What the guides refuse
The South Australian guidelines refuse both on a gravestone: no sandblasting, and no pressure cleaning.
Heritage Victoria’s code approaches the same ground from the masonry side: grit blasting is named among its mechanical methods, and the mortar of old sandstone walls is usually too soft to withstand it.
The code also notes what abrasive methods leave in the air: dust, and old paint dust with a heavy content of lead, which the code calls a cumulative poison.
The answer for old masonry
Where the code names a gentler mechanical answer, it is specific about the limits: steam cleaning may be used on strong concrete, strong cement render, or hard stone such as unpolished granite, and only if repointing is to follow.
Marble and limestone are not in that sentence. The code does not recommend its stronger chemicals for marble, and it places rubbing last, because the method damages the stone by taking its outer layer.
Where a state allows low-pressure water
New South Wales writes the narrowest permission of the three. Under its standard exemptions, cleaning significant fabric with water is to use low pressure and neutral detergents appropriate to the surface being cleaned.
The exemption sets the ceiling for that water, 500 kPa (72 psi), and allows another pressure only on written advice from a suitably qualified and experienced heritage professional.
Its conditions frame all of it: the exemption applies to items listed on the State Heritage Register or under an interim heritage order, and the activity must not alter, damage or remove significant fabric.
Cleaning under it is maintenance, part of the continuous protective care of a place, and the exemptions are for simple things that must have little to no impact on the item’s heritage significance.
The states, side by side
Each answer below belongs to the state whose heritage body gives it.
| State | What its heritage guidance says |
|---|---|
| SA | Sandblasting and pressure cleaning are refused on a gravestone. |
| VIC | Grit blasting is warned against for old sandstone walls, whose mortar is usually too soft for it; steam is named only for strong concrete, strong cement render and hard stone such as unpolished granite. |
| NSW | Low-pressure water and neutral detergents are allowed on State Heritage Register items, with water pressure capped at 500 kPa (72 psi) unless a heritage professional’s written advice says otherwise. |
Where this question goes next
Two guides sit beside this one: How do heritage guides say to clean marble?, and Why does acid damage marble?