Before you clean a headstone, make sure you understand the type of stone it is. Certain gravestones may react with the chemicals in the cleaning product and polished stones can be scratched with abrasive cleaners. Be as gentle as possible when cleaning the headstone and understand you may not be able to clean it as nicely as you had hoped. You should also be aware that signs of deterioration like chipping or flaking might make the stone un-cleanable (cleaning could cause further deterioration).
Be sure that any grass and debris has been cleared away from the stone. On some older stones it is not recommended to remove algae or other fungi from the stone because it can damage the stone further. Try removing a small amount of fungi to see if the stone underneath is still stable, if so, then it is probably safe to remove the fungi.
You will need clean water and a soft scrub brush and possibly a cleaning solution if the headstone is particularly dirty. If you are unsure what kind of cleaning solution to use, you can contact a monument dealer. One method many people use is a clay-like substance known as poultice. It’s applied to a moistened headstone and removes stains caused by metals such as iron. Poultice is used frequently on marble headstones, but it can be used safely on many other types of stones as well.
If you choose not to use poultice as a cleaning agent, then you should use household ammonia (mixed 1 cup ammonia with 1 gallon of water), gentle soap, or a cleaning agent that has been approved for cleaning headstones and grave markers. Never use bleach on a headstone because it can damage the stone.
Have a bucket of clean water (either clean rain water or other water that isn’t considered “hard” with calcium) ready to use or a spray bottle of water. Before you spray or wet the headstone with water, you should knock on it gently to make sure it is not hollow (many older headstones are). If it has a hollow sound, then you should not clean it. If it is solid, wet the headstone in water and gently scrub away any dirt and residue.
If water does not clean the headstone, you can use a cleaning agent listed above to help clean any other stains that may be present.
Finally, give the monument one final rinse with clean water. Older marble or light colored headstones will rarely appear as light as you hoped and you shouldn’t strive for a perfect clean color in order to preserve the integrity of the gravestone. You also don’t need to clean a headstone every year. In fact, many headstones are cleaned once a decade to prevent further, unnecessary weathering.