However, most modern "lead pencils" have a nonpoisonous core of greyish-black graphite mixed with various proportions of clay for consistency, enclosed within an outer wooden casing to protect the fragile graphite from being snapped apart or from leaving marks on the user's hand. White chalk has been traditionally used in schoolrooms to write on a main blackboard at the front of the room. In the 19th century, and indeed well into the 20th century, when paper was less readily available, individual students also wrote with chalk on their own small slates. Both pencils and chalk exist in variants which can create marks in other colors, but colored pencils and colored chalk are generally considered to be art supplies rather than writing instruments. Similarly, although very young children may use colorful wax crayons to write words into their pictures, writing is not considered to be the primary use of crayons. A wax pencil resembles both a crayon and a pencil in that it contains a brightly colored wax core within a protective paper casing, but its proportions are closer to that of a standard pencil. Wax pencils are primarily used to write onto nonporous surfaces such as porcelain or glass. Normal pencils, chalk, and crayons all share the characteristic that they cannot "run out". The useful life of these implements is closely linked to their physical existence. However, specialized accessories such as pencil sharpeners may be required to reshape the working end of the pigment core or to remove the outer casing from around the tip. These require the presence of an added pigment in order to write, and are useless when "empty". The pen is the most common form of writing implement. It has a hard tip which applies ink to a surface. Initially, pens were made by slicing a suitable nib point from the end of a thin, hollow natural material which could retain a small reservoir of ink by capillary action. However, these ink reservoirs were relatively small, requiring the pen to be periodically dipped back into an external inkwell for replenishing. Reed pens were used by the ancient Egyptians to write on papyrus. Quill pens were standard in Europe and the United States up through the 18th and 19th centuries, and are still used in various contexts, such as calligraphy and formal settings such as major bank transactions. The most common quills were taken from the wings of geese or ravens, although the feathers of swans and peacocks were sometimes favored for prestige. A dip pen has a steel nib (the pen proper) and a pen-holder.