Why Salt Crystallisation Happens in Old Walls

Salt damage is one of the most common frustrations in historic building restoration. You repaint a wall, and within months white fluffy patches appear, paint blisters, and the plaster beneath turns soft and crumbly. This is not a failure of the lime plaster itself. It is a sign that moisture is moving through the wall and depositing soluble salts at the surface.

Old walls are naturally damp. They were built to breathe, letting moisture evaporate from their surfaces. When that balance is disrupted, salts dissolved in groundwater or from other sources are drawn into the plaster. As the water evaporates, salts concentrate and crystallise. These crystals expand with surprising force, enough to push plaster away from the wall and break down its surface.

Common sources include rising damp, leaking gutters, faulty rainwater goods, and high ground levels outside. Older repairs with cement or gypsum plaster can make things worse by trapping moisture. De-icing salts, sea spray, and even some cleaning products can add to the problem. Nitrates from historic organic matter, chlorides from ground salts, and sulphates from pollution are the usual culprits.

Recognising the Signs of Salt Damage

Salt damage rarely appears alone. You will often see it alongside damp patches, a musty smell, or crumbling mortar. Look for:

• White, fluffy deposits on the surface, known as efflorescence.
• Blistering or hollow-sounding plaster.
• A powdery, crumbling texture just below the surface, where salts have crystallised within the plaster.
• Recurring damp patches that seem to move or change with the weather.
• Paint or limewash flaking away in patches, often near skirting boards or chimney breasts.

Hygroscopic salts, particularly chlorides and nitrates, are especially troublesome. They absorb moisture from the air, so the wall feels damp even when there is no obvious leak. This is why simply drying out a room rarely solves the problem.

Finding the Source Before You Treat the Symptom

It is tempting to jump straight to a poultice or a new coat of limewash. Resist that urge. If you do not deal with the source of moisture, salts will return. Start with a careful survey of the building.

Check gutters, downpipes, and hopper heads for leaks. Look at ground levels outside: if soil or paving sits above the damp-proof course, moisture can bridge into the wall. Inspect the roof, flashings, and chimney stacks. Internally, check for plumbing leaks and poor ventilation. In older buildings, a lack of ventilation in cellars or suspended floors can keep walls permanently damp.

It also helps to know which salts you are dealing with. A simple test kit can identify chlorides, nitrates, and sulphates. This matters because treatment and prevention differ. For example, nitrates often point to organic matter or agricultural contamination, while chlorides usually suggest ground salts or sea exposure.

Poultice Methods That Actually Draw Salts Out

Once the moisture source is under control, poultices can remove salts from the plaster. A poultice is a paste that draws moisture and dissolved salts to the surface as it dries. You then remove it, taking the salts with it.

For historic lime plaster, use inert poultices such as kaolin, diatomaceous earth, or cellulose pulp mixed with distilled or deionised water. Do not use tap water in hard water areas, as it can add more salts. Mix to a smooth, thick paste, similar to peanut butter.

Work on a small area first. Dampen the wall slightly with a fine mist of distilled water, then apply the poultice 10–20mm thick. Cover it with a breathable sheet or plastic with holes to slow drying, which helps draw salts out rather than just drying the surface. Leave it for two to four days. When it is dry and crumbly, remove it carefully and dispose of it. Brush the surface with a soft brush to remove loose salt crystals. You will likely need to repeat the process several times, especially for heavy contamination.

For very badly affected plaster, a sacrificial lime render may be the better option. This is a breathable lime mix applied to draw salts out, then removed and replaced once it has done its job. Never use cement or gypsum as a sacrificial layer; they trap moisture and make crystallisation worse.

Reducing Future Salt Damage in Historic Buildings

Prevention is always better than repeated poulticing. The goal is to keep the wall dry and breathable.

• Maintain rainwater goods. A leaking gutter can saturate a wall for years.
• Lower external ground levels so they sit below the internal floor and damp-proof course.
• Improve ventilation. Air movement helps moisture evaporate before salts concentrate.
• Use breathable finishes. Limewash, distemper, and clay paints allow walls to dry. Avoid vinyl or acrylic paints.
• Repair with lime. Lime mortars and plasters are softer and more vapour-permeable than cement, so moisture moves through them rather than being trapped.
• Avoid de-icing salts near the base of walls. Use sand or grit instead.
• Monitor after heavy rain. If damp patches return, investigate the source again.

It is also worth remembering that some salt activity is normal in old walls. The aim is not to eliminate every trace, but to manage moisture so that crystallisation happens within the plaster, where it can be carefully removed, rather than beneath impermeable layers where it causes hidden damage.

When to Bring in Specialist Help

If salts keep returning despite your best efforts, or if the plaster is deeply contaminated and failing, a conservation-accredited surveyor or plasterer can help. They can assess the wall, identify the moisture source, and recommend a repair strategy that respects the building's age and construction. With the right approach, lime plaster can be restored and protected for many years to come.