a power washer with just plain water is pretty good at this type of work. Depending on the "type" of stone you need to be careful using any type of chemical cleaners. Many older stone were made from marble which can be damaged by acidic cleaners. many more "modern" stones are made from granite which is a bit more durable.
I'm a bit curious as to how you have these, or are you a care taker at an old cemetery?.
My eighth grandfather died in 1852. I am now 76 but i want to get the stone looking better.
I'm volunteer gravestone photographer for Find-A-Grave. Lichen obliterates markings. One good reason to take photographs now. Power washing can damage fragile old stones. See their guidelines at http://www.findagrave.com/cgi-bin/fg.cgi?page=listFaqs#96
I tried a spray on granite cleaner, although it removed a lot of dirt, it didn't really look much better. I tried car wax afterwards, it helped a bit. I wouldn't risk power washing though.
I just got back from taking pictures of graves stones (for my ancestry account and 1 findagrave request). We just used a butter knife because it was all we had and only used it to make DOB and DOD clearer but I'd think there woujld be some type of brush to get that off. UPDATE: Thank Em Hooper for the link. fortunately the headstone we "cleaned" belonged to family. Do you know what chalked or floured heastones means?
Get the products from Limeworks, they are safe and they WORK!
I recently tried Scrubbing Bubbles foam on a granite stone that is ½ polished. I put water down, then sprayed a light layer of foam and only left it on for about 30 seconds or so. It changed colour immediately. I rinsed well after wiping most of it up (didn't want to pollute the grass or flowers). It was a rainy day and everything came up beautifully. It had years worth of black on it, and It looks fine and new. I wouldn't use it on a fragile, marble or very old marker, but it worked for me...I took a chance/risk . I wouldn't use it regularly just in case; maybe once every 1-2 years.