How Do You Draw by Scratching Off the Top Layer? Colour Underneath

Two or more layers of contrasting colour are applied one over another, and the image is made by removing part of the upper layer to reveal what lies below. On a wall this means plaster, with a dark coat applied first, a pale coat over it, and the design cut through the pale while it is still soft enough to carve and hard enough to hold an edge. The timing is the difficulty, since too soon and the plaster smears, too late and it chips. The result is a line of the lower colour with a physically incised edge, so the image reads as both a drawing and a relief, and it is part of the wall rather than applied to it.

The same principle turns up across several materials and traditions:

• •Building facades, especially across Italy, Bohemia, Bavaria and Austria
• •Pottery, with a coloured slip over a contrasting clay body
• •Byzantine and Islamic ceramics, where the method is centuries older
• •Modern children's art, using wax crayon under black paint
• •Printmaking, where a coated plate is scratched through
• •Contemporary murals, which have revived the wall technique deliberately

The reason this decoration survives on facades for five centuries while painted decoration does not is straightforward. The image is cut into the plaster rather than laid on top, so there is no paint layer to flake, fade or wash off, and weathering removes material evenly from a surface whose design is expressed in depth. The colours are mineral and integral to the plaster rather than pigments bound in a medium that degrades. Repairs are possible by replastering and recutting a section. The technique was therefore chosen for exterior work in climates where painted decoration would have needed constant renewal, and towns in Bohemia and Moravia hold entire squares decorated this way from the sixteenth century.

The same principle applied to ceramics has a longer history and a different character. A pot is coated with a slip, which is a liquid clay of contrasting colour, and the design is cut through that coating to reveal the body beneath before the piece is fired. Byzantine potters used the technique extensively from around the eleventh century, and it spread through the Islamic world, Italy and eventually across Europe, with regional traditions developing distinctive styles. The cutting must be done at the leather-hard stage, firm enough to hold a clean edge and soft enough to cut, which is a narrow window. Glaze applied over the whole surface afterwards makes the incised lines catch pooled glaze and appear darker, which sharpens the design further.

The method imposes limits that shape the imagery strongly. Fine gradation is impossible, since the technique produces one colour or the other with nothing in between, so designs rely on line, on hatching and on areas of solid contrast, which gives them a graphic quality resembling engraving more than painting. Repetition is efficient, because a pattern can be transferred with a pricked template and pounced through, so facades carry rows of identical panels. Figures are outlined and modelled with hatching. Architectural illusions are common, with plain walls carrying imitation stonework, columns and window surrounds cut into the surface. Where more than two layers are used, additional colours become available at additional depths, which a few elaborate examples exploit.

Layers of contrasting colour are applied one over another and the upper one is cut through while still workable, exposing the colour below as an incised line. Building facades across central Europe carry decoration made this way from the sixteenth century, and it survives because the image is cut into the plaster rather than laid on it. The method allows no gradation, so designs rely on line, hatching and solid contrast.