Cleaning Methods in Stone Restoration: Poultice, Micro-Abrasive Blasting and Laser

It is not surprising to see three different cleaning methods used side by side on the same facade at a restoration site. In one corner a thick paste has been applied to the stone, elsewhere a specialist sprays fine powder with a pencil-sized nozzle, and on a relief section a conservator wearing protective goggles slowly moves a laser head across the surface. Each method has been chosen for a different type of soiling and a different surface. In this article we explain how poultice, micro-abrasive blasting and laser work, why each is preferred for particular soiling, what risks they pose to the stone and which situations always require a specialist.

In brief: A poultice uses an absorbent paste to draw dirt or dissolved material out of the stone; it is used for oil, salts and some crusts. Micro-abrasive blasting sprays a very fine powder at low pressure to locally abrade hard crusts. A laser uses short pulses of light to separate the dark crust from the light-colored stone. All three require a test area and an experienced operator.

The Type of Soiling Determines the Method

In restoration there is no single cleaning method that suits every job; there is a method suited to the soiling and the surface. On the same facade you may find loose dust, greasy soot, hardened gypsum crust, rust that has seeped from metal fixings, old paint residue and biological growth all together. Each bonds to the stone differently and calls for a different approach.

To identify the type of soiling, the surface is first examined closely and touched, and if necessary a small area is tested with water and a soft brush. Dirt that comes off with water is only loosely attached to the surface. Dirt that does not come off with water but softens with soap contains grease or soot. A hard layer stuck to the stone that comes off with neither water nor soap is most likely a crust. These simple observations determine which method is worth trying.

The stone itself also influences the choice. A dense, tight-textured marble and a porous limestone or sandstone react differently to the same method. A surface with relief, inscriptions or fine chisel marks calls for a much more delicate approach than a flat cladding slab. Environmental conditions are also taken into account: in winter, when there is a risk of freezing, methods that use water are postponed, and on a street with heavy pedestrian traffic, measures for collecting dust and waste water are added to the work plan. In a museum interior, effects such as noise, dust and chemical vapor are subject to far stricter limits than on an outdoor facade.

Poultice: Drawing Dirt Out of the Stone

A poultice, or compress as it is commonly called, is an absorbent material mixed with a liquid and applied to the stone surface as a paste. The absorbent is usually a clay such as sepiolite, cellulose fibers or a mixture of the two. The liquid part is chosen according to the type of soiling: it may be pure water, a solvent or a chemical solution that reacts with the dirt.

How does it work?

When the poultice is applied to the surface, the liquid in it penetrates the pores of the stone and dissolves the dirt or salt. As the poultice dries, the liquid moves back toward the surface and carries the dissolved material with it into the absorbent. When the dried poultice is removed, part of the dirt goes with it. If necessary, the process is repeated several times.

One method often used on black gypsum crusts is a poultice prepared with an ammonium carbonate solution. This solution reacts with the gypsum and converts it into a water-soluble salt and calcium carbonate; the dissolved salt is then removed by rinsing with water. The concentration of the solution and the dwell time are always kept at the lowest level determined by testing.

What soiling does it work on?

A poultice stands out for stains that have penetrated into the stone and in places where work must be done without abrading the surface at all. Oil and soot stains, soluble salts, some metal stains and thin gypsum crusts are its main targets. We explained the finer points of preparing a poultice for household and business-scale stains in our article how to prepare a stain-removing poultice .

Risks

The greatest risk of a poultice is that the chemical used leaves residue inside the stone. A wrongly chosen or overly concentrated solution can form new salts inside the stone after cleaning. If the poultice stays on the surface too long, moisture goes deep into the stone; if it dries too fast, the dirt may form a ring under the poultice without being carried to the surface. For this reason the drying rate is usually controlled by covering the poultice with a wrap, and after application the surface is rinsed with plenty of clean water.

Micro-Abrasive Blasting: Controlled, Local Abrasion

Micro-abrasive blasting works on the same principle as conventional sandblasting but on a much smaller scale. A very fine-grained abrasive powder is sent to the surface with low-pressure air from a pencil-sized nozzle. Aluminum oxide, glass powder, calcium carbonate or softer organic powders can be used as the abrasive. The powder grains used in micro-abrasion work are generally tens of microns in size.

What soiling does it work on?

Micro-abrasive blasting is considered for hard crusts that do not soften with chemical methods, thick and adhered layers of dirt, and colored or dark stones where a laser is unsuitable. The operator can thin the crust by advancing the nozzle millimeter by millimeter and stop when the sound surface is reached. This control is the main feature that sets it apart from conventional sandblasting.

Risks

Micro-abrasive blasting is ultimately an abrasion and always removes something from the surface. A result that looks acceptable to the eye may show fine damage on the stone surface under a microscope. In a cathedral frieze in England, micro-abrasion carried out with aluminum oxide years earlier was found, when later examined with an electron microscope, to have left damage on the stone at the microscopic level. A hurried application, on the other hand, can leave a blotchy, mottled appearance on the surface, and this becomes more pronounced as the stone ages.

For this reason, in micro-abrasive blasting the powder type, grain size, pressure, nozzle distance and angle are determined in a test area and kept constant throughout the work. On a soft stone, a hard abrasive or high pressure wears away the stone before the dirt.

Laser: Selective Cleaning with Light

Laser cleaning is based on very short, high-energy pulses of light striking the dirt on the surface and vaporizing it or breaking it up and detaching it. The most widely used devices in stone cleaning are Nd:YAG lasers, which emit light in the infrared range and produce their pulses in very short durations. The operator moves the light beam across the surface with a small head.

Why is it selective?

The main advantage of laser in stone cleaning is that the dark crust strongly absorbs the light while the light-colored stone largely reflects it. When the energy is set correctly, the crust is cleaned, and once the white marble surface is reached, the effect drops markedly. This "self-limiting" behavior makes the laser valuable especially on white marble reliefs, sculptures and finely detailed ornamentation. Working without contact, water or chemicals is also an advantage on salt-laden and delicate surfaces.

Risks

It is known that a yellowish tone sometimes remains on white marbles cleaned with a laser. Whether this tone is caused by the laser itself or is the exposure of an organic layer already present beneath the crust is still debated among scientists; some studies point to organic matter that has penetrated beneath the crust. In addition, a laser can cause unwanted color changes on dark and colored stones, on painted or gilded surfaces and in some pigments. Eye safety, enclosing the area and having the device operated by a trained person are also indispensable conditions of this method.

Supporting Methods: Water Mist, Steam and Gels

Poultice, micro-abrasive blasting and laser are often used not alone but together with gentler methods. Water mist keeps the surface damp for a long time with fine droplets, softening the dirt and gypsum, which is then removed with a soft brush. Low-pressure steam can be effective on greasy dirt, but heat and moisture require care on salt-laden surfaces; we discussed when steam is suitable on natural stone in our article steam cleaning and natural stone . Gels, on the other hand, hold solvents or chemicals on the surface and limit how deeply they penetrate.

Graffiti and paint residue form a separate group. For these soiling types, solvent poultices or gels are used depending on the type of paint; micro-abrasive blasting and laser are considered only for the remaining traces. We covered this topic in detail in our article anti-graffiti coating and graffiti cleaning .

A comparison of the three main methods is given in the table below:

Situations That Require a Specialist

Applying a poultice to a simple oil stain is a job an experienced stone crew can easily handle. In some cases, however, choosing and applying the method should be the responsibility of a conservator or restoration specialist.

In the following situations, the work should begin with a specialist's opinion:

• Listed buildings and artifacts: On a building subject to heritage protection legislation, the choice of method requires approval; the application must also follow the approved method and be documented.
• Reliefs, inscriptions and sculpture: Even a small mistake on surfaces with fine detail cannot be undone; supervision by a specialist who knows the limits of the method is essential.
• Paint, gilding and traces of color: If there are remains of old paint or gilding on the stone, they should be examined and documented first, as they can easily be lost during cleaning.
• Blistering and disintegrating surfaces: If the crust has detached or the stone is powdering, consolidation is needed before cleaning; otherwise, cleaning will also remove the original surface.
• Salt-laden and damp walls: Because methods that use water and chemicals can mobilize salts, the source of moisture and the salt load must be assessed first.

How Is a Method Chosen in a Project?

The method is chosen on the building itself, not at a desk. First the surface is mapped and the soiling types are identified. Then, for each soiling type, several methods are tested on small trial areas, starting with the gentlest. Once the results have dried, they are compared in daylight, photographed and recorded. Practice guides such as BS 8221-1, widely used in the UK for cleaning stone, brick and concrete surfaces, also place trial areas at the center of method selection.

In most projects, the result is not a single method but a sequence of methods. First, loose dirt and gypsum are softened with a water mist; then a poultice or laser is used in areas with thick crusts, and micro-abrasive blasting comes into play at the most stubborn spots. Surfaces that have been washed by rain and are already thinned are often left out of this sequence. You can find the principles that guide cleaning in principles of cleaning historic marble surfaces, and how black crust forms in the article air pollution and black crust on stone .

If you are looking for a cleaning method for a facade, fountain or stone ornament, the first step is to see the surface on site and identify the soiling types. Based on this assessment, we can plan together which work should be done by the stone crew and which should be done together with a conservator.