80 year old stained stone

Local college has a few of these as well as several cap stones on brick pillars. I’m interested in approaching them about cleaning. We also have these at my local firehouse and I cleaned a test spot with good results using SH, H2O, and Dragon Juice, however, when i tried to clean the rest of the cap stone it had a very mediocre result. Any tips for best cleaning method before I approach them?

Had the same problem at my firehouse and did a hot mix with low pressure. Cleaned up pretty nice for a 100 year old plus building… u find any other way that works let me know