A corer is a wire instrument for removing the core from artist charcoal.
Charcoal is made by carbonizing tree branches or offcuts from tree trunks. Those made from branches have vessels running through the center that turn into a different type of charcoal to the outer portions when fired. This central part is called the core and because it produces different color shades to the outer portions and does not fix well to the surface, it could affect the artwork. Therefore, removing the core beforehand with a corer will help you draw without experiencing problems later.
The main types of corers available are those made of firm brass wire shaped into a spiral at the tip, and those made of two strips of thin wire twisted together into one with a small brush attached to the tip. Both types are around 20 cm long. Generally, the brass type is used with thicker charcoal and the brush type with thinner charcoal and for applying the finishing touches.
To remove the core from the charcoal, you start with the brass corer. Insert the pointed tip into the core of the charcoal and press it straight through to the other end slowly, without turning. Be sure to apply the appropriate amount of force because charcoal that is bent or has a hard core could break, or the corer could get stuck if too much force is applied. Next, thread the brush-type corer through the hole, inserting the wire end first and the brush end last, to remove more excess core. To finish, blow strongly into the hole and gently tap the charcoal on the floor or a hard surface with the hole facing down, to remove any remaining particles. When coring a thin piece of charcoal, the hole can be made by boring the wire end of the brush-type corer into the core and moving it back and forth several times to widen and remove the core. If the brush fits inside the hole, it can be threaded through for the finishing touches. (The brush could get stuck if too much force is applied.)
Check that the charcoal has a core, and if it has, check its diameter before using the corer. Charcoal made of chestnut and birch trunks has no core and can be used as is. If the corer gets stuck inside, break the charcoal in half to remove it. Corers should be kept in a case so that they do not become bent. Corers can be purchased at general art stores, etc.