Best Practice Guidelines
Before cleaning please appreciate:
Variations in appearance can be due to climate.
The appearance of biological growths is not harmful to the stone.
Staining patterns can be due to water flow down stone and over the lettering.
Consider carefully whether there is a real need to clean.
The possible need to clean to permit other conservation work to proceed.
Removing biological growths
Remove only when:
Monuments are heavily coated and Inscriptions are obscured and unreadable. To enable other repairs e.g. repointing.
If you MUST clean
Think again – does it really need it?
Is it really dirty or is it just dust adding to the patina of age.
Is the soiling due to dirt or soot?
Or is it biological growths – lichens, algae, fungi, moulds?
Use only soft bristle brushes with wooden handles and plenty of water.
A super-clean headstone looks out of place in an historic cemetery.
Cleaning agentsFor biological growths:
Use only preparations based on quaternary ammonium compounds e.g. benzylkonium chloride. Never use anything based on sodium hypochlorite (or bleach) These products take time to act and use the natural weathering process to clean the stone.
NEVER EVER use:
Steel wool, sanders, or other harsh abrasives.
Wire brushes of any sort.
Sand-blasting or high pressure water-blasting.
Acids (e.g. spirits of salts).
Bleaches and mould removers (sodium hypochlorite or caustic soda).