Organ pipes have been constructed from a wide range of materials; whilst some constructors have used stone, glass, paper and plastic, wood or metal is the usual choice.
Metal organ pipes are most often made from a mixture of tin and lead known as Pipe Metal. The percentage (proportion) of tin to lead varies according to the builder and the type of pipe, though generally the higher the proportion of lead, the darker the tone, and the higher the proportion of tin, the brighter the tone. Small amounts of copper or other elements are used to increase the stiffness of the metal. This is particularly important for high lead content pipes where the density combined with the relative softness of the metal would otherwise cause the pipes to deform (or, worse, collapse) under their own weight.
One of the main advantages of a tin/lead mixture for pipemaking is the ease of production combined with the stability of the end result. Pipemakers pour the molten pipe metal into a wheeled trough mounted on a ‘casting table’, a long stone or slate bed down which the trough can travel. The filled trough is rapidly wheeled the length of the table, leaving a thin, solidifying layer of pipe metal behind. The metal is then rolled up and used for pipemaking, either directly or after planing to reduce its thickness. Offcuts from the pipe making process and machining swarf from the planing process can easily be recycled by re-melting in the furnace. For heavier pipes, sometimes a layer of woven cloth (eg. linen) is laid on the casting table. When the metal is removed, it bears the witness pattern of the cloth on one surface. Such metal is sometimes referred to as being ‘off the cloth’.
A second advantage of pipe metal is the ease of fabricating pipes. It can be readily and repeatedly be cut by hand tools, formed into complex shapes and soft-soldered. Finally, the mouths can easily be subject to very fine adjustment by the voicer.
Pipe metal where the percentage lies between 40% tin / 60% lead and 60% tin / 40% lead is sometimes referred to as ‘Spotted Metal’, due to the characteristic appearance of round darker patches on the bright shiny surface of the metal. Metallurgically, Spotted Metal is known as a non-eutectic mixture, which means that as the molten metal drops in temperature, so the tin and lead freeze (solidify) at different times. Since the melting point of Lead is around 327 Celsius and the melting point of Tin around 290 Celsius, it is the lead which freezes first into the darker patches, and the tin second into the brighter surroundings. Spotted metal is frequently used for stops with more pronounced harmonic development.
Rolled zinc is often used for larger pipes. Zinc is cold rolled by the supplier into relatively stiff sheets, from which longer pipes with good rigidity can be rolled and soldered up. Frequently, the area around the mouth will still be made from pipe-metal to facilitate voicing and regulation.
Two other metals are used in theatre organs. The famous Wurlitzer Brass Trumpet and Saxophone resonators pipes were made from flared brass and present an imposing appearance in the chamber. Other builders copied this practice on prestigious classical instruments, for solo reeds. American builders, again notably Wurlitzers, also used a composite layered material called Hoyt’s Metal in which the outer layers are a high percentage of tin, as a substitute for higher tin content pipe metal.