Pitch

Pitch is a dark or dark-brown substance that is very dense and flows when heated or when crude oil, coal, or tree resin such as that from coniferous trees is distilled. It is a natural material that does not allow water to pass through it, and for this reason it has been used for centuries to seal the seams of ships, house roofs, and barrels to prevent water from entering.

The most notable characteristic of pitch is that its appearance is hard and brittle when touched, but if left for a long time it flows very slowly like a liquid. This is called viscoelastic behavior — a material that combines the properties of both solids and liquids.

Types and Origins

Pitch is divided into two main types. Plant-based pitch (pine pitch) is obtained from tree resin, which is heated until light oil (turpentine) is expelled from it. Mineral-based pitch includes bitumen and material produced from the distillation of crude oil and coal (coal tar pitch). The two types resemble each other in appearance but differ in their chemistry.

Historical Uses

Before the development of synthetic materials, pitch was the most important substance used to seal the seams of ships. This technique, called caulking, prevented water from entering vessels. It was also used to seal barrels of wine and oil, and to treat rope and cordage so it would not rot at sea.

Flow Experiments

The very slow flow of pitch has been demonstrated in university experiments lasting many years, in which pitch is placed in a funnel and drops are allowed to fall through. The time between one drop and the next can be many years, showing that pitch is an extremely slow-flowing liquid.