Davies outlines the role of traditional limewashes and modern alternatives
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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
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.
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
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.
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.
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.
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.
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.
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.
article is reproduced from The Building Conservation Directory, 1996
DAVIES is the Export Director
of Keimfarben GMBH and Co of Dieseldorf, Germany, who patented
the first silicate paint in 1878. An engineer by qualification,
he has worked for Keim for the past 12 years, running their UK operation
and, more recently, a world-wide network of Keim dealers.
PRODUCTS AND SERVICES
Paints and decorative finishes
Painters and decorators
Communications Limited 2010