The Pipes of the Organ: Diapasons
Written by Aiden Scanlon, Organ Builder & Organist of St Ann’s, Dawson St
Introduction
This is the first of a series of articles on the pipes which form the stops of an organ. In this, pipe composition and open diapasons or principals are described. In further articles, other types of pipe will be considered.
Pipe composition
The vast majority of organ pipes are made of wood or from metal. In general, metal pipes consist of various alloys of lead and tin although the proportions of each metal can vary considerably.
Common alloys are 75% tin/25% lead, 50% tin/50% lead (commonly called 'spotted metal' since it gives a sort of dappled appearance), 25% tin/75% lead (commonly called plain metal). In older Dutch and German instruments pipes of 3% tin/97% lead can be found to this day. Before being formed into pipes, the metal must be hammered to work-harden so the pipes will not collapse under their own weight. Since circa 1864 zinc has come into general use for front pipes and large basses. Zinc is very strong relative to its weight. A drawback when it comes to working on zinc pipes is that it is tough and brittle.
A mix of 99% tin and 1% lead (very rare and only found in Germany, Holland and France) is used for front case pipes on account of its ability to remain untarnished. It also lends itself well to being cast in very thin sheets, for making gamba, dulciana and salicional type stops of great delicacy. There are only a handful of organs in Ireland containing pipes of such a mixture.
Wood such as pine or deal is commonly used for large pedal stops. Mahogany, oak and other woods (maple, fine fruit woods) are sometimes used for stops shorter than 4’.
Types of Pipe
Pipe families are divided into two fundamental types:
flues (like tin whistles) and
reeds (where a vibrating tongue provides the initial sound which in turn is amplified by the rest of the pipe).
Flue pipes have bodies that can be cylindrical or conical (where the top of the pipe may be 2/3, 1/2, 1/3 or 1/4 of the diameter at the mouth). Occasionally flue pipes are wider at the top than at the mouth. Wood pipes may be four-sided, three-sided, cylindrical (turned on a lathe) or four-sided but tapered, although the latter three types are rare. Another oddity is pipes (either wood or metal) with two mouths.
(See a series of different types of pipes here:
http://commons.wikimedia.org/wiki/File:Organ_pipe_types.png).
Pipes may be
open (at the top) or
stopped, this being done internally by a turned wooden handle sealed with leather, or externally using a metal felted cap. A stopped pipe is approximately half the length of an open pipe of the same pitch.
Open Diapasons
The tone of diapasons is specific to the organ, in that it does not mimic any other instrument. The word diapason, literally translated means
'a concord through all tones'. The tone embodies a unique combination of pure fundamental with an even spread of upper partials (hence the reference to 'all tones').
The Open Diapason consists of a set of open cylindrical pipes. On the manuals these sound unison (or 8’ pitch) and on the pedals at 16’ pitch. The pipes may be of either wood or metal, or both. On organs standing on rood screens (such as in many cathedrals and some large churches), there are two diapasons, one on each façade facing away from each other (one East and one West). This trend continued (but for different reasons) up till the mid 20th century where organs had several diapasons (e.g. St Patrick’s Cathedral, Dublin).
The
Open Diapason is the organ’s foundation stop, and on which a chorus of diapasons at different pitches sit: 4’, 22/3’, 2’, 13/5’, 11/3’, 1’ and even higher. Whilst all of these pitches may be found as separate stops on some organs, it is usual to find some (two or more) gathered together and brought into use in a compound stop (e.g. a
Mixture or
Sesquialtera), drawn by a single stop knob.
On larger organs the manual diapason chorus may be based on 16’ pitch, giving more gravitas, and possibly the pedal at 32’. The topic of diapason (or principal) choruses and particularly of how different organ builders treat them, is really the subject of a much more complex and intriguing concept: namely the tonal structure of the organ. The constituent stops is akin to …. say the ingredients for a Christmas cake! You can give six different people the same recipe, and end up with six very different cakes.
The 4’ diapason or
'Octave' is often called the
Principal. In Germany a diapason at any pitch is entitled principal. In Ireland (and UK) only the manual 4’ pitch and pedal 8’ is so named. This is a hangover from the mediaeval organ.
From 1851 the German builder Edmund Schulze built organs in Yorkshire and since then, we have diapasons called
Geigen or
Violon Diapasons. Later in the century the quest for more power and smoothness continued. It was Arthur Harrison or Robert Hope Jones who discovered that by putting a thin covering of leather on the upper lip of the pipes, they could be blown harder without misbehaving. This was applied to 8’ stops only.
I’ll end by posing a question –
'when is a diapason not a diapason?' — to which the answer is:
when it’s a stopped diapason, which leads us to the next article about flutes, open and stopped.