Way back before the pencil was invented, artists wanting to put down an elaborate drawing before commencing a portrait used something called a metal point. This was a very common way of creating a delicate mark on prepared grounds like gesso.
The metal point simply consisted of a piece of gold, silver or lead which was fashioned with a small tip allowing it to make a mark, similar to a hard pencil. A few different metals were used for this but the most commonly used were gold, silver or lead. Gold makes a yellowish brown line, lead a dull grey one, but in all likelihood the most favoured metal was silver. Silver produces a finely rendered line that starts off grey, though over time, turning a brownish black due to oxidisation. Metal point was used particularly for creating an underdrawing for portraits and other compositions that required great detail and, because of it’s faint and delicate line, it was easily obscured by the subsequent layers of paint, even quite thin layers and glazes. Leonardo da Vinci employed silver point to great effect when he created “The Condottiere” (British Museum, London), a very detailed drawing with a surprising amount of light and shade, given the medium being used.
It is possible to purchase metal points commercially, but it is very simple to replicate one yourself. All you need is a thin piece of silver wire and a clutch pencil, instead of using the pencil leads, use the silver wire. The tip of the silver needs to be relatively fine without actually being sharp, as this may scratch the surface rather than lay down a line. Before putting point to paper, it is generally best to rub the tip with an emery board to remove sharp edges or roughness from the tip. The another benefit for the artist using a metal point, is that it’ll not need to be reshaped or sharpened for a long time, as the tip suffers very little wear.