I have Granite slabs and they are bowed

[pkg]

I have 5 granite slabs, 3/4 inch thick which are bowed anywhere from 1/4 to 5/8 inch. Can these be installed without cracking? What could cause the granite to bow like this? I purchased them over a year ago and they have been in storage with the fabricator. Could they have been cut from the mountain wrong? Can granite actually bow?

[Mark D.]

Some granite (and other stone) does warp with moisture. This problem is seen more in green granite than any other color. If you take them to a fab shop with a surfacing machine they can be resurfaced, but you will lose so much thickness that I would not pursue that (3/4" granite with a 1/4" bow resurfces to 1/2" thick granite, which is a recipe for disaster on slabs).

If you can live with the bow you can still use them as they are. Make sure when you install them that you have support in the high spots as well as the perimeter.

When granite is cut from the mountain it is then milled to be slabs, so they were once true.

Nark

[pkg]

Interestingly enough, these slabs are green, with amber crystal veining, garnet and black flecks running through them. Thanks for the reply, when you say that they were once true, does that mean that perhaps the bowed while in storage? Or perhaps from sitting in the rain at the granite yard? One side of the slabs is polished  and the other is not. Would the unpolished side be the more porous side?

[Mark D.]

The warping is a process. It could have started prior to your purchase and continued while in storage. Especially if they were wet during storage. The unpolished side is always more porous, but these green slabs are very porous when compared to other slabs, like Absolute Black for instance.

Like I said before, if you can live with the bow they are usable. If not for a countertop then perhaps for a fireplace? Just don't use them to line your shower stall 

Mark

[pkg]

Thanks Mark for the information. I don't think I'll be able to use them since they are for my kitchen countertops and island. I wonder if there is a way to flatten them back out?
Thanks again~Paula

[Mark D.]

Time & Pressure....But I will have to get some answers on this one for you,
Mark

[pkg]

standing by~

[pkg]

Mark~any luck on researching whether or not these bowed slabs can be flattened out?

[Mark D.]

The best I can give you is either flip all 5 slabs and let them unwarp, but if you don't occasionally check on them they will go past flat to warp the other way. This is one way. The other suggestion is to take 1 slab and put it on your 'A' frame and then lay a straight slab over it. No more than 1 slab because too much weight will cause the stone to crack.

After straightening these out and knowing they are prone to warping, make sure you use a substrate under the entire counter as opposed to merely gluing it to the pieces of the cabinet it would touch as the hollow areas will warp again.

This information was provided to me by 2 different fabrication shops here in town, and I trust them both.

Mark

[pkg]

Thanks Mark, after a lot of research we have decided to go ahead and install the slabs. Here is the most recent response I received which may be of interest and help to others. This comes from the Technical Director of the Marble Institute of America:

Paula,

It is very rare that a stone slab exhibits a true "warp" as one would
encounter in wood, where the warp is the resolution of an in situ stress
that is released when cut into thinner sections.  What does happen commonly
in stone slabs is the slab bends in response to a load.  When a 3/4" slab is
placed on an A-Frame, the angle of the frame can be enough to cause the slab
to bow just due to its own self-weight stress.  I suspect that is what has
happened to your slabs.

A 5/8" bow in 9 feet isn't all that much - a 9 foot 3/4" slab may be capable
of bowing up to 1½" before fracture is initiated.  Normally, this type of
bow is reversible if the slab is stored the other direction.  In many cases,
placing these slabs on saw horses overnight with the camber up may yield a
contraflexure bow by morning.  Sometimes it is necessary to saturate the
slabs to have the bow corrected.

I would believe with the right handling, your slabs would be usable for the
intended application.

Regards,
Chuck

[Mark D.]

Very similar information.