muller and slab of the type commonly used today for grinding pigments.
Shown here, clockwise from top right are: French ultramarine (blue)
and three earth pigments; yellow ochre (on the slab with a pool
of turpentine in it, ready for grinding), raw sienna (left) and
burnt sienna (centre).
Photo: Jonathan Taylor
can arise from the multitude of 'historic' paint ranges that are now on
the market. Most of these reproduce the colours from former periods but
do so in a modern alkyd or emulsion paint. Whilst these may be aesthetically
pleasing, they should not be confused with the true traditional paints
that are very similar to (though not perhaps identical to) the composition
and performance of those that were used over the past few centuries.
WHAT IS PAINT?
does two jobs - it protects and decorates. Its decorative use goes back
to the cave paintings of the Stone Age, but the protective properties
of paint became more important from the 17th century as scarcity of good
hardwoods lead to cheaper and less robust materials being used for building.
In order to carry out these two tasks, all paints are made up of three
- pigments that colour
and protect the surface to be painted - the substrate
- binder that holds
the pigments together and binds them to the substrate
- solvents that thin
the mixture of pigment and binder sufficiently for it to be applied
to the substrate .
Many different materials
have been used as pigments. The earliest examples include natural earth
pigments, such as umbers, sienna and yellow ochre, which could be changed
in shade by heating or 'burning' them. Other naturally occurring mineral
pigments include chalk, china clay and barytes, all of which were white.
These minerals were relatively cheap and easy to find as well as having
good 'light fastness' - that is to say they were less likely to fade in
sunlight. More exotic colours made from vegetable dyes or other natural
materials could be very expensive and their light fastness was often poor.
In the 18th century,
as a proper understanding of chemistry began to emerge, a number of metallic
salts which had better light fastness were discovered. These pigments,
such as lead chromate and Prussian blue, gave a much wider range of colours,
but as they were often based on metals such as copper, mercury, lead or
arsenic they could have unhealthy sideeffects. Many modern synthetic pigments
have since been developed that are safer to use.
Some pigments alter
the structure of the paint without necessarily changing its colour. These
are known as extenders as they can help to bulk out a paint and to give
body to a pigment, but they can also be used to change the finish from
gloss to matt. Other pigments are added for their special attributes such
as anti-corrosion properties.
The earliest types
of binders were often derived from organic matter such as eggs, milk (casein),
bones (glue) or lac, which is produced by an insect on certain trees in
the East Indies, and is the resin used to make shellac. Later, oils were
used, particularly linseed, walnut, poppy and latterly soya and tung oil.
These could be modified by heating (or boiling) to change their characteristics,
and, more recently, have been reacted with chemicals to produce the highly
versatile family of alkyd resins.
the 20th century, synthetic resins such as epoxy, polyurethane and chlorinated
rubber were developed, which have excellent protective properties. The
emulsion resins of the now ubiquitous vinyl and acrylic families are perhaps
the most important of these.
such as turpentine were initially used to thin the mixture of pigments
and binder. Other chemical 'additives', were included to improve the performance
of the paint, for example by speeding up drying, preventing skinning,
or improving stability and by imparting a wide variety of other useful
we define traditional paints as those materials used for internal and
external decoration (and protection) between the late 17th and the early
20th centuries, then there are three main families: limewash, distempers
and oil paints.
LIMEWASHES AND DISTEMPERS
has been used for many centuries and is a thin gelatinous (colloidal)
suspension of calcium hydroxide in water, the principal ingredient of
lime putty. It is highly permeable and is widely used as a 'shelter coat'
to consolidate and protect old limestone.
or 'soft distemper' to give it its original name, was a very simple, cheap
and widely used interior paint which is very permeable and resistant to
strong alkali, making it ideal for coating lime plaster. It is a water-based
mixture of chalk bound with glue. Casein paint, which first appeared in
this country in the early 19th century, is similar, but it also contained
casein, a binder of milk solids, and for this reason casein paints are
sometimes referred to as 'milk paints'.
addition of raw linseed oil, emulsified in water using borax (sodium borate),
produced 'oil bound' distempers. These were significantly more useful
than previous materials. They had greatly improved hardness and were the
first washable water paints. They were still very permeable, allowing
new plaster to dry out, and the small amount of oil was not liable to
be 'saponified', or turned to soap, by the alkali in the plaster.
oil, obtained by crushing flax, was the most important oil for use in
oil paints. Its rather yellow colour was a drawback, and for more delicate
shades other more expensive oils, like walnut or poppy seed, were sometimes
used. These are all 'drying oils' - they absorb oxygen from the atmosphere
to form a hard flexible film. This reaction could be accelerated by the
addition of driers, notably litharge or lead monoxide, to the linseed
oil. Grinding white lead (basic lead carbonate) with linseed oil produced
a mixture called 'lead soap', which was an outstandingly flexible and
adhesive coating. It also had excellent opacity, or covering power, whilst
many other white pigments, such as chalk, became almost transparent in
oil. White lead also helped the linseed oil to dry, unlike some other
pigments, such as lampblack, which slowed the drying process.
lead had been known since antiquity to be the best white pigment available
for use with drying oils. It was made by suspending sheets of lead metal
over vinegar in covered pots. These were then laid in a dung heap to keep
warm for several weeks to allow the fumes from the vinegar to react with
the lead. After removal, the white lead powder (lead carbonate) which
formed on the surface of the sheets was ground to a fine powder. This
was called the Dutch or stack process and was used until the late 19th
century when the more efficient chamber process was developed.
white lead was originally ground with the linseed oil by hand using a
Muller and Slab, then later by machines such as cone mills or edge runners
driven by horse or steam power. Paints typically contained over 80 per
cent white lead with the balance made up of the linseed oil binder and
turpentine as the solvent. The balance between these two dictated the
properties of the paint. More oil than turpentine gave a well-bound but
glossy paint that was more resistant to the weather, and was suitable
for outside and inside use; more turpentine than oil gave a matt finish
that was suitable for indoor use only.
technology improved, so the composition of paints and their ingredients
altered and their performance and ease of use improved. By the early 20th
century, alkyd resins joined linseed oil as binders, improving the flow,
gloss and appearance of the finished film, while the production of titanium
dioxide as an additional white pigment boosted the covering power. These
advances to the formulations produced lead paints of outstanding flexibility
and durability: the internal surfaces of the main supporting tubes of
the Forth Rail Bridge have only been repainted once in 110 years, despite
the extreme climatic conditions and the attentions of nesting pigeons.
people consider that the performance of modern gloss paints remains inferior
to that of lead-based paints. However, concern over the potential hazards
of using lead continued to grow, and when titanium dioxide became available
in commercial quantities in the mid 20th century it soon started to replace
white lead. By 1970 the use of white lead in decorative paints was voluntarily
withdrawn by the paint industry and in 1992 its use was prohibited except
for approved applications on Grade I and II* listed buildings (Grade A
in Scotland), and their use is monitored by English Heritage, Cadw and
Historic Scotland. Information on the requiremnts can be obtained from
these bodies or from the white lead paint manufacturers.
and where traditional paints such as lead paint and limewash should be
used is a matter for endless debate between conservationists. Although
they can be difficult to use and apply in a satisfactory and safe manner,
there is little argument that they give a texture and look that cannot
be matched by modern paints. Their use also helps to preserve the integrity
of a building in the general context of its original design, appearance
and purpose. It could even be argued that because lead paint will last
externally much longer than modern paints, its use is more economical.
Traditional paints may not always be the best or right solution, but their
use should always be considered within the overall context of a project.
paints are a fascinating topic and it has only been possible to scratch
the surface here. Those interested in further information should contact
the Traditional Paint Forum. Founded in 1992 'to promote a better understanding
and appreciation of traditional paint', it is a unique and important forum
for the exchange of information and ideas about traditional paint.
- Ian Bristow,
Interior House Painting Colours and Technology 1615-1840, Yale University Press, London, 1996
- Noel Heaton, Outlines of Paint Technology, Charles Griffin, London, 1928
guide on repainting and removal of old lead painted surfaces is available
free from The British Coatings Federation, James House, Bridge Street,
Leatherhead KT22 7EP
article is reproduced from The Building Conservation Directory, 2002
The late COLIN
MITCHELL-ROSE spent four years in the Army before joining family firm Craig and Rose in 1973. Initially employed as a chemist in the laboratory, at the time of writing this article he was
Technical Director of the company.
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