COPPER/ZINC (BRASSES SCB, DCB, PCB, HTB, CZ SERIES, 60/40, 70/30, ISO CuZn SERIES & NICKEL SILVER)
The copper/zinc group of alloys contains the largest number of variants of all the copper based materials. This group includes alloys formulated specifically for sand casting (SCB), die casting (DCB), pressure die casting (PCB); as well as high tensile (HTB) versions and at least thirty six wrought (CZ) alloys.
Copper/zinc alloys are usually divided into one of two broad classes. The first class ALPHA BRASSES, are copper rich alloys containing no more than about 30% zinc. Alpha brasses are more commonly referred to as ‘70/30’ brasses (of which the sand casting grade SCB2 [US similar C85400] and the wrought alloy CZ106 [ISOCuZn30] are typical). Alpha brasses are noted for their DUCTILITY, especially useful for COLD FORMING processes (deep drawing, spinning and other machining operations). 70/30 brasses include some of the most important architectural and decorative grades.
Alloys with a higher zinc content (in excess of 37%), are referred to as the ALPHA-BETA, or BETA type brasses, variable according to the precise quantitiy of zinc. A more common term for these beta class brasses is ‘60/40’ alloy, of which the die casting alloy DCB1 (US, SAE 41/C85500) and wrought alloy CZ109 (ISO CuZn40) are typical. The beta class brasses generally have a higher tensile strength than alpha brasses and are more easily HOT WORKED (forged), usually becoming quite plastic when heated.
The common usage of brass as a decorative and architectural design/detail material has encouraged an extensive range of ‘off the shelf’ wrought sections, extrusions and fastenings. These can be incorporated into fabricated art and design works using basic non-ferrous workshop techniques (a feature shared with many aluminium alloys).
Good general purpose wrought brass alloys (CZ series alloys) for fabrication are found in the BS 2870/75 group which includes CZ108 (basic brass [ISO CuZn37]), and CZ109 (Muntz metal [ISO CuZn40]). CZ101 (gilding metal [ISO CuZn10]) is used for the creation of jewellery based designs and as the name suggests, this alloy is especially well suited to GILDING with metal foil. CZ114 (ISO CuZn39/AlFeMn) and CZ115 (ISO CuZn28AlFeMn), are both high tensile brasses (containing iron, aluminium and manganese additions), these alloys may be used for decorative/structural applications.
Preformed brass sections are generally available at a lower cost and in a greater variety of forms than similar sections in other copper alloys. This encourages the cost effective use of this material for fabricating plinths, frames and other ancillary items. By carefully selecting an appropriate PATINA finish (heavy greens and blacks are ideal), it is possible to combine a fabricated brass feature with a cast gunmetal or silicon bronze design, without unduly drawing attention to any difference in the alloy’s underlying colour.
Having approximately one third the conductivity of high purity deoxidised coppers (see COPPER), brasses are rather easier to weld in terms of heat input – though one particular difficulty associated with zinc rich alloys is their tendency to loose quantities of this light metallic element during casting and welding operations. Zinc loss is due to the metal’s relatively low boiling point (bp 911°C), which is significantly lower than the melting point of the parent metal (copper mp 1085°C), therefore zinc vapourises before the copper base melts. For this reason alone many art founders avoid casting brass alloys (melting can evolve significant quantities of metal fume). Grades containing small additions of aluminium or silicon (sometimes added by the founder themselves to the charge), are more resistant to zinc loss effect (DEZINCIFICATION), than alternative alloy versions.
The welding of brass alloys, is usually done with a TIG process using either a C14 or C15 filler rod (phosphor bronze C10 & 11 will also suffice); alternatively gas welding with an oxidising flame and silicon bearing filler rod (to minimise dezincification) is practiced in some workshops. The wrought brass grades CZ 101/2/3 [ISO CuZn10/15/20] are among the easiest to weld. Brass filler rod is also used for a variety of non-fusion joining processes, in particular for brazing cast iron sections or when uniting elements constructed from dissimilar metals.
Most brass alloys are readily identifiable by their yellow tint (though tone will vary according to composition). Cast and wrought designs are usually finished to a mirror polish, this is especially common for architectural and decorative features. Alternatively, many brasses will take a patina, though the ability to accept a chemical colouring depends as much upon the quality of the alloy and the surface condition of the cast as it does the upon the type of chemical used. With a few notable exceptions, cold patinations often perform poorly on brass, especially if applied to a smooth or polished metal surface. Many brasses that initially resist a patination can be made more receptive by first shot blasting the surface of the cast, then applying a hot (torch) COPPER NITRATE solution. This can act as a ground for further treatments with subsequent colouring compounds.
Although the copper/zinc based alloys are highly regarded and extensively used for producing decorative and architectural metalworks, the material is considered by many (rightly or wrongly), to be inappropriate as an alloy for fine art sculpture making. Whilst the alloy has clear merits in it’s own right, it is not recommended that brass be specified simply on the basis that it may be less expensive than one of the more usual art foundry alloys. Any saving in metal ingot cost is irrelevant if the art founder then surcharges for the use of a non-standard alloy, or if the finished art work is inherently devalued.
Portsmouth Cathederal West Doors.
Brian Kneale RA (1997).
Forged leaded brass & phosphor bronze.
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