A gravestone can remain clean by simply applying nano titanium dioxide.
Titanium dioxide when applied to a gravestone or any surface for that matter will remain clean through photo-catalytic activity.
Wire brushes, metal instruments and abrasive pads may give you instant satisfaction but, if you clean with anything that is harder than the stone, you risk scratching the face of the stone and causing more damage in the long run. The same goes for acidic cleaners, which includes most household cleaners. Power washers are also NOT recommended. Anything that puts out greater than 60 psi is too much force to use on older fragile stones. A garden hose putting out a gentle soaking spray is enough to clean and flush the stone but mold and mildew will still grow in the microscopic cracks and crevices of the gravestone but with titanium dioxide the stone will just stay clean for years at a pop.
The titanium dioxide when exposed to UV light constantly makes hydrogen peroxide therefore will not let mold or mildew to grow on the stone surface.
Below this sign in St Augustine Florida had to be pressure washed once a year. After being treated with titanium dioxide the sign has remained clean for eleven years.
How would you apply titanium dioxide to a stone?
Simply dissolve the powder form into a pump up sprayer and mist it onto the surface. That is all it takes and the titanium dioxide will work constantly to keep the stone clean.
I would recommend on granite to use the spray and wipe product which is used on granite counter tops.
Just spray down the stone with the liquid titanium dioxide and depending on the time of year the stone will self clean itself with zero abuse to the stone. This normally takes about six months for this to self clean. Once clean reapply the titanium dioxide for long lasting results.
The titanium dioxide comes in many delivery systems and some will even strengthen the stone with potassium silicate.
Potassium silicates are microscopic sheets of ambient temperature cured glass.
There are several people offering business opportunities for cleaning stones and I am sure it would be a lucrative business.
With titanium dioxide you could offer your customer a product that would keep the stone clean for years at a pop.
Actually you could offer a yearly service to come and spray the stone once a year for a nominal charge that would keep their loved ones stone immaculate.