Silverpoint is a drawing technique which predates the simple graphite pencil, having become common around 1400 in Renaissance Italy. An early Renaissance painter would draw his subject in charcoal, on a prepared surface, then use silverpoint to establish a precise drawing.
Drawings are made using a pointed metal rod - originally of silver, but can also be a more cost-effective alloy of lead and tin or other metals - to draw marks on paper coated with white pigment. As the rod draws on the surface it leaves minute particles of metal embedded in it. This produces a pale line which becomes progressively darker in time as the particles tarnish. Different metals tarnish differently, and so produce subtle differences in colour in the finished product.
The method as practiced in the fifteenth century does have several disadvantages, and as it has become disused over time few resources are available to inform a successful technique. Traditionally, disadvantages include that there is little latitude in the range of marks which can be produced; the time and skill needed to prepare the special ground; and the difficulty of making corrections. These can largely be overcome by modern methods.