Bronze, copper and nickel silver

At a glance

From UK stockholder and mill datasheets and a US trade-body manual (NAAMM/NOMMA, 2006), not the EN standards: confirm against the current edition and the mill certificate. Colour words are the sources' own, not a match.

Sort the names first: what "bronze" means in a specification

Strictly, bronze is copper and tin and brass is copper and zinc. In architecture the words have drifted. The NAAMM/NOMMA metal finishes manual says the old distinction “no longer holds” and that architects and fabricators have for years called the principal architectural copper alloys “bronzes”, although of the five main alloys none contains tin and three are true brasses. The same manual groups the nickel silvers as “white bronze”. A specification that says only “bronze” can therefore mean a tin bronze, a brass, an aluminium alloy, a silicon alloy or a finish.

The drawing sorts the names you will meet by the one question that decides whether the word is literal: does the alloy contain tin?

Name confusion. “Antique bronze”, “bronze effect” and “oiled bronze” on a hardware or finish list are finishes (patinated, plated or PVD) and can sit on brass, steel or stainless. “Architectural bronze” is, in the stockholder glossary we read, a loose term for several brasses coloured like bronze. Ask for the alloy by EN number and the finish by name, as two separate lines.

The alloys, one by one

The chart adds copper content to the brass chart in our brass alloys sheet. Dashed bars are worked out by difference from the additions read, so they are estimates, not specification limits.

• Copper (CW004A). The stockholder sheet gives about 99.9 % copper, good to excellent corrosion resistance in most atmospheres including marine and industrial, and attack by ammonia-based solutions, oxidising acids, halogens and sulphides. It cold works readily. The NAAMM manual calls its natural colour salmon pink, while the stockholder describes it as yellowish gold: colour words are not a specification.
• Phosphor bronze (CW451K, also written CuSn5). Hard, strong copper-tin alloys with a little phosphorus: wrought grades such as this carry 4 to 8 % tin and cast grades 9 to 12 %, and leaded phosphor bronzes, mostly cast, up to 20 % lead (stockholder glossary). The datasheet gives it excellent cold working, poor hot working and good corrosion resistance, and lists gears, bushes and springs, so it is a mechanism alloy more than a surface one.
• Gunmetal. A casting bronze of about 10 % tin and 2 % zinc with excellent resistance to sea water and steam; the leaded version, with up to 7 % lead, is a plain-bearing metal (glossary). Some finish lists use “gunmetal” as a dark colour name, which is not the same thing.
• Aluminium bronze (CW307G). No tin: aluminium 8.5 to 11 %, with iron and nickel. The datasheet describes high corrosion resistance, particularly in marine conditions, immunity to chloride stress corrosion cracking and full weldability, for marine valves, pumps and fasteners. The glossary recommends it where an alpha brass would risk stress corrosion in ammonia-laden air.
• Silicon bronze (CW116C, CuSi3Mn1). Named here from its EN symbol, which gives about 3 % silicon and 1 % manganese. We read no datasheet for it, so the page gives no properties.
• Nickel silver (CW409J, CuNi18Zn20). Copper, nickel and zinc and no silver: “their copper content is similar to that of brasses but up to 20 % of the zinc is replaced by nickel” (glossary). Two mill sheets give 18 % nickel, 20 % zinc and about 62 % copper, very good cold forming, resistance to tarnishing and use for cutlery, tableware, keys and springs. One describes a silvery colour, both stress resistance to tarnishing, and a lead-free version is offered.

Colour: what the sources say and what they leave out

Copper alloys are the only architectural metals with integral colour, the NAAMM manual says, from the salmon pink of copper through the golden yellows of its alloys 385 (extruded architectural bronze), 280 (Muntz metal) and 230 (red brass) to the warm whites of the nickel silvers. Those are US CDA alloy numbers and 2006 wording. In the natural finish the manual calls the yellow alloys “yellow bronze”, the intermediate brown weathered state “statuary bronze” and the final weathered state “green bronze”. It adds that a green patina, once formed, is a protective film needing no maintenance.

What the datasheets leave out. The stockholder sheets for phosphor bronze and aluminium bronze give no colour at all, and the two sources disagree about copper. Neither sheet nor any source we read gives a measured colour for any alloy here. Specify by a physical sample in the alloy and finish.

Uniformity, in the manual’s words. A supplier delivers the alloy ordered but the items can come from different sources, which raises the chance of variation, and acceptable colour match is much better if all material is from the same source. The colour of bronze changes with time and changes differently with different exposures. Statuary bronze finishes are harder to produce in a given shade than any other standard finish. Standard hardware finishes from two sources compare reasonably only at arm’s length, about 0.6 m apart. Our sheets on patination, lacquered or unlacquered brass and copper and sample approval cover the ageing, the sealing and the approval record.

Choosing for a door, handrail, table or inlay

• A door or door furniture. Handled metal changes. A living copper alloy darkens and shows hands; a lacquer slows that but wears where hands land (see the lacquer sheet). No source we read compares these alloys for hardware wear, so ask the partner for the alloy, finish and a worn sample. Our high-touch finishes sheet covers hospitality use.
• A handrail. The finish is chosen for touch and ageing, the section and fixing for the rules in our guarding and handrail sheet. A bronze-coloured rail is often a brass (NAAMM's naming) or a coloured finish on another metal, so name which.
• A table or bar top. Copper is a living surface and acidic food can attack it (see zinc, pewter, copper and stainless bar tops). Nickel silver is pale and, by the mill sheets, tarnish resistant, but we found no food-contact or hospitality wear data for it.
• A mechanism, spring or bearing. Phosphor bronze, aluminium bronze and leaded gunmetal are the working alloys in the datasheets, not the decorative ones.
• A pale, silvery look. Nickel silver is a copper alloy and ages like one; stainless steel is a different family and does not (see types of steel). Neither datasheet we read compares their corrosion in a given room: see metal by environment.
• Contact with other metals. Our galvanic corrosion sheet reports aluminium against brass, bronze or copper as a pairing to avoid or isolate. For movement see thermal expansion of metals: copper about 17, phosphor bronze about 18 and nickel silver about 17 millionths per kelvin on the data read.

What to put in a specification

• The alloy by EN number and name, for example CW451K (CuSn5), never “bronze” alone.
• The form and its standard, as the datasheets cite them: EN 1652 for sheet and strip, EN 12163 for rod, EN 12167 for profiles, EN 12449 for tube and EN 1982 for castings. Editions to be confirmed.
• The temper where it decides forming or springiness.
• A lead statement if the part is handled or in contact with water or food; some bronzes are leaded and a lead-free nickel silver exists.
• The finish by name and sample, separately from the alloy, and whether it is a living finish.
• Source control: visible parts of one run from one source and batch where possible, and no change of alloy or source without written approval.
• A material certificate kept with the project file.

Metal: [alloy name and EN number, e.g. phosphor bronze CW451K (CuSn5)] in [sheet / bar / profile / casting] to [EN 1652 / EN 12163 / EN 12167 / EN 12449 / EN 1982, edition to be confirmed], temper [ ]. Lead content [not above __ %] per the certificate. Material certificate [EN 10204 type 3.1] per batch. Visible parts of one run from one source and batch where possible; no change of alloy or source without written approval.

Colour and finish as approved physical sample [ref] in [alloy], finish [name], viewed at [arm's length, about 0.6 m] in the project lighting. The finish is [a living finish that will darken and show handling / sealed with [ ]]; the client accepts that it will not stay as the sample. Alloy and finish are stated on separate lines of the schedule.

Appearance [bronze] by [patinated brass / PVD on stainless steel / solid aluminium bronze CW307G], as the partner confirms in writing with the alloy proposed and whether it contains tin. Not to be described as a bronze alloy unless that is what is supplied.

Standards and sources

• NAAMM/NOMMA Metal Finishes Manual, AMP 500-06 (2006), Chapter 2: US trade body; naming, natural colours, uniformity precautions.
• UK stockholder datasheets (CW004A, CW451K, CW307G) and glossary: alloy facts; not the EN standards.
• Mill datasheets for CuNi18Zn20 (CW409J): composition, tarnish resistance, uses.
• BS EN 1652:1998; BS EN 1982:2008: BSI Knowledge: 1652 current (plate, sheet, strip); 1982 (castings) withdrawn 10 Oct 2017, no replacement listed. Texts not read.

Questions we are asked

What is the difference between bronze, brass and copper?

Copper is the pure metal. Brass is copper and zinc. Bronze, strictly, is copper and tin, but architectural usage is loose: a trade manual says three of the five principal architectural alloys it covers are true brasses and none contains tin. Ask for the alloy by EN number.

Is nickel silver the same as stainless steel?

No. Nickel silver is a copper alloy of copper, nickel and zinc with no silver in it; one mill sheet gives about 62 % copper. Stainless steel is an iron alloy. Nickel silver is silvery white and, by mill sheets, tarnish resistant, but it is a copper alloy that changes as one, which stainless steel does not.

Is aluminium bronze a true bronze?

No. The stockholder glossary says it contains no tin: it is copper with 5 to 12 % aluminium, sometimes with iron, nickel, manganese or silicon. It is strong and corrosion resistant and is used for stressed parts in corrosive places.

Is gunmetal a colour or a metal?

Both. As an alloy it is a casting bronze of about 10 % tin and 2 % zinc. On finish lists it is a dark colour name that can be produced by plating, patination or PVD on other metals. Check which is meant.

Will bronze go green?

Copper alloys weather: the NAAMM manual describes yellow, then brown (statuary) and finally green stages, and says a natural patina needs no maintenance. How fast, and whether it goes green indoors, depends on the room and the hands, and no source we read gives a timescale for an interior. Sealing changes it: see the lacquer sheet.

Which alloy for a handle or a handrail?

We found no source that ranks these alloys for hardware wear. Decide the colour and how much ageing the client accepts, name the alloy and finish on separate lines, and ask the partner for the alloy, the finish and a worn sample.

Can I mix bronze, brass and copper parts in one scheme?

Only if the colours are approved side by side as finished samples. The NAAMM manual warns that items from different sources can vary and that a colour match is much more likely when all the material comes from the same source.