The pupils of Room 2, at Woodlands School, asks :-

What is ink made of?

Jim Johnston, a chemist at Victoria University of Wellington and who works to reduce the environmental aspects currently associated with the de-inking part of paper recycling, responded.

There are many different types of inks in use today. These are needed because of the many different types of printing methods and papers used. However, all inks essentially comprise a liquid carrier, often referred to in the printing and paper industries as the vehicle or carrier, and a pigment which provides the colour.

For the black printing ink used in newspapers the carrier is generally a mixture of a lightweight oil and the black pigment is finely powdered carbon. After the ink is transferred to the paper it dries as a result of the oil being absorbed into the paper and also by some chemical setting of the ink. This type of oil is often referred to as a drying oil. For colour printing the carbon is replaced by appropriate coloured pigments, which may be coloured metal oxides (eg. iron oxide for red; zinc chromate for yellow), inorganic compounds (eg. prussian blue for blue), or complex organic molecules known as dyes (eg. phthalocyanin green for green).

For inks used for higher quality printing applications such as glossy magazines and books, the carrier or vehicle is a polymer material rather like glue which dries by chemical action, rather than by absorption into the paper. When this ink dries the pigment is concentrated at the surface of the paper and provides a sharper and more glossy print.

The ink present in felt-tip pens is usually a mixture of organic dyes, a vehicle which contains both a volatile component which evaporates upon exposure to air (you can smell this) and a polymer material which dries and sets the ink onto the surface.

The ink used in biro pens is similar to that used in felt-tip pens but dries by more of a chemical mechanism rather than by evaporation. (The ink of biro pens do not evolve significant amounts of organic solvent vapours).

Black photocopy and laser printer ink is different again. It comprises fine particles of carbon which are encapsulated with a plastic material. After the image and ink is transferred onto the paper, using the photocopying technology, the paper is heated and the plastic melts and binds the carbon particles together and onto the paper surface.