Some of the earliest examples of glazes and stains can be found on medieval European polychrome sculpture (Serck-Dewaide, 1991). There are, however, countless examples on later European painted or gilded furniture. Theophilus, a twelfth century monk, recommended coating metal leaf with 'boiled oils added to a quart of gum called glass by the Romans'. The De Meyerne Manuscript of 1620 describes the use of shellac, saffron, aloe and gum arabic. Some common transparent coatings were lacquers or glazes of coloured resins or gums (e.g. gamboge) applied in a solvent such as spirits of turpentine over silver gilding to give it the appearance of gold. Watin (1728) describes making a good 'vernis or' from gutte, sandarac and wine spirits, while Stalker and Parker (1688) advise the use of 'size, seed-lac-varnish or lacker', which, for a deeper colour, should be mixed with dragon's blood and saffron. Pacheco (1649), Cennini (Thompson, 1960) and other well-known period sources all give such recipes, many of which were highly coloured with madder, indigo or other dyes to be applied over metal leaf or paint.
The process of distillation necessary for the production of solvents was known to the ancient Greeks but not extensively or efficiently employed. Purified alcohol was widely available in Europe after the twelfth century. Various volatile essential oils such as 'oil of spike' (spike lavender) and 'spirits of turpentine' (distilled from conifer resins) were also being prepared in Europe by the late fifteenth or early sixteenth centuries. Several clear resins could be dissolved in 'spirits of wine' (ethanol) including shellac, sandarac, mastic, gum benzoin, accroides and the brightly coloured red and yellow resins dragon's blood and gamboge. A smaller number, such as mastic and dammar, could be dissolved in turpentine. Turpentine was the primary solvent used to thin out both drying oils and oil—resin varnishes.
Until the end of the eighteenth century, the relatively high cost of alcohol soluble resins as well as the expense of the solvent confined these 'spirit varnishes' to such rarefied and time-consuming crafts as japanning and the finishing of small decorative objects. The technique of 'French polishing', by which shellac-based varnishes are applied with a cloth pad in a complex series of operations, was probably not developed until the early years of the nineteenth century when both shellac and rectified alcohol became more commonplace. It is worth noting however that shellac must have been available from at least the late seventeenth century since it is referred to by Stalker and Parker as being unsuitable for varnishing. The application of shellac with a pad creates a surface coating known as French polish. This finish technique has its origins in an eighteenth century process called French polishing which used bundles of abrasive equi-setum stalks called a 'polissoire' and wax to create a high gloss finish (Halee, 1986; Mussey, 1982a). By the early nineteenth century, obtaining a high gloss finish with a pad and shellac became the preferred technique. In an 1827 American finisher's manual, it was noted that friction, or French polishing, was 'of com paratively modern date' (Mussey, 1987). The Mechanic's Register (1837) claimed that 'We were the first to publish any accurate information on the "French Polish" for wood, now become so universally employed'. It has been a part of the finisher's and restorer's trade ever since. For further discussion on the introduction of 'French polish', see Penn (1984).
Synthetic polymers became available as coating products toward the close of the nineteenth century with the introduction of cellulose nitrate. This and the products that have followed in the twentieth century are synthetic materials intended as improvements on traditional natural materials. These products vary greatly in chemical make-up. They can be found in transparent coatings, paints, and other finishing materials. In general, synthetic resins are high molecular weight long chain or network polymers. Synthetic resins that have been used as coatings on wood include alkyds, urethanes, phenolics, polyesters, epoxies, methacrylates (acrylics), ketone resins and cellulose nitrates.
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