Filling a crack in a piece of granite

[teddysnips]

Fitted a granite window sill at the weekend but during the process the wretched thing cracked across the middle into two pieces. It's now firmly in place with a very small crack (~1mm) running from back to front. I'd like to fill this with something, but what? A stonemason friend suggested microcrystalline marble wax, but that sounds like a bit of a pfaff.

Thoughts?

Thanks

Edward

[Stuart Noble]

Not a lot different to a crayon. Microwax is very similar in texture, and at least you'd have a wide choice of colours to choose from. A hair dryer might help when applying it.

[teddysnips]

Excellent - hadn't thought of that (had, however, thought of a candle)

Thanks

Edward

[The Natural Philosopher]

IF its polished granite, try an epoxy plus pigment. You can PROBABLY use powder paints, but ISTR some issues with carbon and epoxy.

Ive done a fair bit of what follows in the modelling arena, and mostly it works, but test it first.

First of all get your epoxy. A model shop is good, as they carry several sorts. The 5 minutes stuff is a bit to hasty, and the wing bandage sort takes forever to dry, so something like 30 minutes is pretty good, although the wing bandage sort is nice and runny.

Mix up the epoxy well..it reacts not by catalytic action, but by physical contact between the two parts.

Then cut in your powder pigment. As much as it will take without getting absurdly stiff.

Now to get it down the cracks. Paste it on and use a heat gun or hair drier to lower the viscosity and rub in into the crack. This also gets its setting pretty well, so be careful.

As far as getting the surplus off, a bunch of tissue soaked in acetone seems to work and of course once part-set you can take it down while rubbery with a sharp scalpel, and later on with ultra fine wet and dry, followed by rubbing down compound to blend a sheen into the adjoining material.

[Andy Hall]

You can get a special epoxy filler for granite.

a supplier of a U.S. range called Bonstone which works well.

This does need to be done properly, or the result will look like crap. It is time consuming to do properly and getting a professional may be a better option if you don't want to spend the time. However, depending on the degree of damage, it may then prove less expensive to have a new piece cut.

[Paul D.Smith]

...snip...

...

What is "catalytic action"? Isn't epoxy a mixture of a "pre-polymer" (wish I could remember the correct term - monomer?) and an initiator, typically a peroxide of some sort. The reaction is that under light or heat, the peroxide breaks and then reacts with the monomer causing it to polymerize. I suppose you're sort of correct saying that it's "physical contact" but that alone won't do it - you need something to trigger the peroxide to break, which is why the heat gun gets it setting.

However I would not recommend speeding up the process unneccessarily. Doing so can result in a brittle end product so letting it set at its own pace as much as possible is preferable.

Paul DS

[Stuart Noble]

The advantage with crayons/candles is that you can see what the end result will be colour wise. Crayons possibly have a higher melting point, which could be an issue on a south facing window. As Andy says, there is great scope for making a mess with 2 part fillers in such a tiny crack

[The Natural Philosopher]

well yes..howver 'special epoxy fillers' that are in fact standard epoxy plus pigment may wotk out more expensive. Check pricing first ;-)

I knew someone years ago who used to supply 'formatted floppy disks' at a fiver a hit...to custiomesr who doidn;t know how to type format a:/s..

[The Natural Philosopher]

No, I dont think EPOXY is..

I cant remember the details, but unlike polyester resis, where ultimately there is everythig in ther resin to do that hardening once the 'hardener;' kick starts it, the epoxy is an equal mixture of two things that have to react in a normal chemical fashion..failure to do that resuklts in pockets of unset resin.

The reaction is that under light or heat, the

I have found that heat actually seems to encourage better mixing. And a setting time in days rather than years ...

[Paul D.Smith]

I was very imprecise - very poor for an ex-scientist. I meant that it's a mixture as you're applying it but it comes in two tubes and you mix before applying.

Paul DS.

[The Natural Philosopher]

Mm,. thjats true, but what *I* meant was that the reaction is not catalytic..that is yiu need to fully mix the two parts or it never sets properly.

As opposed to polyester, where a drop of hardener seems to set a whole tub, no matter how badly its mixed.