Introduction

Paints almost always consist of pigments, which provide the color, and an adhesive material that binds the pigment particles together and joins them to the substrate to which they are applied. Rarely, one material could serve as both pigment and binder. For example, Paleolithic cave paintings (Figure 3.1) at least in some cases were probably done with clay-containing earth pigments; the clay component, when moistened, would have given the earth pigments enough stickiness to adhere to the cave walls. But in most instances, even in the earliest times, the adhesive or binder was a separate material that had to be mixed with the pigments. Animal fat has been identified as a binder in other Paleolithic cave paintings.

Traditional binding materials are all natural substances produced by living things (plants or animals). Some require no processing, while others have to be extracted from their source by some means. In modern times, naturally occurring materials have been supplemented by compounds synthesized in the laboratory, such as the resins used to bind modern acrylic paints and latex house paints. In this chapter only the natural organic binders will be discussed; Chapter 2 discusses modern acrylic paints.

One method of classification of natural binders is by the types of organic compounds of which they consist, as shown in Table 3.1. Another way is to group them by their solubility in water, as in Table 3.2. Paints that contain water-soluble binders can be diluted with water. Some paints that contain water-soluble binders remain soluble in wa-

Black Bison Cave Painting

Fig. 3.1. Black bison of the "Salon Noir" in the Niaux cave.

Upper Magdalenian period (10,500 B.C.) and have been found in a number of caves in France and Spain. The people who did this painting used a limited range of pigments, probably for the most part, locally available natural earths. At least in some cases, fatty material (perhaps animal fat) was mixed with the colors to serve as a binder.

Fig. 3.1. Black bison of the "Salon Noir" in the Niaux cave.

Upper Magdalenian period (10,500 B.C.) and have been found in a number of caves in France and Spain. The people who did this painting used a limited range of pigments, probably for the most part, locally available natural earths. At least in some cases, fatty material (perhaps animal fat) was mixed with the colors to serve as a binder.

ter after they dry (such as modern tube watercolor paints, which contain gums). Others are quite insoluble after they dry (for example, egg yolk). The non-water-soluble binders are substances that cannot be put into solution in water, although many can be dissolved in organic solvents such as turpentine or mineral spirits. It is possible to mix various binders together, something that has often been done. Even some water-soluble binders can be mixed with water-insoluble ones to produce emulsions that have attractive handling properties or optical qualities that appeal to artists.

The solubility, application, and handling properties of the different binders is ultimately related to their general chemical composition, and thus the scheme of Table 3.1 will be used to group the binders.

Classification of Natural Binders by Composition

CARBOHYDRATE-CONTAINING BINDERS _I_

Binders that contain simple sugars

Honey

Binders that contain complex sugars (polysaccharides)

Plant gums

PROTEIN-CONTAINING BINDERS _L

Animal glue

Egg white

Drying oils consist of triglycerides

Egg yolk I

also contains lipids (oil)

OTHERS

Beeswax contains esters, hydrocarbons, and fatty acids

Casein I

if not purified, also contains sugars and lipids

Natural resins most contain complex molecules known as diterpenoids or triterpenoids

TABLE 3.2

Classification of Natural Binders by Solubility

WATER-SOLUBLE l

Occur in water solution May be dissolved in water 1 1

Egg white Egg yolk ■ 1

1 1 Honey Plant gums i i

Animal glue Casein i i

i i somewhat insoluble

i i soluble soluble

i i soluble in may or

soluble after

after after

hot water may not

after drying drying

drying drying

after be soluble

drying after drying

NOT SOLUBLE IN WATER l

Beeswax

Natural resins i

Drying oils i

soluble in organic solvents

i most are soluble in organic solvents;

i when fresh (liquid), may be

(such as turpentine); may

most become less soluble as they age

dissolved in organic solvents;

also be melted

insoluble thereafter

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Freehand Sketching An Introduction

Freehand Sketching An Introduction

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