Gareth Davies outlines the role of traditional limewashes and modern alternatives.

Under normal atmospheric conditions in western Europe, external masonry walls have a moisture content ranging from 10 to 20 per cent. Broadly speaking the higher the porosity of the material, the higher its potential to retain moisture will be. Moisture content will vary according to external influences including wind, rain and sun, and the rate of water ingress, either as rising damp or percolation due to some failure in the fabric of the building. In addition where a structure may have become saturated through damage or neglect and subsequently repaired, there will be a drying out process until an equilibrium is reached, which could take weeks, months or even years. The choice of finish applied to the walls needs to take these factors into consideration.

At Alexandra Palace, for example, the brickwork structure became saturated as a result of extensive fire damage to the building which left the building without a roof for several years. Conservative estimates calculated that there was approximately 50,000 litres of water contained in the structure, a high proportion of which would have to evaporate through the internal walls of the building following renovation. The specification required both a plaster system and decorative coating that would allow the walls to dry out gradually over several years without harming the new finishes.

If we consider the role of an external coating, the primary aim of the specification is to decorate and enhance the appearance of the building. Secondly the specification should also provide a degree of permanence and if possible, some protection to the underlying material not only from varying climate conditions but also from potentially harmful environmental pollution. Ideally it should allow for the free passage of moisture vapour from the underlying material, whilst at the same time preventing the ingress of further moisture - in other words it should allow the underlying material to 'breathe'. If this were not the case and the coating retained moisture in the underlying material, the risk of damage from frost action would increase, due to the effect of the expansion of ice as it melts on the structure around it. Coatings can also fail under damp conditions and the underlying material can deteriorate, as a result of salt crystallisation in particular. This is caused by the migration of soluble salts to the point of moisture evaporation where they crystallise out of solution. Masonry decays under the force of crystallisation pressure within the pores.

Where the underlying material is particularly vulnerable to decay, such as soft brick, friable stone, render and cob, the need to achieve a permeable coating is paramount.

FILM FORMING PAINTS

Rather than categorising coatings as being breathable or impermeable, it is better to understand the mechanisms by which certain coating systems achieve the required visual and performance characteristics. In simple terms a coating either sticks to the surface to which it is applied, or soaks into it in either a physical or chemical way - or performs by means of a combination of both of these methods.

When a coating sticks to underlying material it is usually an organic polymer, petrochemical in origin and forms a skin, or film, that has a permeability that is lower than that of the underlying material to which it is applied. The higher the porosity of the underlying material (weak and friable materials), the greater the difference in permeability. The breathability of polymer paints can be improved through the use of inorganic fillers resulting in a more open or porous system - the concept of many of the modern microporous paint systems.

As well as the breathable aspects of the coating it is important to understand how it performs in relation to movement of the underlying material caused by climate temperature changes. Artificial resins contained in film forming coatings have thermal expansion coefficients ranging from 10 to 20 times higher than that of the mineral underlying material to which they are applied. This differential thermal movement causes stresses in the coating film which can lead to cracking and subsequent coating failure.

Organic based systems may also fail due to the action of solar radiation causing UV degradation of the coating. This initially can result in a loss of colour with an organic pigmented system, colour fade and then eventual embrittlement of the film causing cracking and failure.

Of the alternatives available, water-borne metha-acrylic emulsions provide perhaps the best performance as they allow some vapour movement through their polymeric structure, without the need for inorganic additives, and are least likely to suffer from UV degradation or to become brittle within a typical 10 year repainting cycle.

LIMEWASH

This traditional coating provides a breathable, decorative finish that soaks into the underlying material to which it is applied. The material is primarily composed of slaked lime (calcium hydroxide) usually with a low proportion of an organic binder such as tallow. The calcium hydroxide sets slowly by combining with carbon dioxide to form calcium carbonate, the principal component of limestone and marble. Less commonly used alternatives to tallow include casein which reacts with the slaked lime to form calcium caseinate and produces an insoluble bond, and pozzolanic additives such as fly ash which cause it to set on contact with water through a more complex reaction. As a coating for lime-based render and stucco, and limestone in particular, limewash is in many ways comparable in nature to that of the underlying material, with similar porosity, alkalinity (pH value) and coefficient of thermal expansion.

In the past, periodic redecoration of building facades with limewashes every few years was very much the norm and specific compositions varied almost on a building by building basis, relying heavily on past experience, local availability of raw materials and what had previously proved successful.

A number of factors conspired to the reduction in use of limewashes. With the advent of modern film-forming paints in pre-pigmented colour shades, the skill and knowledge required for both the production and application of limewashes rapidly diminished, although it is now enjoying a resurgence. When limewashes are applied to underlying materials of varying porosity, such as a brickwork facade having a relatively soft jointing mortar and hard faced brick there will be a tendency for the limewash to exhibit differential crack movements allowing water ingress and eventual coating failure. Limewash should not be used on sandstone which has not previously been treated with limewash, as the introduction of lime can lead to its deterioration.

The rapid increases in pollution since the industrial revolution, especially acid rain, has lead to a very rapid degradation of external limewash facades. Firstly run marks are noticeable on such areas as sills, copings, etc, due to the acid reacting with the limewash and dissolving it. Then eventually the entire facade will be attacked and degraded. Limewash provides protection for the surface as a 'sacrificial' coating, in that it is destroyed more rapidly than the material below it. It also can help to consolidate friable limestone surfaces. Limewash is most suitable for smaller historic buildings which are rendered or constructed of limestone masonry, cob or wattle and daub, where there is a need to maintain the historic appearance and where it is accepted that regular maintenance will be required. In the short term at least it is also relatively economical.

SILICATE PAINT SYSTEMS

As with limewashes, silicate paints soak into the underlying material, but in addition the potassium silicate binder chemically reacts with the mineral underlying material to form a microcrystalline silicate bond which is insoluble. Secondary chemical crystallisations also take place between the binder, the colour pigment and carbon dioxide in the atmosphere.

The resultant microcrystalline structure has a pore size that allows the free passage of vapour (silicate paints have a very low diffusion resistance equivalent to approximately two centimetres air layer thickness), but the pores or 'holes' are small enough to prevent the ingress of driven rain. The inherent nature of a silicate paint is that of a semi permeable membrane.

Combined with this basic structure, silicate paints employ inorganic fillers and earth oxide colour pigments that are unaffected by the action of UV degradation. The microcrystalline structure is comparable to that of the mineral structure to which it is applied and it has a comparable coefficient of thermal expansion. The insoluble silicates formed in the chemical reaction are resistant to strong acid and alkali attack in the same way that silica sand is a highly resistant product.

As a highly porous protective coating, silicate paints offer a valuable alternative to a limewash, and can provide for much longer life expectancies. There are documented examples of such paint systems performing satisfactorily on lime based render facades in Germany, Switzerland and Norway for periods in excess of 100 years.

ENVIRONMENTAL ISSUES

There is a European wide trend to move towards solvent free coatings. In addition, coatings that are non-petrochemical based are also seen to be attractive. Both limewashes and silicate paints are preferable in this respect to either solvent borne or aqueous acrylics and other resin based systems.

Lead-based paints, which were widely used in the 19th Century on stucco and in London in particular, are more resilient to pollution than limewash, but today they are effectively banned for almost all applications due their toxicity. Gloss paints such as the microporous acrylic systems provide the closest visual match to a lead-based paint, although less permeable and less durable than silicate systems.

In conclusion, typically limewashes require maintenance every few years; film forming systems from 5-15 years; and silicate paint systems perform from 15 years and upwards. For historic buildings the need to provide the most durable long-term protection will often be the prime consideration, particularly where scaffolding will be required for maintenance. However a balance may need to be struck between the need for durability and the need to maintain historic integrity and, where lead paints were used, the character of an area.