After arrival at the veneer mill the logs have to be made ready for slicing. Firstly
the bark is stripped away and then, depending on the method of production to be
employed, the log may be sawn into flitches in preparation for being mounted on
to the slicing or peeling machine. |
In order to make the slicing operation easier most logs, with the exception of the
whitest species such as Sycamore, then have to be softened by steaming or "cooking"
in large tanks of hot water. This process may take anything up to three weeks for
certain very dense timbers, but more usually lasts between 24 and 48 hours. The
length of time together with the temperature and even the hardness of the water
all affect the ultimate colour of the veneer. The log is then ready for conversion
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FLAT CUT/SLICED | QUARTER CUTTING
| TRUE QUARTER CUTTING |
ROTARY CUTTING | RIFT CUTTING
STAY-LOG CROWN PRODUCTION |
HALF-ROUND CUTTING | BURR PRODUCTION
Most decorative veneers are FLAT CUT (or flat sliced). This is to say that the knife
passes straight over the log, slicing from one side to the other as shown in the
illustration. The result is a veneer showing a "crown" (sometimes known as a "heart").
Depending on the growth of the tree this crown may be quite uniform, when it is
often known as a "cathedral crown" (as in cathedral windows), or it may be variable
or wild. Each can be desirable to the specifier according to the effect required
for the finished article or scheme.
Normally the area in the middle of the tree (the core) is either rotten or at best
defective throughout its entire length and this will be trimmed away to produce
some straighter grained material from each side. These veneers are commonly referred
to as "halves" or "quarters".
The leaves of veneer are always kept in sequence as they come off the knife, ensuring
ease of consecutive matching throughout.
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If there is a requirement for much straight-grain material and the log is deemed
suitable for this purpose, then it will be QUARTER CUT. To achieve this the log
is marked at the end and then sawn into quarters. Each of these quarters (or flitches)
is in turn mounted on the knife at the appropriate angle for the blade to slice
across the annual growth rings at roughly 45° to the log's centre. This exposes
the radial face of the wood and the total yield will be straight grained, i.e. not
showing the crown of the wood. Obviously, the actual straightness of the grain still
depends on the growth of the tree throughout its lifetime, and logs chosen for quartering
will be carefully selected with this in mind.
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TRUE QUARTER CUTTING
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A few decorative veneers are ROTARY CUT (or peeled) in order to reveal the particular
growth characteristic that is close to the outside of the log. Bird's Eye Maple,
for instance, is almost always cut in this way. In addition some smaller growing
species such as Birch are often peeled; otherwise they would be excessively narrow.
Here the log is secured centrally at each end and rotated against the knife blade,
the veneer coming off rather like unrolling a roll of paper. After each turn the
knife moves closer to the log by the chosen thickness of the veneer being cut. Most
veneer produced for the manufacture of plywood is rotary-cut since the resulting
extreme wildness of the grain does not matter and, additionally, far greater widths
may be achieved by this method. For convenience the veneers are clipped in the width
to ensure easier handling.
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A variation on full rotary cutting is RIFT CUTTING. Here the log is first cut lengthwise
into four, as in quartering, then each quarter is mounted a little off centre on
to a staylog machine and rotated against the knife. This produces a growth pattern
similar to quartered material but can yield greater widths. However, the stripy
effect may also be a little broader than with true quartered flitches and a certain
amount of "half-crown" is commonly seen on one edge of some rift-cut veneers.
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STAY-LOG CROWN PRODUCTION
Very big logs that are too large for flat cutting into crown veneers in the normal
way can be cut first into thirds or quarters before being mounted on to the staylog
machine and cutting from the inside to the outside to produce very nice crown grain
material in more manageable sizes than would otherwise be possible.
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Yet another way of producing crown featured veneers is the HALF-ROUND CUT. This
is particularly useful when a log is a little small in diameter for normal flat
cutting. The log is first cut through the centre into two halves. One half is mounted
on the staylog machine and rotated against the knife so that the arc of the cut
produces an increased width compared with flat slicing. However, usually the crown
itself has a wider spread and shows less true cathedral pattern.
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The versatile staylog machine is also used to cut burrs, the rotation of the log
against the knife ensuring burr veneers of a larger dimension than would be the
case were they to be flat sliced.
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Because of the cooking process the veneers, as they come off the knife, are still
wet and must be dried as soon as possible. Therefore they pass, still in the sequence
they were sliced, into a warm air drying machine. Here, depending on the species
being produced, different programmes are used to reduce the moisture content to
the required level. Too dry and they become brittle and unworkable - too moist and
they may develop mould which will undoubtedly leave staining. |
Modern drying machines, known as press driers, also flatten veneers that may be
somewhat buckled after the slicing process. Should the veneers be severely buckled,
as some species are prone to after drying, then they must be further pressed before
being offered for sale.
Now, having been cut and dried, the sequenced veneers are trimmed at each end and
along each edge and bundled for convenience of handling. Usually a bundle will contain
24 or 32 leaves of veneer but occasionally they may be packed in as few as 16 or
as many as 40. Each mill has its own preferred way of bundling and even the dimensions
of the veneer will be taken into account.
The length and width of each bundle is then measured electronically and a computerised
specification produced so that the total square metres of the log's surface area
Our log's journey from arrival at the mill to being presented to the customer is
completed when it is carefully inspected, bundle by bundle, and assessed (or graded)
by an experienced evaluator and priced according to its overall quality.