Newbee has question about bending slate!

["Disbrow Consulting" <diz@zzzzzzzzzzzzzzzzzzzzz>]

Hi Rick,
I am sort of the weird one of the group, and I too have heard that you can
bend slate, but as John suggests... hold onto your leg.  I have seen some
kinds of shale that some really evil contractors call slate, be bent.
Unless this is for interior work though...pull your own leg to save time.
If left outdoors this stuff comes unglued after less than five winters,
looks terrible, and simply can't be repaired.  Customers really hate this
kind of news.  If you really must bend this kind of shale (Don't get me
wrong.. it's pretty and has some mica in it generally) you will first need
to cut it to size.  Cut cleavage axis first then bedding axis.  If you are
using a circular saw (wet or dry), axis cuts doesn't seem to
matter...although wet gives a nicer finished cut to my little pea brain.
Definitely wear a mask when cutting with a circular saw.  You don't even
want to know what's in the dust in addition to asbestos.  Then you will need
to lay- out your piece according to the way that you want to bend it.  Kerf
(Cut into) the backside of your piece perpendicularly to the direction of
your proposed bend.  Never kerf the piece more than half way through .  It
will go to pieces during the bending.  For the early experiments, double
kerf (cut a double wide kerf with your circular saw) so that the bending
will be more forgiving.  Next, kerf around the edge making a channel for
your mortar material of choice.  This will additionally keep the piece from
curling like a potato chip later.  Next, place the piece in a jig that will
approximate the bend you want with the kerfs up to the convex face of your
jig.  a warm environment is better than a cold one.  You will want to clamp
the work on each end with some kind of screw clamp to bring the curve to
where you want.  Tighten your clamps by hand just to snug, then every few
hours to days as you walk by, give the clamps a little hand twist as evenly
as you can.  Over a period of time you will eventually get the piece to
curve the way you want.  When it is at full curve you may remove it from the
jig, but install it immediately, and brace the materials in place until your
mortar choice is dry.  The remaining kerfing in the back will additionally
help to hold the piece to your other work because your mortar choice will
penetrate the kerfing as well.  When I say "immediately" I mean that before
you remove the clamps from the piece, have EVERYTHING ready to lay, attach,
and brace the piece during drying.  If you aren't ready, it won't work.  The
piece will want to go back towards it's original shape.

When you cut your original piece, cut at least 2 extra.  The material
frequently decides that it would rather explode crumble, or sort of
fold-over than conform to your wishes.  That's part of why I won't do this
kind of work.  When you set and brace your piece in shape leave  a little
room for final pointing after the mortar choice is cured, but don't forget
to at least "catch" the kerf around the edge into your mortar.  It needs to
be held in place at all times.  Indoors or out this will eventually
delaminate along the bedding plane.  It just plain will.  I know of no way
to prevent it.  They used this kind of material on the fancy homes of "Back
Bay" Boston back in the "way back machine" days.  If you take a walk on
Beacon, Commonwealth, or Marlboro Streets there you will see lots of this
stuff flaking to pieces even though these pieces were unbent.  They tended
to use the nicer materials indoors.  I have seen the bent shale used for a
couple of outdoor cooking grills, and stair sets.  The heat of cooking and
later cooling makes them flake like Phyllo Dough quickly, and the stairs
tend to draw water, Ice, and Calcium Chloride or Rock Salt and comes unglued
fast too.    As mentioned earlier, this stuff is almost impossible to repair
later.  If you want to give it a try for kicks and cookies.. go for it, but
I personally wouldn't waste my time using this material for anything other
than landfill.  I have never seen it on the market, but I know where the
evil contractors yank it out with back hoes and excavators.  If you want to
have nice curved stone, talk to the folks here who will help you learn to
carve your curve or consider featherwedging and finishing if the material is
something other than slate.
Good Luck,

Craig Disbrow
Week-end Warrior