Rock art has been practised for tens of thousands of years and the techniques by which the paintings and engravings are created, while varied, conform to basic principles related to resources and technology. An equally important factor is where the image is placed. Not every rock surface is appropriate to inscribe, some may need preparing beforehand. While much research focus is generally given over to subject matter, dating and meaning in rock art studies, consideration of the techniques of production can provide some insightful observations into the methodologies by which images are created. Technically speaking, images can be broadly classified into two main groups: petroglyphs and pictographs. Petroglyphs are engraved or carved depictions on a rock surface; pictographs are painted images on a rock surface.
Pictographs - Painting
The pigments used in paintings are dependent on local resources, but typically have been made of minerals such as ochre that comes in a variety of red, brown and yellow hues. White is derived from silica, china clay and gypsum and black is obtained from manganese minerals, charcoal or specularite. Once collected these minerals were ground to form a powder and mixed with liquids such as egg albumen, urine, blood, saliva or water to make the pigment easier to apply and to act as a binding agent. Current research is exploring new methods to be able to date these ingredients. A variety of tools have been used to apply the pigment onto rock surfaces including birds’ feathers, animal hair, grasses or reeds and fingers. This process is not simply a technological one, and for many cultural groups the pigment itself and the method of application are imbued with symbolic meaning.
A very specific type of image is the handprint. These can be made in two ways, positively and negatively; by covering the hand in paint and applying it to the rock surface or by placing the hand on the rock and blowing paint through a tube or from the mouth over the hand to produce a stencil effect. Most images were made just in one colour, although examples of bichrome or polychrome are relatively frequent. Again, although very rare, there are some examples of figures combining engraving and painting.
Petroglyphs – Engraving
Three techniques were generally used to remove the rock surface; incising/engraving (lines are cut into the rock with a sharp tool); pecking (hammering and chipping away at the rock surface) and scraping. Rock faces were sometimes ‘prepared’ by scraping, allowing for an improved surface on which to inscribe the image. Incising or engraving involves scratching into the stone surface using a sharp lithic flake or metal blade. Pecking is a form of indirect percussion whereby a second rock is used like a chisel between the hammerstone and the rock face, while scraping is created using a hard hammerstone, which is battered against the rock surface. A last, much rarer method of engraving is the bas-relief technique, where the area around the image is lowered (usually by polishing) thus making the figure raise over the surface.
These methods of engraving are sometimes combined, as happens in the Sahara where the outline of pecked figures is polished afterwards. Although for the most part the engravings just depict the figures’ contour, on other occasions all or part of the image is infilled with pecking or polishing. In all these techniques, after differing periods of time the engraved surfaces weather and acquire the same darker patina as the original surface.