Chronology
When studying the evolution of medieval building sites, the stone processing traces left by former stonemasons on the visible stone surface are a useful source of information. A study conducted in Brabant has indeed shown that the stone cutting technique changed twice during the 15th century.
We refer to the three successive, yet partially overlapping stone carving techniques as phase I, phase II a and phase II b. The transition from phase I to phase II a occurred between 1400 and 1420; that from phase II a to phase II b between 1430 and 1450. This thus means that by locating the transitional lines or zones between two stone carving techniques in a 15th-century building, we can get a fairly clear idea of the state of the building site.
Carving
In medieval buildings, a carved stone surface often features a separately carved border. This border is not decorative, but has a technical function. When roughing out the stone surface, the first step was to locate and mark out the two opposite edges of the future rectangle in one plane with a ruler and to cut them straight with a chisel.As a next step, these two initial edges were connected with the chisel.
The final border dressed around the edge of the rectangle, is called the drafted margin. The same operation was then repeated for the adjacent surfaces, which were carved out perpendicular to the first surface. At that point, the middle area of the stone surfaces to be cut still protruded from the chisel-drafted margin. Surface irregularities could be flattened out with a stone axe, a polka hammer or a drove chisel. Chisel drafting a margin beforehand was important in order to prevent other tools from coming too close to the edge and from damaging the edge of the carved stone surface.
During the 15th century, the chisel gradually took over the role of the stone axe, the polka hammer and the drove chisel. This transition did not occur simultaneously for all types of stones.
Sometimes, the strokes that the stonemason traced across the stone with his chisel are clearly visible. These strokes are often quite curved, despite the generally regular aspect of the carving. The width of these strokes indicates the minimum width of the chisel used for giving the stone surface a boasted (or droved) finish.