowl shows the species and the data.
If you do not prefer to weigh your pieces, you can also use a moisture meter to determine the overall moisture level. Moisture meters can be difficult to use on turnings because of the curved surfaces. However, if you limit your readings to the flat tennon or rim area you should be able to get an accurate reading. Avoid the pin style of meters, as these require you to hammer the pins into the wood to get an accurate reading. This leaves two nice holes that you will have to fill later, unless you can turn the area away when you are completing the piece on the lathe.
Pinless moisture meters do not cause damage to the wood and are a better choice for woodturners, but can easily cost $200.00 or more for good models. I do not use either of these methods (weighing or moisture meters) as I have several thousand bowls drying at any time and there is simply not enough time to individually weigh each piece, or measure their moisture content with moisture meters. Therefore, I developed a super fast and accurate way to tell when a piece is ready to finish turn I call it the visual observation method.
When the bowl has reached EMC, remove it from the bag.
|Thru the years, my “Visual Observation Method” (VOM) has been a very accurate and simple method to employ in my studio when paper bag drying. This method relies on visual observation of the shape of the tennon on the bottom of each piece. Since the tennon is usually the thickest part on the piece, it is the last part to reach EMC. When it does, it will be oval, just like the rest of the piece. By carefully observing the degree of ovalness on the tennon, it is easy to determine when the piece has reached EMC.|
When the pieces have reached EMC, you can safely remove them from the bags and store them in the open air. At this point, it is OK to nest the dried bowls or platters together to save space. My dried bowls and platters are stored on open-air racks that allow easy, fast locating of the specific species and sizes required. While the paper bag drying method is not right for every green wood drying situation, it is an easy, effective and inexpensive method to employ. To this day, I still use the paper bag drying method to dry most of my green wood roughouts in the studio. Remember to ask for paper bags at the grocery store instead of plastic, to keep up your empty bag inventory.
A few years ago, I began experimenting with different synthetic materials to replace the paper bags I use for drying roughouts in my studio. One of the problems you must overcome when replacing paper bags is the alternative material must allow the moisture vapor to freely pass through the material, to allow the wood to dry. Results were mixed with many of the materials in the testing.
|One day I was at a demonstration and another turner friend of mine and I were talking about drying wood and he asked if I had every tried using Tyvek as a substitute bag material. I said that I had thought about testing Tyvek, but had not yet had the chance to as yet. The next day, he brought me a few Tyvek bags he had made from some scrap material and encouraged me to test them in my studio.|
Homemade Tyvek bowl drying bag.
If you have new home construction in your area, chances are they are wrapping the homes with Tyvek before they apply the stucco, brick or siding. Usually, there is a small amount left on a roll when they complete the house wrapping, which is usually thrown away. Ask the construction supervisor in the new home area if it is OK for you do a little “dumpster diving” to obtain the scrap Tyvek.
This is a very easy way to get some of the expensive Tyvek for free. Of course, you can just buy a roll, or share a purchased roll among your turning friends and have a bag making party. In Texas, we love any excuse for a party…The bags can be made to almost any size you need. I staple the bag edges, and apply a tape over the stapled edges on the side of the bags. The top is left open for loading and secured with clips when the bag is loaded for drying.
Tyvek works very well, but so do the free paper bags. Whichever material you choose, bag drying of your roughouts will serve you well and save you enormous amounts of time when working with green wood roughouts.
Paper grocery bags are available at most grocery/food stores. Just remember to ask for paper, not plastic when your groceries are being bagged. If you prefer, you can contact any large paper manufacturer in your area (look in the yellow pages) and purchase the Kraft paper in large rolls, approximately three feet wide. Tyvek house wrap is available from home improvement centers and building supply stores nationwide.
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 a regular 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 monthly articles covering technical topics, or project based articles in each issue.