I think you guy already know this but before WWII for the most part all that was used was steel or CI for surface plates. Since there was a shortage of iron the industry turned to granite. After the war this continued since granite has great wear resistance and never rusts. I am fortunate I have both. The one I actively use is the granite one, its around 12 x 18 and is a B grade. I plate I think that that one is 18 X 24. At Least I think it is. I have built covers for both of my plates and keep them covered unless I am using them.
Big John, I for one would like to know how they check the surface plate for flatness. Thanks for the offer.
http://www.qualitydigest.com/aug03/articles/03_article.shtml
Here's a good article on plates and gives some history.
As I explained, plates are checked for flatness and repeatability.
Repeatability is checked with a "Repeat-o-meter" (I didn't make up these names). Basically, it's a sled about a foot long that flexes in the center. An indicator measures the deflection of the end of the sled as it is drawn over the plate. You can do the same test with a gage block, a height gage and a good dial indicator. The block is placed at different spots around the plate and a measurement is made. This will tell you the repeatability of the plate. The Repeat-o-meter is just a lot faster and makes more repeatable measurements.
Here's a picture I.... ummm... borrowed off the web.
The one I use has a better Federal gage head and electronic readout.
Flatness is another measurement. There are a few ways to do this. Laser, electronic levels, autocollimator and a straightedge system called "planakator" is used. I've personally used all those methods. Most common though are autocollimators and planakator. I use both, but for most field work, the planakator system works best for me.
Here's another borrowed image:
Basically it's a granite straight edge that is leveled to the top of the plate and an indicator is run between the plate and the straight edge. A plane is established and a grid of check points laid out. Unfortunately, many of the "fly by night" calibration outfits don't do this important test.
Now if the plate doesn't pass these tests and isn't too far gone, it can be repaired by lapping. A cast iron lap (sometimes an old cast iron surface plate) is charged with diamond or carbide grit and the table is lapped back into tolerance.
Here's a video of one of my guys working up a sweat.
If this really appeals to you, here's where you can buy the stuff.
http://www.tru-stone.com/pages/price_lists/price_list_popup.asp?id=8
Most likely, not in reach of the guy in his home shop, but that's what you need to really do it right.