46 Joinery joints
how the timber has been cut (converted) from the log,
wide boards may also take on a shape known in the
trade as cupping. This, as illustrated in Figure 3.45(a),
is when the board becomes convex on the heart side.
By examining the end grain of a piece of timber,
you can usually detect what is likely to happen
by remembering that the greatest amount of
timber- shrinkage takes place in the direction of the
annual growth rings. Therefore, the longest annual
rings on a particular cross section of timber will
shrink more than the shorter rings on the same
section. As illustrated in Figure 3.45(a), this will alter
the shape of the timber. In this example, the board
has been cut tangentially and, because of this, it is
also likely to shrink more in thickness on the outer
edges than in the middle area. This is because timber
shrinks more in the sapwood area than in the middle
heartwood.
Figure 3.45(b) shows the effect of shrinkage known
as diamonding, which can happen when the annual
rings run diagonally across a section of timber. Again,
the greater shrinkage along the longest annual rings
has caused this.
Figure 3.45(c) shows a section of timber which
has been quarter sawn or rift sawn. This type of log-
conversion creates short annual rings of equal length,
radially square to the face, which will ensure equal and
minimal shrinkage without distortion.
Butt joints
Figure 3.46: Butt joints are sometimes referred to as
rubbed joints, as this was the traditional method used
for joining two boards edge- to- edge with reheated
Scotch glue, a toffee- like glue made from recycled
animal bones, sinews and hides, etc. The glue was
applied to both the prepared edges; then, with one
board held in the vice, the other placed on top, the top
board was rubbed backwards and forwards a few times
until the excess glue was exuded and the boards were
stuck firmly together.
Sometimes, especially if there was a shortage of
available sash cramps in the workshop, a small iron
dog would be driven across the joint at each end, into
the end grain (as illustrated), to help hold the joint
together until the glue had set.Gluing with PVA glue
When the edges have been hand- planed and there
are no rotary cutter- marks from planing machines,
this traditional method of jointing has been scien-
tifically proven to be stronger than machined- and-
glued butt jointed edges. Although animal glues
are not used for jointing nowadays, rubbed butt
joints can still be successfully made by using PVA
(polyvinyl acetate) adhesives. There is not the initial
‘grab’ experienced with animal glues, but there is
enough – provided only one of the jointed edges is
glued.
Of course, holding the freshly- jointed boards
together with a few sash cramps would be better
than relying wholly on the iron dogs and would also
promote alignment across their face- sides. However,
the top edges of the metal cramps can stain the wood
badly – where the glued- surfaces are still wet from
being wiped with a damp cloth and/or more glue
has exuded – so cover the cramps’ metal edges with
masking tape or pieces of paper before laying the work
into them.
This method requires only the face- side and face-
edge of each board to be planed true prior to gluing.
When planing the edges, both boards can be held
together in the vice, face- to- face, and separated for
testing. The main point is that each board must have a
perfectly straight edge. Being square to the face- side is
ideal, but, providing the face- sides are together when
planing (or both on the outer surfaces, if the boards
have been thicknessed), a slight lean one way or the
other will still achieve perfect face- alignment across
the boards when they are put together. Testing for
straightness can be done by eye, with a straightedge
and/or by trying the edges together. Planing should be
done with a long plane – preferably a try plane, but a
jack plane will do the job. Unless very short lengths of
board are being shot for jointing, a smoothing plane is
not recommended.Gluing technique
When the boards’ edges are together and judged to
be ready for gluing, the mid- area of the face- side
should be freehand- marked across the joint with two
closely- spaced pencil lines. One piece is then placed
in the vice and glue applied; the other (dry) piece is
Figure 3.46 ‘Iron dog’ across butt joint (at each end). placed on top and rubbed backwards and forwards (by