On Feb 8, 5:14*am, Clive George wrote:
On 08/02/2011 00:03, Andy Dingley wrote:

By "slice", I'm guessing you mean a transverse disk. You have pretty
much no chance of this drying without splitting (anything over 4"
diameter). The reasons are slightly complicated to explain in detail,
so if the usual pinheads could please go and read Bruce Hoadley before
arguing, we'd all save some time.

Assuming I don't have Bruce Hoadley to hand, but don't see any reason to
argue, is this because the thin stuff isn't strong enough?

Sorry, but every time this comes up, the usual idiots start arguing
about how they've dried a giant redwood in their shed and it didn't
split.

If you don't have Hoadley, buy it, as it's a damn good book. If you
don't want to do that, the US Forest Products Handbook is on-line for
free (and also available printed and bound for a reasonable price).
OTOH, Hoadley is clearer to read.

Timber drying highlights how much some species differ in some aspects,
but also (surprisingly, to me anyway) how much other aspects are
consistent between species. Moisture content (EMC) has a consistent
relationship with air humidity (RH). Moisture content of a felled log
varies a lot (why felled ash will burn, but others needs to be dried).
Shrinkage with MC varies across species, but total shrinkage from
"felled log" to "dry board" ends up consistent again. The breaking
point of timber varies a lot measured as a stress (i.e. force) but is
consistent as a strain (i.e. dimensional change). The ratio between
tangential, radial and longitudinal shrinkage is consistent, even
though the absolute values vary.

Tangential, or hoop, shrinkage is twice the radial shrinkage (and
lengthways is near zero). If they were the same, then wood would
shrink isotropically, by the same in every direction. The total
shrinkage is about 10% tangentially and 5% radially, for a wet log to
a dry board, for any species. Considering the log as a set of "onion
rings", you should realise that it's now increasingly difficult for
the outer rings to stretch all the way to reach round the inner layers
- and so they crack radially, from excess tension.

Why does it crack? Well the shrinkage will hit 10%, which will
generate some unknown tension in the rings. The tensile force is
enough to break the timber. Now I know neither the force generated,
nor the tensile strength (in force units) of the timber, but I do not
that the maximum strain (as a dimension change ratio) for all timber
is about 8% (AFAIR, can't remember the precise figure). So _whatever_
the species, wet to dry is enough to break a constrained piece of it.

We can avoid this in a few ways.One is a radial cut, or halving the
board. Note also that a log that develops a split early just develops
the one major split. That split relieves much of the tension. Another
way is to take the centre out of the log and to allow it to collapse
as rings.

Another way is to crush the central core of the log (by an
imperceptible amount). If the central core is simply small, then it's
crushed by the larger outer ring. The square law for cross-section
area is such a small core surrounded by a ring an extra inch thick is
far less cross-section than the ring, but for larger cores that extra
inch of ring becomes a progressively smaller cross-section compared to
the core. Small cores get crushed, large cores burst the ring. This
varies by species, as it depends on the species-varying ratio of
tensile strength vs. crush strength. When a species, like lime, is
easy to dry without cracking, it's usually down to this ability to
crush the core slightly.

Elm, noted for its interlocking grain, has a much stronger tensile
strength in a large piece than a small piece, so drying elm will tend
to generate many microcracks, rather than a single big crack.

It's not about varying moisture content radially.