What a poultice does that a solvent cannot
The South Australian guidelines draw the contrast: solvents can carry inks and dyes further into porous stonework, while a poultice pulls the staining material out.
The same guidelines put a test before it: any cleaning process the guidelines describe is tried first on a small, hidden portion of the stone.
The stains the guides name
Each mark below is one the guides name for the stone, with the body that names it.
- Red dust can leave white marble pink, the South Australian graves guidelines warn.
- Water-soluble salts gather on sheltered exterior stone and can leave it flaking or fretting; left on, the Victorian code says they can soak back into the masonry and cause further deterioration.
- In marble, the same code singles out gypsum in the efflorescence as difficult to remove.
- Graffiti is the mark that wants speed and skill: the guidelines say it is to be removed as soon as possible, and that its removal is technically complex work for skilled people using the right techniques and materials.
- Spalling, fretting and delamination are traced by the same guidelines to rising damp, corrosive salts or rainwater, with drainage and run-off to be improved.
The test, and who does the work
A test comes first, on a small hidden part of the stone, and the guidelines put a second condition before it: a headstone that is brittle or fragile is not cleaned at all.
Graffiti removal is the clearest case of work the guidelines reserve for skilled hands, and Heritage Victoria’s code says the same of cleaning in general: carelessness or the wrong method can cause damage that cannot be undone.
What a poultice is not
A poultice is not a licence for a harsher cleaner. The guidelines set acid and bleach aside on this stone, and refuse household bleach for whitening marble.
Nor is it for every stone. The guidelines say to leave a headstone alone when it is cracking, splitting, flaking or scaling, or when its surface is grainy.
The South Australian guidelines warn that the wrong cleaning materials or practices can leave a gravestone damaged beyond repair.
Where this question goes next
Two guides sit beside this one: How do heritage guides say to clean marble?, and Should old marble be sealed or coated?