Are there are any techniques to flatten out bowed sandstone slabs in a relatively short period of time (e.g. laying them out in a flat spot of the driveway and keeping them wet for a few weeks)? Or, would I be looking at high potential for cracking and/or years of weathering to undo the years it took to get them to bow in the first place? None of my Google searches have turned up any results with related discussions or theory on how to correct for bowing in sandstone (I mostly get hits on flattening sharpening stones by grinding them down or straightening bowed wood).
Background:
I have sandstone steps supported by brick (above grade, cinderblock for the foundation below grade) that are original to my 1950's home.
Since no one seems to even believe it's possible to have warped sandstone slabs, my hopes that there will be tips on how to salvage these slabs have diminished. Below are pictures of the warped sandstone slabs with (the bottom slab is shown in reference to a straight piece of 1 1/4" EMT with close up measurements to show that the slab is still approximately 2 1/4" at the edges and the middle with a roughly 1/4" bow):
The brickwork has been patched poorly over the years by previous owner(s) and as a result, the 2 1/4" x 16" x 72" sandstone slabs that are the surface of the steps have been unsupported in the center at different points in time and have significant U-shaped bowing (about 1/2" difference between the lowest point in the center compared to the outer edges in the top slab with the worst bowing). As you can see, the rest of the steps are also in rough shape and I have them blocked off with caution tape until I can get them structurally sound again. I am hoping to reuse the slabs if possible as part of the step rebuild if it can be done easily by a home owner and in a relatively short period of time (maximum of a month or two).
Best Answer
"Itacolumite is a naturally occurring porous, yellow sandstone that is flexible when cut into thin strips." –Wiki
"Most of the known flexible rocks are also fine-grained; in some cases they are said to lose their flexibility after being dried for some time, probably because of the hardening of some interstitial substance, but many specimens kept in a dry atmosphere for years retain this property in a high degree." Can A Rock Bend? –geocaching.com
I think you're on to something with getting it wet, but I've no idea how long it might take to straighten; probably long enough that you'd need new stairs in the interim anyway.
Shore them up as best as possible and use a 20 amp grinder with a diamond wheel to flatten them? Sounds like a lot of work... but sandstone is awfully soft.
Really though, if they're all off by exactly the same amount, then danger is minimal IMO. Grinding them down will ruin their patina, so from an esthetic view point, I'd leave them alone after shoring them. That's assuming that walking up and down them doesn't feel funny, and that they're all true to each other heightwise within 3/8 of an inch at any given point.
I'd prefer you put your efforts into installing railings. Hopefully that's part of your rebuild plan. If you have to move them make sure you have help and that you both choke-up on it so that they don't crack in half under their own weight when you pick them up.