What is poultice cleaning?

The basic principle

DOFF and TORC are surface methods. They remove soiling that has settled on top of the stone — soot, biological growth, old paint, sulphate crusts. Poultice cleaning solves a different problem: contamination that has migrated into the stone itself. Rust from a corroding iron cramp. Oil from a leaking gutter bracket. Soluble salts drawn up through rising damp or driven in by de-icing salt spray. None of that sits conveniently on the surface where steam or aggregate can reach it.

A poultice works on capillary action, the same mechanism that lets a paper towel draw up a spill. An absorbent carrier — sepiolite or attapulgite clay, cellulose fibre, or paper pulp — is mixed with water or a solvent chosen for the specific contaminant, applied as a thick paste directly onto the affected area, and covered to slow evaporation. As the poultice dries from the outside in, moisture inside the stone migrates outward to replace what is evaporating, carrying the dissolved contaminant with it. The stain moves from the stone into the poultice. Once the poultice is fully dry, it is scraped or brushed away, taking the problem with it.

What it removes

Poultice cleaning is specified for a narrow set of problems, and it is worth naming them precisely, because using the wrong method on the wrong stain wastes time and can make the mark worse.

Rust staining is the most common call. Iron cramps, dowels and fixings were used extensively in Victorian and Edwardian masonry to tie stone together, and as they corrode they expand and bleed orange-brown staining down the face of the stone. A chelating agent — typically a form of EDTA — carried in a clay or cellulose base binds to the iron ions and pulls them out as the poultice dries. Oil and grease staining, common on cills and around historic ironwork, responds to a solvent-based poultice or, on more delicate stone, a poultice built around a fuller's earth or similar absorbent clay with no added chemistry at all.

Soluble salt contamination is the other major use, and the one with the most consequence if it is ignored. Salts — from de-icing spray, from sea air, from rising damp carrying ground salts, or from previous repairs using the wrong mortar — crystallise near the stone surface as it dries out and cause spalling, the flaking and crumbling that eats away carved detail and ashlar faces. A desalination poultice draws the salts out into the poultice material rather than to the stone's own surface, over repeated dry cycles, before the salt has the chance to do further damage.

The process, step by step

The stone is assessed first, not treated first. The contaminant is identified — rust, oil, salt, or something else — because the poultice recipe depends entirely on getting this right. A trial patch is applied to a small, unobtrusive area and left to run its full cycle before any decision is made about the wider job.

Once the recipe is confirmed, the poultice is mixed to a thick, workable paste and applied in an even layer over the affected area, usually a few millimetres thicker than the visible extent of the staining to draw from the full depth of the contamination. It is covered — often with plastic sheeting, taped at the edges — to slow evaporation and force the drying process to happen from the stone outward rather than from the poultice surface first, which is what makes the capillary draw work.

Then it is left. This is the part that surprises clients used to DOFF or TORC, where the work happens in a single visit. A poultice needs time, and rushing it defeats the method. Once fully dry, the poultice is removed by careful scraping and brushing, the area is washed down, and the result is assessed. Stubborn rust or long-standing salt contamination often needs two, three, or four cycles before the stone is clear, with the recipe adjusted between applications based on how much progress the previous cycle made.

Why it matters for London's building stock

Iron cramps and dowels are everywhere in London's Victorian and Edwardian stonework — they were the standard fixing method before stainless steel became routine, and most are now well over a century old. Where they were installed close to the surface or without adequate cover, corrosion staining is one of the most common defects we see on Portland stone facades and dressings, and it is very often mistaken for general atmospheric soiling that DOFF or TORC should be able to shift. It cannot — because the source of the stain is inside the stone, at the cramp, not on the surface.

Soluble salt damage shows up differently but with the same root cause: something has carried salts into the stone, and cleaning the surface without addressing the salt does nothing for the underlying problem. It resurfaces. On buildings near main roads salted in winter, or with a history of rising damp, this is a recurring conversation — and one where poultice extraction, sequenced correctly with repointing and any necessary damp-proofing, is the only method that actually solves it rather than masking it.

Poultice, DOFF and TORC: how they fit together

These are not competing methods. On a real project, they are frequently used in combination, each addressing what it is suited to. DOFF and TORC clean the general soiling across a facade — the atmospheric grime, the biological growth, the sulphate crusts. Poultice cleaning is brought in afterwards, or beforehand, for the localised problems that a general clean will not touch: the rust bleed at a cramp point, the oil stain under a bracket, the salt contamination at a cill.

The distinction that matters is between surface soiling and embedded contamination. If the mark wipes clean with steam or aggregate, it was sitting on the surface. If it keeps reappearing after cleaning, or is a distinct orange, brown or dark patch with a clear point source — a fixing, a bracket, a damp line — the contaminant is inside the stone, and poultice is very likely the correct answer. For a full comparison of the two main surface methods, see our article on DOFF vs TORC; for method detail on each, see what is DOFF cleaning and what is TORC cleaning.

When poultice is the right choice

Poultice cleaning is the correct method whenever the contamination is localised and embedded rather than surface-wide: rust staining from ferrous fixings, oil and grease marks, isolated biological or algal staining that has penetrated a porous stone, and soluble salt contamination identified during a survey. It is also the method of choice on the most delicate carved detail, where the risk profile of any mechanical or thermal method — however low — is still higher than a paste that is simply left to dry.

It is not the right tool for general facade cleaning. Poulticing an entire elevation of atmospheric soiling would be slow, expensive, and unnecessary when DOFF or TORC will do the same job in a fraction of the time and at a fraction of the cost. Matching the method to the actual problem — not defaulting to whichever method is easiest to sell — is the same diagnostic discipline that governs every method choice on a Trebor project.

Common questions

What is poultice cleaning used for?

Poultice cleaning is used to draw out localised staining and contamination that sits within the stone rather than on its surface — rust from corroding iron fixings, oil and grease, biological staining, and damaging soluble salts. It is a targeted method, used on specific areas rather than whole facades.

How long does a poultice need to stay on stone?

Dwell time depends on the contaminant and the poultice recipe, ranging from a few hours for light staining to several weeks for deep-seated salt extraction. The poultice is left to dry slowly, and drying time — not application — is what determines the schedule.

Can poultice cleaning remove rust stains?

Yes. Rust staining from corroding ferrous cramps, cills or railings is one of the most common uses for poultice cleaning. A chelating agent such as EDTA, carried in an absorbent clay or cellulose base, binds to the iron and draws it out of the stone as the poultice dries.

Is poultice cleaning safe for listed buildings?

Yes, and it is often the preferred method for listed buildings precisely because it is so targeted. There is no steam, water pressure or abrasion involved. The main risk is getting the recipe wrong for the specific stone and stain, which is why a trial patch is standard practice before any wider application.