Andy Dingley wrote:

On Tue, 2 Nov 2004 16:43:19 +0000 (UTC), "Richard"
wrote:


I want to stick the two bits together to make a repair.


Not a hope. Use a metal splint underneath. You'll be able to hold it
together with a glued butt, but any strength will rely on the splint
being in tension.

Fasten it to the slate with short drilled blind holes from beneath,
then bond in short stainless steel studs or parallel-thread
woodscrews. Screws need to be put in place "wet", studs can be glued
in, left to harden, then bolted up later.

To drill it, turn the slate upside down on a few layers of carpet or
foam underlay. Drill it _without_ hammer action, as many slates are
quite soft. If you do need hammer action, run the drill at the slowest
speed and use the lightest action you can. Blow the dust out
afterwards.

You can use either epoxy or the right grade of polyester. Araldite
isn't particularly good, as it's already over-thickened, but it would
do the job. Screwfix sell several suitable resins in a cartridge,
specially for use with wall fixings. Personally I'd use a better
grade epoxy, like West Systems. Epoxy is also good for bonding the
slate itself.

I agre with all of this unreservedly, except consider liquid metal (or
polyester car body filler) as a way to BOND the splints to the slate. I
have used this with slate floor tiles and it sticks like buggery.

You can also use it to fill the scarf joint as its fairly close in
colour to grey slate.

wipe any surplus off with white spirit or cellulose thinners before it
sets, make sure surfaces are dry and grease free, and scarpe off any
oozes as it sets 'rubbery'

This will achieve what Andy is describing without the need to drill. But
remember your splints have to take the full stress of the broken slate -
don't rely on scarf joint adding any strength at aall.