The purpose of this article is to respond to a situation I've noticed in which
turners familiar with the basics of turning have little if any information about
the media in which they are operating. I'm not suggesting that this article is
a comprehensive treatment of the properties of wood or even the procedures that
are used in making bowls, but instead, it is an attempt to provide information
that I've received from seminars, reading, and personal practice that I've found
useful in making bowls with relative success.
One hears horror stories about experiences of loosing many bowls in the drying
process; wasting wood, acquired by great effort, by storing it improperly prior
to turning it; or expending great effort to turn a bowl only to be unable to
finish it properly because of unforeseen effects. While it's unlikely that one
could turn forever without some loss, it's my opinion that loss should be the
rare exception. Therefore, I felt that a little information in compact form
would provide perspective wood turners with some necessary facts about making
bowls out of green wood and help them minimize unnecessary loss.
In that spirit we will not go into the basics of woodturning such as the need to
keep sharp tools, riding the bevel, using the proper tool, and other important
items. Instead we'll stay with basics about the wood, where to get it, how to
handle it, and how to orient your bowl in the wood to succeed in getting the
desired end product.
Green wood, for our purposes, is wood that hasn't been
dried. Wood in log form is generally green, since it will usually hold it's
moisture for a long time, if not forever. It is usually available at low or no
cost and handy for wood turners to use.
Unless one is independently wealthy and can buy wood ready to mount or has very
good friends who will provide wood in ready to turn form, he/she will need a
means of sawing logs into a form suitable for mounting on the lathe. Wood
available by the side of the road or from tree trimmers is in whatever form that
was convenient for disposal. A husband and wife, both members of our
woodworking club, walk in the morning for exercise. As they walk, they keep an
inventory of the tree cuttings along the way. When suitable wood appears they
harvest it using a small trailer. Another source of wood is from club members
and their acquaintances. Many times members will call with a neighbor or friend
who has alerted them that a tree is on the way out. Another way to acquire wood
for turning is to go to a local sawmill and buy it before it's milled. This is
a good, and economical, way to get wood for projects that require a lot of the
same type and size of wood. Woodcutting and milling is a major industry in our
area, so getting wood from mills is not too hard to do. One just needs to be
aware though that the major job of the sawmill is to produce boards for market,
not to provide raw wood for the turner. So treat the mill personnel with
respect and gratitude.
However one acquires raw wood, it is, needless to say, not ready for the lathe.
The raw wood needs to be sawed to size and shape to fit on the lathe and turn
without major imbalance. While a buck saw might sound like an answer, consider
the amount of cutting one will have to do to make one bowl; multiply it by the
many that will follow; and go buy a reasonable chain saw. I won't go into
what's reasonable here. Suffice it to say that one person's favorite saw will
probably be much different than his neighbors. My experience is that the small
homeowner saw didn't suffice. I burned one out and then went back to buy a
better one. The rule: "Buy the best tool you can afford" is probably a good
guide in the case of a chainsaw. Many people cut the log down to size with a
chainsaw and then use a band saw to round it. Others use the chain saw to round
it. Either way works. Both require a moderate degree of caution to complete
the job with all of your limbs still attached, so make sure you understand the
operation of these tools and follow the directions provided with them.
Green wood begins to crack and check the minute it is cut. The ends of logs
will almost always have visible breaks. This requires that the turner take some
precautions in his/her acquisition and preparation of wood to turn. Generally
the best strategy is to leave the wood in the largest form practical until
you're ready to turn it. Sometimes, driven by the situation (for example: you
have an opportunity to share a cutting with several others), the only practical
solution is to cut a short block (sometimes called a bolt) from a log. If
driven to this point, one needs to take into consideration the final form of the
turning and saw the bolt to be six to eight inches longer than the diameter of
the log so the inevitable checks and cracks can be cut away. If you aren't able
to turn the bolt within the immediate next few days, you must do something to
protect it; such as, putting it in a plastic bag, painting the ends with a
sealer, or some other means of protection. If you do use the plastic bag method
be aware that holding the moisture in will result in creating an environment
favorable for mold and insect infestations. The longer you leave the bolt in
the bag, the more you are likely to have decay of some kind. Also, when you
take the wood out of the bag to turn it, it'll have nasty stuff from bugs to
mold spores from which you'll need to protect yourself with masks and protective
clothing.
There are a number of turned
objects you can turn from green wood such as closed vessels, open bowls, vases,
and other turnings, but let's narrow our discussion for now to the creation of
simple decorative bowls.
Bowls are usually, but not always, turned on side grain. What does that mean?
The grain runs up and down a tree. Cell fibers run with that grain. If you
sheared wood from the end of the log section you'd be cutting the end of the
cells of the wood fiber. We'd call that "cutting end grain". If you cut the
log section in half longwise down the center thereby making two half logs, and
then sheared wood lengthwise down the cut, you'd be cutting generally in
parallel with the cell structure. We'd call that "cutting side grain".
Generally, when end grain is cut the waste comes off as powder or dust. When
side grain is cut the waste is generally in the form of shavings. Let's just
state, for now, that it's easier to shear side grain than end grain.
Notice the use of the word shear as opposed to the more generic word cut. Most
cutting in turning wooden bowls is done with a shear cut. We use tools that
shear off the wood fibers rather than chopping them apart. This accomplishes
the job of severing the wood fibers without pulling them away from surrounding
fibers thus leaving voids in the finished product that are difficult or
impossible to remove in later stages.
If one cut a bowl from a log so that the rim of the bowl ended up pointing at
one end of the log and the bottom of the bowl at the other end, that would be an
end grain orientation. If, on the other hand, the bowl were to be cut with the
rim to the side of the log and the base to the other side of the log, that would
be a side grain orientation.
One doesn't always turn bowls on side grain because some designs require they be
turned on end grain. However, the general rule is to cut on the side grain.
Let's look at the side grain options. There are two generally acceptable
methods of turning side grain bowls. Consider the log section again. We've
split it down the middle creating two pieces. We can choose from two different
orientations for our bowl in each half. We can put the base of the bowl toward
the center, or pith, of the tree or towards the outside, or bark of the tree.
If we put the base toward the pith, the rim will be toward the bark. If we put
the base toward the bark then the rim will, of course, be toward the pith.
There are as many bowl designs as there are turners and logs. Each bowl turned
by each turner is somewhat different from any other. Although there are many
exceptions to this rule, there are generally two classifications which we'll
call symmetrical and non-symmetrical bowls. The symmetrical bowl has a rim that
is round and closes on itself. In general, the symmetrical bowl is expected to
be round and level at the rim. The non-symmetrical bowl has a rim that is made
up of the outside of the tree or, in some cases, the cut edges of the raw
material. Its rim follows the available surface that it's cutting. In most,
but not all cases, that will be the outside of the tree and will often include
the bark. When the bowl rim includes the outside of the tree it's referred to
as a natural edge bowl.
Now that we've decided to stick with bowls on the side grain and have narrowed
the choices down to symmetrical or non-symmetrical, let's look at the
differences in turning requirements.
Symmetrical bowls are the
most common of the two designs. When you turn a symmetrical bowl from green
wood you must take into consideration the changes that will occur in the drying
process. Without getting too deep into the different grain orientations, it's
easy to say that during the drying process wood will shrink more across the
grain (i.e.; across the tree) than along the grain (i.e.; along the length of
the tree). Also, wood will shrink in a direction around the tree; that is in
the direction of the growth rings (i.e.; tangent to the rings themselves). So a
bowl cut out of a tree in the side grain orientation described above will shrink
more across the rim of the bowl perpendicular to the growth rings than in the
direction that was along the centerline of the tree. But it will also shrink
around the bowl on a line drawn from the rim down the outside of the bowl across
the base and up to a point on the rim directly across from the starting place.
To make things more interesting, all this shrinkage will occur unevenly. So
what does one do? A symmetrical bowl is supposed to have an even rim and be
round. There is an answer.
Unlike the non-symmetrical bowl whose edge follows the available surface, most
often the outside of the tree, the edge of the symmetrical bowl needs to be
even, the rim round, and the base level. Since shrinkage will not permit this
during the time immediately following the first turning, there needs to be a
second turning after the bowl has dried. Therefore, the bowl must be at first
turned to an intermediate stage leaving enough material to allow subsequent
turning to a final stage which will accomplish the even, round, and level goals.
Turners therefore rough turn a bowl and set it aside to dry. It is important
that the walls of the bowl be left in a state where the wall thickness is
relatively even so that drying is even around the bowl. I try to make the
walls even but the base slightly thicker. This gives me some wiggle room in
correcting problems that may occur around the base without adding a lot of risk.
The questions of how thick to leave the various parts of the rough turned bowl,
how to dry it, how long to dry it, and how to keep it from cracking during the
drying process present a complicated situation to which there is no simple set
of answers.
First let's think about how thick the rough bowl should be left. It must, of
course, be left thick enough so that when all the shrinkage is done there's
enough wood left to turn it round leaving the wall thickness desired. Depending
on the type of wood, it will shrink across the grain several times more that it
will shrink with the grain. This will leave an oblong rim. If not enough
thickness is left, the finishing cut will enter the inner wall before the outer
wall is round resulting in a failed bowl. If too much thickness is left the
drying time is significantly increased, and the chances of cracking increases
dramatically. It's hard to determine a hard and fast rule for how thick the
wall should be left since shrinkage depends on the type of wood and how much
water it holds -- the more water, the more shrinkage. I try to leave about an
inch in every twelve. That is, if the bowl is smaller than 12", I leave
proportionally less. If it's more than 12", I leave proportionally more than
the inch. A sixteen inch bowl would have 1 1/4" walls. I haven't experienced
any failures with this rule, but there are woods that may shrink more than my
local choice.
The question of how long to dry it is answered: "Until it's dry!" But, how dry
is dry? Green wood will continue to dry until it has reached equilibrium with
the surrounding air. If you store your work in your heated shop, the bowl will
be as dry as it's going to get when the moisture in the bowl has reached
equilibrium with the shop air. If the bowl is stored in a moist environment
such as the back porch of your shop during a wet spring it will have different
moisture content from the bowl stored in a warm dry shop. If you take the bowl
from the back porch and immediately turn it to finish, the bowl will resume
drying when taken in the warm dry environment of your living room. The result
will be that the rim and base that were round and true when you finished the
bowl will now change shape. Similarly, if you dry the bowl to a very dry state
(for instance in a kiln) and immediately finish it in that state, and then send
it to Aunt Martha in Florida to use on her sun porch, the wood would begin to
absorb water from the damp environment until it reaches equilibrium, and the
result will be an untrue bowl. While the wood movement would be in opposite
directions in the two examples, the result is that the bowl will have an untrue
rim and wobble on its base.
So drying is a question of judgment and compromise. Most venues for finished
bowls will, on average, present a narrow range of relative humidity and thus
drying your rough turnings to somewhere in that range will suffice. About 8 to
10 % is a reasonable goal. The use of a moisture meter will help but isn't
essential. A second method, assuming that your storage environment during the
drying process averages near the target range, is to take and record
measurements across the rim of the bowl. When the measurements stop moving, the
bowl is ready.
The thicker one must leave the wall of the bowl to accommodate movement, the
longer it'll take to dry. A 10" bowl with a 3/4" wall will be ready to finish
turn in 3 to 4 months in a shop environment. A 16" bowl with 1 1/4" walls will
take a month or two longer. You can speed the process up somewhat by putting
the bowl in a warm, dry place, but you do so at the risk of cracking the bowl.
The other variable in how long it will take to dry a bowl is the method used.
There are as many methods to dry a bowl. Most work. Some work better than
others.
Some turners dry the bowl by throwing it in a pile in the corner of their shop
and going back for it in a few months. Others go to great effort to control the
process. The rate of success between the two extremes isn't as great as one
might at first figure, but it is great enough to warrant some precautions and
preparations. The primary concern is to get the bowl through the first several
days after rough turning without damage due to 'too rapid' drying. My own
experience is that some limited control of the drying process is helpful on most
woods and essential on others. My usual method for small bowls, less than 8",
is to put them into a box or paper bag for a few to several days and then put
them on a shelf to finish the process. Larger bowls, or bowls that have some
special attribute, will usually get further treatment. I've put some in bags
for a few days and then coated them with a waxy product called Anchor Seal to
dry. Others I've kept in bags or boxes for several weeks to keep the rate of
drying slower than it would be in the open. My experience with red oak is that
very careful attention is required and a slow drying rate is critical while
walnut or soft maple require less attention.
Some other drying methods bear mentioning. Some folks will put a bowl into a
plastic bag and turn the bag inside out every day. This process controls the
rate of drying by keeping dry air away from the wood. Turning the bag inside
out gets the water outside of the bag in a controlled manner. This technique
should be considered for problem woods or very special projects, but it's not
usually needed. Others will coat the bowl with a product like Anchor Seal or
wax the minute it comes off the lathe. Still others, as mentioned, will coat
the bowl after a few initial days of drying.
Whatever drying method chosen, one needs to pay attention during the first
several days to make sure the bowl isn't beginning to crack. Hairline cracks
can often be stopped with thin cyanoacrylate (CA) glue if caught right away.
Just flow the CA glue into the crack and let it dry naturally. On the other
hand, if a crack gets out of hand, it may be time to relegate the bowl to the
burn pile.
As previously stated,
non-symmetrical bowls incorporate either the natural edge of the tree - the bark
or outside surface that may have been stripped of the bark - or some cut edge.
Either way, the rim isn't expected to be round and true. The base, however,
must be true or the bowl will not be stable. Since the rim symmetry isn't a
factor in this design, the non-symmetrical bowl is often turned nearly to
completion in the first session. This serves to permit almost immediate
completion of the bowl avoiding the lengthy drying process and, usually, the
risk of cracking.
Since non-symmetrical bowls are usually turned to a final thickness immediately,
most have a thin wall thickness. This facilitates a short drying time - from a
few hours to a few days - before final finishing can be accomplished. Even
though the drying time is much quicker, shrinking still occurs across the
grain. This means that the finishing procedure must be much different than that
of the symmetrical bowl. Since the walls are thin and shrinkage turns the bowl
into an oblong shape, the bowl cannot be shaped further with lathe tools.
Therefore, the final shaping must be accomplished in the initial turning and
planning must include a means of dressing the base of the bowl in the second
session. Usually enough material is left in the base for straightening it out
in a final turning. The bowl is usually remounted on the lathe using the same
mounting as used in the final stage of the initial turning session. Sanding is
then done using sanding sheets and / or power sanding equipment. When sanding
is complete on the inside of the bowl and most of the outside, the bowl is
reversed and the base is turned to its final shape and sanded to completion.
Providing a jamb chuck of sorts usually made of a round block mounted in a chuck
or on a faceplate facilitates reversing a non-symmetrical bowl. A non-marring
material is mounted between the inside of the bowl and the block, and the
tailstock is brought up to hold the bowl in alignment. To help center the bowl,
a nub is usually left on the base during the initial turning onto which to bring
the tailstock to bear in the reversal procedure. This process sometimes
requires a little patience to get the bowl placed on the jamb chuck so that it
runs true. The idea is to get the base aligned so that it can be turned to
final shape and the sanding of the outside of the bowl completed. This
sometimes requires some compromise since the bowl has distorted from the drying
process.
A non-symmetrical bowl can also be left with walls thick enough to turn in a
second turning and dried as described in the section on symmetrical bowls.
There are, however, some problems with this procedure. One problem in the
natural edge design is keeping the bark on for a second turning after the bowl
has been dried. Another is to keep the bowl from cracking since, given the
bowls odd shape, the forces are less predictable than in the symmetrical bowl.
These problems, plus the point that the bowl can be turned to near completion in
one session, argue against this procedure.
Once the bowl is completely
turned and sanded it is usually finished with one of the many available products
for that purpose. You can leave the bowl with no finish whatever or add several
layers of finishing materials. A turner can arguably use any wood finish for
the bowl. The following are some of the finishes I use.
Lacquer and Wax
My own favorite finish is nitro-cellulose lacquer with wax. You can spray on several coats of lacquer in just a few hours. The method I use is to spray on a light coat of lacquer, let it dry for about half an hour, and then sand it lightly with 220 grit sandpaper. Wipe off the dust with a rag or tack cloth or blow it off with compressed air. Then put on several coats of lacquer, allowing each coat to dry for several minutes before going to the next coat. After several coats of lacquer are applied, allow them to dry for an hour or so before sanding with 320 grit sandpaper or the white 3M abrasive pad. I avoid using steel wool on light colored wood since it sometimes will leave dark smudges. Once the sanding is complete, coat the bowl with paste wax and rub it on with another piece of the 3M pad or steel wool. Then allow the wax to dry for 10 minutes or so before power buffing. I use a linen buffing wheel on my lathe for everywhere it will reach and a buffing head on my electric drill for those areas that the lathe buffing wheel doesn’t reach. Depending on the results, you may want to repeat the wax application to get a better look or feel. I usually use a hard floor wax like the Bruce wax product for hardwood flooring, but Johnson’s Paste wax works well and is less costly.
An oil and wax finish is one way to impart a satin look and feel. Putting oil on a bowl couldn’t be simpler. My method is to put on a pair of rubber or vinyl gloves. Pour a bit of tung or linseed oil into the bowl, slosh it around until it has coated the entire inside of the bowl. Wet a piece of cheesecloth in the oil remaining in the bowl, and use that to wipe the oil onto the outside surfaces. If there isn’t enough oil in the bowl, just wet the cheese cloth from the container. Completely soak the surfaces, inside and out, with the oil. You can either coat it, let it dry a bit, and recoat the driest places, or you can continue to rub the surface with the wetted cloth until you’re satisfied that the surface has been fully saturated with the oil. At that point, wipe off the excess oil, and set the piece aside for 15 minutes or so before buffing it with a soft rag. Dispose of your oil rags in a safe manner since they are candidates for spontaneous combustion, which can cause a fire. I usually take mine outside and spread them out to dry in my burn barrel. After the bowl has dried overnight, or at least several hours, repeat the above process as many times as you feel necessary to get the desired effect. When finished with the oil procedure, use the wax procedure given above. I will often substitute and Oil/Varnish such as Deftoil Danish Oil and follow the procedure given for Oil and Wax.
A variant of the above two procedures will produce a finish with the characteristics of both. Using the procedure for applying oil, followed by the procedure for lacquer and wax will produce a fine looking and hardwearing finish. The oil will enhance the grain pattern of some woods and add depth to the finish. There are a few cautions however: The first is that oil will impart color to wood and sometimes make it look splotchy. The second is that oil finishes must be allowed to dry completely before applying the lacquer over them.
I use shellac for small turnings such as ring boxes and ornaments. I have not used it for larger bowls. One can use shellac in a number of ways though. One use is as a barrier coat to seal the wood before adding other coatings. Another is to produce a fine French polish finish on art pieces.
Some turners use varnishes and water base products for bowls. Some results are pleasing and others not so pleasing. I avoid the hard varnish finishes due to the difficulty of application and my taste in results. The only guidance I can offer is to read and follow their directions.
All wood finishes are toxic to some degree. Use them in
well ventilated spaces and follow directions that come with the product.
Dispose of the wastes properly. The most important caution, in my opinion, is
to dispose of all applicators (rags, steel wool, abrasives) properly. My
approach is to get them out of my shop to a safe place. I spread them out to
dry where they will not contact anything that will burn. As previously stated,
I spread out rags in my burn barrel. I include in this category all rags, steel
wool, and abrasives used to wipe on oil, varnish, stain, or any other finish
containing oil or wax.
Another safety concern is the use of gloves and long sleeves. Many turners use
gloves to protect their hands from shavings and contaminants. A glove or sleeve
is dangerously close to the rotating workpiece or chuck. A catch of either of
these can result in serious injury.