Colin Mitchell Rose
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
Considerable confusion 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?
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 components:
- 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.
In 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.
Solvents 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 attributes.
If 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
Limewash 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.
Distemper, 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'.
The 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.
Linseed 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.
White 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.
The 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.
As 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.
Many 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.
When 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.
Traditional 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
- A 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