Look at the piece above. what type of medium was used to create this piece? how were the different values achieved? what makes this medium challenging?

If this is the picture of the bird with the columns on the sides of the picture, then this is the answer.This piece is an example of silver point. The values in a silver point work are created by using a thin wire that is held by a cylinder-shaped holder. The artist must slowly build up scratches overtime, in order to created darker values. The greater the etching or scratching, the darker the value. This medium is challenging because it cannot be erased and it takes a long time