Carbohydrate Containing Binders

Carbohydrates are compounds that contain carbon, hydrogen, and oxygen. The basic building blocks of carbohydrates are simple sugars, or (in the nomenclature of organic chemistry) monosaccharides. Some simple sugars contain five carbon atoms, others six. The six-carbon simple sugar known as glucose is probably the most abundant organic compound on the earth. Another common simple sugar is fructose, which is found in honey and many fruit juices. Simple sugars can be bonded together to form larger molecules. Disaccharides are carbohydrates that consist of two simple sugars bonded together (for the sake of convenience, we can call these individual simple sugar building blocks "units"). The most ubiquitous example in everyday life is table sugar (sucrose), a carbohydrate found in many plants. Each molecule of sucrose consists of one glucose unit and one fructose unit bonded together.

Many simple sugar units can be bonded together to form very large molecules, known as polysaccharides. Starch, found in grains such as wheat and corn, is a polysaccharide consisting of many glucose units bonded together. Like all natural polysaccharides, starch has no fixed chemical composition, but each "molecule" of starch appears to consist of from about one thousand to several thousand glucose units bonded together. Another common polysaccharide is cellulose, the principal structural material of plants. Like starch, cellulose consists of many glucose units bonded together, about 1,500 or more per molecule of cellulose.

3.1.1 Honey: Honey, while not a widely utilized binder, has probably found use in paintings at many times in the past. It may have been used in ancient Egypt; nearly 3,000 years later it was described as one of the binders in use by the painters of medieval European illuminated manuscripts. Also mentioned in association with manuscript illuminations were plant or fruit juices, which owed their stickiness to simple sugars or disaccharides. Like modern table sugar, honey and the sugars of fruit or plant juices can be easily dissolved in water. Once the sticky sugar solution dries, it can be readily redissolved in water. It seems likely that binders containing simple sugars would not have been used on artifacts that were intended to be subjected to the elements, as the paint layers they produce are very sensitive to moisture and are quite brittle.

3.1.2 Plant Gums: Plant gums are another example of natural polysaccharides. These are exudates from trees of many types. Although varying considerably in composition from one type of tree to another, they consist of very large molecules, each probably consisting on average of several thousand simple sugar units bonded together. In addition to simple sugars, gums usually contain sugar acids.

Some plant gums are soluble in cold water, some in hot water, while some are partly soluble in water and some are completely insoluble. Those that have been used as paint binders can be put into solution in water. The behaviors of starch, cellulose, and plant gums when placed in water are quite variable, and yet all of these materials are polysac-charides. The extreme variability, from the total insolubility of cellulose to the complete solubility of some gums, is related to the structure of the molecules. Simple sugars can be bonded together in several different ways, from long, chainlike molecules (as in cellulose) to highly branched, approximately spherical molecules such as the soluble plant gums. The polysaccharides with branched structures can be dissolved in water.

A great many types of plant gums were probably utilized in different times and places as binders for paints. Probably the most common is the material now known as gum arabic, which at this time can be found as an additive in many foods and is the binder of many modern commercial artist's watercolor paints (Figure 3.2). Gum arabic comes

Fig. 3.2. Winslow Homer. The Adirondack Guide, 1894.

Watercolor over graphite on paper. Paintings like this one were carried out with commercial watercolor paints, which were sold in pans or tubes. The paints were bound with natural plant gums, most commonly gum arabic. In this type of watercolor painting, the paints are usually thinned with water and applied in washes. Bequest of Mrs. Alma H. Wadleigh. (Courtesy, Museum of Fine Arts, Boston)

Fig. 3.2. Winslow Homer. The Adirondack Guide, 1894.

Watercolor over graphite on paper. Paintings like this one were carried out with commercial watercolor paints, which were sold in pans or tubes. The paints were bound with natural plant gums, most commonly gum arabic. In this type of watercolor painting, the paints are usually thinned with water and applied in washes. Bequest of Mrs. Alma H. Wadleigh. (Courtesy, Museum of Fine Arts, Boston)

from a particular variety of the acacia, a shrublike tree of which there are hundreds of individual varieties that grow in many parts of the world. Acacia was one of the few trees that grew in ancient Egypt, and it is not a surprise that acacia gums were used as paint medium at that period. Gum arabic served as a major binder in European medieval manuscript illuminations and in traditional Indian manuscript paintings (Figure 3.3). Adding a little honey or sugar to a gum binder was recommended, probably for flexibility of the dried paint. Mixtures of more than one gum may have been used.

Among other gums that were used are the gums of fruit trees, such as peach, apricot, plum, cherry, and almond. All of these trees belong to the genus Prunus. Among examples of their applications are Central Asian cave paintings.

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