Why Brass: Material Properties for Optical Instrument Bodies
By Rocco (Roi) Ramon - licensed architect, founder of Studio Yabaye
The body of a kaleidoscope is a structural part. It holds a set of mirror strips at a fixed angle, keeps stray light out and carries the eyepiece and the object wheels. The material of the tube decides how well it does these jobs, and for how long.
Brass has been a standard material for instrument bodies since the 18th century. This article sets out what brass is, which of its properties matter for an instrument body, and how it compares with aluminium, steel, wood and plastic.
What brass is
Brass is an alloy of copper and zinc. The proportions vary, and the name covers a family of alloys, not one material. Three grades show the range:
Cartridge brass (UNS C26000, "70/30"): 68.5-71.5% copper, the remainder zinc. A single-phase alloy that bends, draws and forms well when cold.
Common brass ("63/37" or "64/36"): about 63-64% copper. A general-purpose grade for cold working.
Free-cutting brass (UNS C36000): 60-63% copper, 2.5-3.7% lead, the remainder zinc. The Copper Development Association notes that lead makes the chips break up readily under a cutting tool.
Across the family, density runs from about 8.4 to 8.7 g/cm3. Colour shifts with composition: more copper gives a more golden metal, more zinc a paler, more silvery one.
How brass became the instrument material
In the middle of the 18th century, small optical instruments were still built from mixed materials. One documented example is the pocket microscope of Benjamin Martin: the early form, dated to about 1760, combined lignum vitae, pasteboard, ray skin and brass, and an all-brass version is dated to about 1780.
Supply changed in the same period. William Champion patented a process for distilling metallic zinc on an industrial scale in 1738. Metallic zinc made it possible to produce high-zinc brasses that were difficult or impossible to make by the older cementation process, and these alloys were used in scientific instruments and clocks.
The kaleidoscope arrived into this tradition. Two instruments of about 1820 survive in museum collections. The Whipple Museum in Cambridge describes its kaleidoscope by Robert Brettell Bate as a tapering brass tube with a rotating collar that adjusts the angle between the mirrors. The Science Museum Group lists a Brewster's Patent Kaleidoscope by Philip Carpenter of Birmingham, black enamelled, with brass among its materials. For the wider story, see the history of the kaleidoscope.
The five properties that matter
Workability. Brass cuts, drills and threads cleanly, and the free-cutting grades were developed for exactly this. A tube end can be brought square and a cap can be fitted to it closely.
Dimensional stability. Brass does not absorb moisture, so its dimensions do not follow the humidity of the room. Its response to temperature is small and predictable, as shown below.
Corrosion behaviour. Brass contains no significant iron and does not rust. Its copper reacts with sulfur compounds in the air and forms a brown, eventually black, surface layer of copper sulfide. Brass has limits. It can lose zinc (dezincification) in water that is hot, high in chlorides or very soft, and it can crack under stress in the presence of ammonia. Neither condition is part of normal indoor use.
Density. At about 8.5 g/cm3, brass is roughly three times as dense as aluminium (2.7 g/cm3), and it is also denser than steel. More mass means more inertia, so a given small hand movement displaces a heavier tube less.
Surface. Brass accepts a brushed or polished finish directly on the metal. I offer three finishes: matte, dark and glossy. Brass also takes engraving with a clean edge, and I offer engraving on the brass body, up to 8 words.
Thermal expansion in plain terms
Every material grows when it warms and shrinks when it cools. The coefficient of linear thermal expansion states how much: micrometres of growth per metre of length per degree Celsius. The values below are in units of 10^-6 per degree C, followed by the growth of a 200 mm part over a 20 degree C rise.
Plate glass: 8.5-9, about 0.035 mm
Carbon steel: 10.8-12.5, about 0.045 mm
Brass: 18-21, about 0.076 mm
Aluminium: 23, about 0.092 mm
PVC, ABS, acrylic: 52-110, about 0.2-0.4 mm
The growth figures are my own arithmetic from the coefficients, using 19 for brass and 11.2 for steel, to give a sense of scale.
Two points follow. First, all the metals move by less than a tenth of a millimetre over a temperature swing larger than a room normally sees. Second, the mirrors are glass, and glass moves about half as much as brass, so the tube and the mirror system change length by slightly different amounts.
Four alternatives for one job
Aluminium. About one third the density of brass, easy to cut, and it does not rust. It expands about 20% more than brass and is less stiff: Young's modulus is about 69 GPa, against about 100-125 GPa for brass. It is often anodised. It is a sound material for an instrument body, and a lighter one. My range includes aluminium pieces as well as brass ones.
Steel. Stiffer than brass (about 200 GPa) and lower in expansion. Plain carbon steel rusts and needs plating or paint. Stainless steel resists corrosion and is generally harder to cut and thread.
Wood. Low density. Wood is not uniform: oak expands about 3 x 10^-6 per degree C along the grain and about 54 x 10^-6 across it. Wood also takes up and releases moisture, and its dimensions change with humidity.
Plastic. Low density and easy to form in quantity. Expansion is roughly three to six times that of brass, and many plastics deform slowly under a constant load.
Why the tube wall matters
The image in a kaleidoscope is built from repeated reflections, so a small error in the angle between two mirrors is repeated in every sector of the pattern. The mirror configurations are described in two-mirror vs three-mirror kaleidoscopes.
The tube wall stands between the hand and the mirror system. A wall that flexes when gripped passes that movement inward. A rigid, round wall gives the mirror system a fixed reference along the full length of the tube.
Resistance to flexing depends on the stiffness of the material and the thickness of the wall. In my studio the body is a brass tube, cut, fitted and hand-finished, and it carries front-surface mirror systems in several configurations.
What to check in a brass instrument body
A general checklist for any brass instrument:
Tube material is named as brass. "Brass finish" or "brass tone" describes a colour, not a material.
Tube does not flex under finger pressure.
Caps seat evenly, with no gap and no rocking.
Nothing rattles when the tube is tilted.
Engraving, if present, is sharp and even.
Surface darkening on uncoated brass is tarnish, not a defect. It can be left or polished back to bright metal.
FAQ
What is brass made of?
Brass is an alloy of copper and zinc. Common grades contain about 60-70% copper. Cartridge brass (UNS C26000) holds 68.5-71.5% copper with the remainder zinc. Free-cutting brass (UNS C36000) holds 60-63% copper and 2.5-3.7% lead, added so that the metal cuts cleanly. Density is about 8.4-8.7 g/cm3.
Why are scientific instruments made of brass?
Brass cuts and threads cleanly, does not rust, does not absorb moisture and accepts engraving. Supply improved after William Champion patented the industrial distillation of metallic zinc in 1738, which allowed high-zinc brasses to be produced. Benjamin Martin's pocket microscope of about 1760 still mixed wood, pasteboard and brass, and an all-brass version is dated to about 1780.
Does brass rust?
No. Rust is iron oxide, and brass contains no significant iron. Brass tarnishes: the copper in the alloy reacts with sulfur compounds in the air and forms a thin brown layer of copper sulfide that darkens toward black over time. The layer is on the surface and can be polished away.
How much does brass expand with temperature?
The coefficient of linear thermal expansion of brass is about 18-21 x 10^-6 per degree C. At 19 x 10^-6, a 200 mm brass part grows by about 0.076 mm over a 20 degree C rise. Aluminium (23 x 10^-6) expands about 20% more, carbon steel about 40% less, and common plastics roughly three to six times more.
Brass vs aluminium: which is better for a kaleidoscope body?
Both work. Brass has a density of about 8.5 g/cm3 against 2.7 g/cm3 for aluminium, so it has roughly three times the mass at identical dimensions. Brass is also stiffer and expands slightly less with temperature, and its bare surface tarnishes. Aluminium is lighter and is often anodised. The choice is mainly between a heavier instrument and a lighter one.
Why does the tube matter for the mirrors?
The image is built from repeated reflections, so a small error in the angle between the mirrors is repeated in every sector of the pattern. The tube is the fixed reference for the mirror system. A wall that flexes or dents moves that reference. A rigid, round tube keeps the mirror system in the position set at assembly.
About Studio Yabaye
Studio Yabaye builds kaleidoscopes and teleidoscopes with front-surface mirror systems in a brass tube, cut, fitted and hand-finished in the studio. The studio builds several mirror configurations, including two-mirror and three-mirror systems, and the range also includes aluminium pieces. Kaleidoscope lengths range from 9 to 28 cm, in matte, dark and glossy finishes. Each object wheel is poured and composed by hand, so no two pieces are alike. The studio is based in Israel and ships worldwide. The current range is at all products, and questions can be sent through the contact page.
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