DOFF, TORC, or poultice: which does my building need?

DOFF is very likely the right method

DOFF uses superheated, low-pressure steam — no abrasive, no aggregate — which is exactly why it's the default choice for carved stonework, listed buildings, and anything soft or friable. At around 150°C it kills biological growth at the root and lifts general atmospheric soiling, while using very little water volume, so there's no risk of saturating lime mortar or porous stone the way a pressure washer would. If what you're looking at is general blackening, algae, or atmospheric grime rather than a hard bonded crust, DOFF should do the job on its own — and if it doesn't fully shift on a test patch, that's the signal to look at TORC for the flat sections, not a reason to reach for anything abrasive or high-pressure.

TORC is very likely the right method — once a test clean confirms it

TORC adds a fine aggregate to the cleaning stream, which gives it a mechanical edge that steam alone doesn't have. That's exactly what's needed for a calcified, carbonated crust — soiling that has chemically bonded to the stone's surface rather than just sitting on top of it — on a robust, flat surface. The caveat that matters: TORC isn't used on soft or friable stone, or on fine carved detail, because the aggregate can abrade the surface. And on any building of significance, the right sequence is a DOFF test first, TORC only once the test confirms steam alone won't shift the crust. If you're not at that point yet, a test clean on a small section will tell you for certain rather than guessing.

Poultice cleaning is very likely the right method

DOFF and TORC are surface methods — they shift soiling sitting on top of the stone. A stain that keeps reappearing, or traces back to one spot rather than a whole elevation, usually means the contamination has migrated inside the stone, and that calls for a different approach: an absorbent paste left to dry slowly, drawing the contaminant out through capillary action rather than removing anything from the surface.

Orange or rust-brown streaking almost always traces back to a corroding iron cramp, dowel, or fixing — extremely common in London's Victorian and Edwardian stonework. A chelating agent such as EDTA, carried in a clay or cellulose base, binds to the iron and draws it out as the poultice dries. A trial patch always precedes full application, since the right recipe depends on the specific stone as well as the stain.

Poultice cleaning is very likely the right method

DOFF and TORC are surface methods — they shift soiling sitting on top of the stone. A stain that keeps reappearing, or traces back to one spot rather than a whole elevation, usually means the contamination has migrated inside the stone, and that calls for a different approach: an absorbent paste left to dry slowly, drawing the contaminant out through capillary action rather than removing anything from the surface.

An oily or greasy patch, often near a bracket, gutter, or old fixing, generally responds to a solvent-based poultice, or on more delicate stone, a plain absorbent clay with no added chemistry at all. Either way, it's drawn out rather than scrubbed off. A trial patch always precedes full application, since the right recipe depends on the specific stone as well as the contaminant.

Poultice cleaning is very likely the right method

DOFF and TORC are surface methods — they shift soiling sitting on top of the stone. A stain that keeps reappearing, or traces back to one spot rather than a whole elevation, usually means the contamination has migrated inside the stone, and that calls for a different approach: an absorbent paste left to dry slowly, drawing the contaminant out through capillary action rather than removing anything from the surface.

White, powdery or crumbling staining is usually soluble salt contamination — from de-icing spray, rising damp, or a previous repair with the wrong mortar — and it's worth treating as more than cosmetic, because the crystallisation is what causes spalling: the flaking and crumbling that eats away carved detail and ashlar faces. A desalination poultice, sequenced correctly with repointing or damp-proofing where needed, is the method that resolves the cause rather than cleaning the symptom.