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Dovetails are an
interlocking joint. They function a
bit like locking your fingers together. The joint is
named dovetail because the interlocking comes from
the wedging of one piece, with has the shape of a
Dove's tail, into a receiving piece which has a
reciprocal or negative shape. The wedge piece is
called the tail, the receiving piece the pin and the
resulting joint a dovetail.
Dovetails are an extremely strong joint.
Mechanically they are almost impossible to pull
apart in the interlocking direction. They must
always be cut such that the mechanically strong
interlocking direction is aiding the most stressed
direction in the application. For example, the
carcass of a chest wants to bulge and pull apart at
the sides. Dovetail joints are used in the top,
bottom and horizontal drawer rails to keep this from
happening.
The joint provides a lot of surface area for gluing
with this area growing linearly as the number of
pin/tail combinations are increased. This
combination of mechanical strength and the large
surface area for gluing makes the dovetail the king
of joints.
A mortise is a rectangular hole in milled stock. It
receives a milled stock, called a tenon, that is the
same size as the hole. Glue is used to hold the
joint together. The combination of the milled hole
and milled stock is called a
mortise
and tenon joint. The mortise and tenon is used
largely in frame and panel construction such as
doors and drawers. However, it is often used
wherever a panel is required such as a dust panel in
the
Shaker Tall Clock or the drawer support frame in
the
Six Pane Oak Hutch.
The mortise and tenon joint provides more surface
area for gluing than a simple butt joint would.
Further it provides for face grain to face grain
gluing which is much stronger than end grain to face
grain such as a butt joint. In antique furniture you
will often see a mortise and tenon additionally
secured with pegs. This was because the old hide
glues were not trusted to last over long periods,
especially if exposed to high temperatures or
moisture. Today's technology eliminates the need for
pegs, but they are still often used for their visual
attraction and to reflect the period of the
furniture. One place where the combination of pegs
and mortise and tenon joinery are required is in the
breadboard ends of tables or
secretary slant tops.
The simple answer is absolutely yes.
Today's technology provides for glue joints that are
stronger than the wood itself provided that the
joint is properly made. I use yellow glue (polyvinyl
acetate) from Titebond. It has a high shear strength
which makes it particularly applicable for edge to
edge glue ups. There are several factors that go
into making a strong, long lasting glue joint, such
as temperature, spreading of the glue, pressure,
setup time, grain direction etc. One factor that is
seldom discussed is the freshness of the joined
areas just prior to gluing. For example, if a table
top is to be made by edge gluing several boards
together it is important that the edges be fresh
just prior to glue up. Edges loose their freshness
when they have sat around for a few days - oils are
released from the board to the surface and dust
particles come to rest on the surface, both
weakening the glue joint. I have a small block plane
I carry on my belt which I use to create a fresh
edge just prior to spreading glue. If this and the
previously mentioned factors are obeyed the
resulting glue joints will last for hundreds of
years.
There are times when you can not use glue as a
fastener. This mostly occurs when there is a cross
grain situation that is large in area or length. The
problem is that wood expands and shrinks with
seasonal changes in temperature and humidity. This
occurs almost entirely across, or perpendicular to,
the grain, typically the width of the stock (while
some expansion/shrinkage occurs over the length of
the grain it is extremely small and can be ignored
in typical furniture dimensions). The forces
generated by wood expanding/shrinking can be large
and cannot be stopped. Therefore, if two pieces of
stock are glued together and the surface area is
large or long, then this expansion/shrinkage will
occur in orthogonal directions and will eventually
stress the joint to the point of breakage or the
wood will split. Cross grain situations must be
dealt with in the design of a piece. There are many
joinery techniques that can be employed to alleviate
these problems and they must not be ignored if the
piece is to last for hundreds of years.
I know of no scientific testing that has been done
to explicitly answer this question. However, Chinese
hardwood furniture from the Ming and Qing Dynasties,
circa 1500 to 1750, are readily available to anyone
who want to pay the price. When correctly designed
and crafted using today's glue and fastener
technologies, traditional joinery techniques and
appropriate treatment, there is every reason to
expect a custom piece to last 300+ years.
This is a two part question really. First, why not use
softwoods? Second, why not use engineered materials
like plywood and particle board?
Classifying wood into hard and soft is a little
misleading. The hard/soft classification is not a density
distinction but rather a botanical one. Hardwoods
are flowering trees while softwoods are conifers.
There are many hardwoods that
are less dense than softwoods and vice versa.
However softwood is generally less dense (hence less
strength) and more susceptible to bug attack.
Hardwoods tend to have better colors and more
interesting grain patterns including figures such as
curly, bird's eye,
blistered and
spalted. Hardwood grain tends to be richer and
finer textured than softwood. Lastly, desirable
hardwoods are more scarce than softwoods and that
makes them more valuable. All of this tends to make
hardwoods the wood of choice for fine furniture.
That said, there is a lot of early American
furniture made from pine and a custom piece make
wormy pine can be very striking indeed.
Engineered wood is another beast altogether. Most
engineered wood is held together with lots of glue
of one kind or another, for example particle board.
They tend to be extremely heavy. Often the surface
is a very thin veneer, subject to scratches and digs
that expose less desirable grain and figure
underneath. This is particularly true of plywood.
Exposed edges are simply ugly requiring the use of a
veneer tape or molding to disguise it. Many
engineered woods are susceptible to heat or moisture
and will delaminate over time. Lastly, let's face
it, engineered wood are anything but scarce, and
that alone makes them less desirable. After all,
fine furniture is in part a snooty concept.
I stick with tradition and use only hardwoods (with
the exception of some very selected softwood on
occasion like wormy pine), mostly of native New
England or exotic variety. I never use engineered
woods, not even as backs or drawer bottoms.
Figured wood is any wood that produces a desirable
and unusual grain pattern when cut a certain way and
finished. Examples are figures such as
curly, bird's eye,
blistered and
spalted.
Unusual doesn't necessarily mean abnormal. For
example, white oak has large ray cells that produce
long horizontal ribbons which extend radially from
the pith. If the lumber is quartersawn (grain is
perpendicular to the face) these rays are exposed
and show up as beautiful one to six inch lines
running across the grain. This is a normal
occurrence in white oak but only shows up when the
wood is cut a certain way, which makes it unusual.
An example of unusual and abnormal figure is spalted
maple. As white rot develops dark zone lines form.
If the decay is caught at the right time, i.e. the
tree is harvested and dried while it is still hard,
a beautiful pattern is captured.
Other figured patterns such as curly and blistered
maple are striking and unusual. The cause of these
patterns is not well known. It can be difficult to
tell that a tree has these patterns and you will
often find they have been missed and the wood ends
up in a palette.
Because figured wood is unusual or rare, it is very
desirable (and expensive). Figured wood is often
used in fine furniture in combination with other
hardwoods, especially when the colors contrast to
provide a striking look such as blistered maple and
walnut, or spalted maple and walnut.
Wood is made up of cells and tubes that are full of
water in green or live wood (cherry has a relative
moisture content of 58% when green, black walnut
90%). It has to be dried to remove the moisture
before it is ready for milling into stock for
furniture - typically to 8% or less. As it dries it
shrinks and the cells and tubes are left relatively
empty.
When exposed to hot humid air these structures tend
to fill with water again and expand much as a
balloon does when it is blown up. In cold dry
weather the moisture leaves these structures and,
like a balloon that looses air, they contract or
shrink. This expansion/shrinkage occurs almost
entirely across the grain. Extremely little
expansion/shrinkage occurs along the length of the
grain.
In a wide board this expansion/shrinkage can be
fractions of an inch to inches. An attempt to resist
this movement, with say a clamp or vise, will result
in either the wood splitting or crumbling. The same
would be true if glue were used to resist movement,
only in that case the glue joint itself may also
fail.
The forces produced by this expansion/shrinkage can
be large. The situation that must be avoided is a
cross grain joint that has a large surface area
(e.g. mortise and tenon joints) or are very long
(e.g. bread board ends of table tops). The designer
has rules of thumb and techniques for avoiding this
problem such as limiting a mortise and tenon joint
in size before partitioning it into multiple mortise
and tenons or using pegs in a breadboard and only
gluing it at the middle.
Dealt with properly during the design and the
crafting, a custom piece can breath, avoiding the
build up of forces that would otherwise drive it
apart. That is how and why a custom piece lasts for
hundreds of years.
Frame and panel construction is another technique
that allows for expansion/shrinkage without
destructive forces being generated. The problem is
most easily demonstrated in the example of a door. A
door constructed of solid wood would push against
the door jamb sides as it swells causing the door to
stick closed. In home construction this is avoided
by either using a hollow core door (the outside is a
thin ply wood veneer that can't produce much force
and hence is held in place by a frame to which it is
glued) or a door with a frame constructed of mortise
and tenon joints which houses two floating,
non-glued, panels held in place by slotted groves in
the frame. The panels are sized smaller than the
outside most dimensions of the groves to allow for
maximum expected expansion.
This latter technique is also used in fine furniture
and kitchen cabinets. That is why your kitchen
cabinet doors may rattle when you slam them shut. In
fine furniture construction we eliminate that by
packing compressible apace balls between the panel
and the walls of the grove. You will notice I used a
frame and panel to construct the sides of the
Shaker A-V Center shown on the Gallery page.
Breadboard ends are another technique to account for
expansion/shrinkage. Breadboard ends dresses up the
ends of a table by hiding the end grain. Take a look
at the
breadboard end design I used in the construction
of a secretary slant top. Notice there are three
tenons each of which will have a peg through it and
its mortise. But the two end tenons are slotted so
as the panel expands/shrinks the peg will ride in
the slot, keeping the breadboard secured to the
panel. The middle tenon can be glued to its mortise,
but no glue on the end ones. I am sure you have seen
breadboard table tops where the breadboard is either
longer or shorter than the top. This is not sloppy
craftsmanship. Hang around six months or so and you
will see that at some point they are exactly the
same dimension and then the opposite is true. This
is good design and craftmanship.
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