Copper has been used for making organ pipes since the eleventh century, but until the tracker revival it was something of a rarity as a pipe metal. Even the encyclopedic Art of Organ-Building by George Ashdown Audsley does not mention it except for a historic reference.
The modern use of copper for organ pipes began in the 1930s when Marcussen and Holtkamp and others used it to replace the zinc which had been common for larger pipes for about a century. By the late 1950s, the use of copper for pipes was widespread in new tracker organs. Entire facades of copper pipes were installed, and even entire ranks of principals as well as many ranks of reeds and flutes were made in copper.
By the late 1970s, the use of copper had declined from its earlier peak, and was mostly confined to larger bass pipes and some reed resonators. Copper pipes became a useful substitute for the large, expensive tin facade pipes, as well as a replacement for zinc.
Organbuilders do not cast or roll the copper themselves, but purchase it in a variety of thicknesses from rolling mills. A pipe metal with a wide variety of surface appearance, copper can be polished to a wonderfully warm glow and brilliance which is usually protected by a clear lacquer or other transparent coating. It may be flamed to a wide range of colors (blue, brown, gold) using various heat treating sources, and it may be etched with acids and oils to form a range of colors and surface textures from glossy to flat. It can also be used "as is" from the rolling mill. Copper does not present the visible crystal growth seen in old zinc pipes, even though it is completely crystallized.
Copper is a long lasting material with good strength and integrity. It requires a lot of heat to solder since it has the second highest thermal conductivity of any metal.
The tonal characteristics of copper are a little hard to define, and every builder will have a different opinion on the uses and the sound of the metal. In reed pipes, copper seems to add reinforcement to the upper partials, producing a brilliance in the sound of some ranks, and in others a harshness or a brittleness. It may also produce a singing sound, or a quaint buzz or rasp. Copper is a harder material than the tin/lead alloys used for the majority of organ pipes, and this can make for a somewhat "harder" sound as well. But there are many factors that influence the sound of organ pipes; and the builder's scaling, type of metal, wall thickness, voicing, chest action and arrangement of the organ all contribute to the final effect. Copper is another metal available to the builder, which can be used when its particular sound and appearance fit into the design of the organ.
As with zinc and aluminum, copper is preferred in alloys of rather high purity. The high purity produces a soft metal which makes for an easier tonal transition to the very soft tin/lead alloys. Where strength is needed for large or mitred pipes, a small percentage of other metals is alloyed, as with zinc, so that the finished pipes will retain their shape.
Recent changes in the prices and availability of tin, lead, copper, zinc and aluminum seem to point to future changes in the usage ratios of these metals for organ pipes.