When present, paint usually carries a vital part of the information the artist, designer or craftsman wished to convey. Paints consist essentially of a pigment, a film-forming medium or binder, and a liquid component composed of solvents and diluents that makes the mixture more or less fluid during application. Together, the film-forming and liquid components are often called the vehicle. Various additives may be used to give coatings additional structural or aesthetic properties. Many of the materials used to make transparent coatings are also used to make paints. Due to the presence of pigment particles, opaque coatings are generally more stable than transparent coatings (Roff and Scott, 1971) and generally provide better protection of the substrate due to the more complete exclusion of light, moisture and environmental fluctuations. A harmonious relationship between the ground, the pigment and the binding medium is essential (Wehlte, 1975). The ground is not only the foundation, but the underlayer for all subsequent colour. The visual character is determined by the binding medium, whether the pigment particles are simply attached or whether they are embedded in it. The absorption of the medium by the ground, and the distribution of the pigment in the medium (opacity) are critical factors. Finally the surface coating is important, not only as a protective layer, but also in giving the paint its optical properties.
Pigments are fine, solid, particles that do not dissolve in the vehicle. They are used to provide colour and body. Materials that provide body without providing colour are called extenders. Extenders are normally used because they are cheaper than prime pigments but they also contribute useful qualities to the paint, for example by improving adhesion, strength and ease of sanding.
A medium or binder is the substance that holds the pigment particles together in a paint. This must be capable when dry of forming a solid coating in a continuous film over the surface. This can be achieved by evaporation of solvents, by reaction of pre-polymers, or by change of temperature. The term medium may also be used for materials suitable for adding to or combining with paint to change its properties to the requirements of the artists particular techniques; for example to make it thicker or thinner, quicker or slower drying. Additives that may be used for this purpose include thickeners, flow promoters and reducers, matting agents, catalysts, accelerators, inhibitors, wetting agents and dyestuffs. Binders for pigments must satisfy most of the same criteria as for a clear finish except that they need not be transparent. Mixtures of pigments and binders, or paints, can be categorized by the ratio of pigment to binder. Paints with a high ratio of binder to pigment are called fat and vehicular and those with a low ratio are lean or granular. This terminology may be extended to lightly coloured mediums which are described as pellicular.
The liquid part of the vehicle can consist simply of a true solvent for the medium but frequently it contains diluents which are not true solvents for the binder but which contribute other desirable properties to the vehicle.
The formulation of paints is critical to their success and many different factors play a part. Paint must have the required appearance when dry but must permit ease of handling to allow application in the fluid state. All paints require an appropriate degree of adhesion to the substrate or preceding paint layer and good flow characteristics to wet the surface and level out to the required degree. They must then stop flowing and dry to produce hard tough durable films that will retain their decorative properties in service. Generally, it is desirable that they should be relatively easy to repair but they must be resistant to common solvents in use. Normally if a paint is being formulated for a particular application one starts with the required properties of the dry film. This determines whether the film will be thermoplastic or thermosetting and consequently the choice of drying mechanism and application techniques required. Formulation of paints in relation to application characteristics is discussed by Bentley (1998).
Pigmented varnishes are sometimes called enamels, lacquers, finish coats or topcoats. Enamels are normally based on thermosetting materials that give a hard finish superficially resembling vitreous enamel. The term lacquer is normally used for thermoplastic solution paints or varnishes but is sometimes confused with oriental lacquer (e.g. Japanese urushi), a quite different material. Although top coats can be applied directly to the support, it is often difficult to achieve the desired result with a single formulation. Stoppers, fillers, primers and undercoats which have a composition suited to fulfil their function in the system of surface decoration, are usually applied to form a ground to give optimal results from the finish layers. The function of stoppers and fillers has been discussed above. Primers promote adhesion, reduce absorption by porous surfaces and usually impart some corrosion resistance over metals. Undercoats, which frequently contain significant quantities of extender, form a highly pigmented layer that provides body to the paint and help to level out minor imperfections to permit easy smoothing and give a good base for the topcoat. Undercoats must adhere well to both the primer layer and top coat. Sealers may be applied to the substrate over another coating layer to prevent movement of material out of the substrate into the paint or from one layer of paint to another. Sealers may also be used to improve adhesion between layers where this would otherwise be weak. Suitable paint media may be used in thin layers with little (transparent) pigment to make glaze layers which permit subtle colour variations to be achieved.
A wide range of substances and mixtures of substances is found in historic paint media used in the 'polychromy' or coloured decoration of furniture. Among these are animal glue (distemper), egg tempera, casein tempera, wax emulsions, wax resin mixtures (encaustic) and various drying oils and oil-resin mixtures. Water colours are also frequently encountered in retouches. Watin's L 'Art du Peintre, Doreur, Vernisseur, first published in 1755, gives an excellent account of artist's materials and their application. The pigments he recommended include lead and chalk whites, red and yellow ochres derived from clays, lapis and Prussian blues and lamp and ivory blacks to name only a few. He cites binders made of animal glues, egg white, oils, such as linseed, walnut and olive, resins such as mastic, copal and san-darac, and gums. These materials are discussed in more detail at the end of this chapter. A French paint system that Watin called Chipolin (from the Italian for garlic), consisted of parchment glue to which garlic has been added and whiting (ground) followed by two coats of a glue-based paint, and then by two coats of weak glue size. Finally, three coats of spirit varnish were applied (Watin, 1728). The garlic may have served to promote adhesion and acted as a fungicide.
Fascinating and valuable historical reference is also provided by the period treatises of Pacheco (1649) and Stalker and Parker (1688). Massey (1967) gives over 200 formulas for making paints, glazes, mediums and varnishes for a variety of painting techniques including tempera, oil, acrylic, gouache and encaustic with instruction on their purpose, manufacture and use. Knowledge and awareness of traditional paints has led to the formation of organizations such as the Traditional Paint Forum in the United Kingdom which focuses on the materials and techniques utilized for architectural paint schemes and associated furniture. The materials and techniques of medieval painting are discussed by Thompson (1956) and by Hulbert (1987). Mayer (1981) provides the artist with a comprehensive review of all aspects of the materials and techniques of painting. A good introduction to paint chemistry and the principles of paint technology is provided by Bentley (1998).
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