< COPPER & COPPER ALLOYS
GUNMETALS (G & LG SERIES, 85/5/5/5 ALLOY)
Leaded gunmetals are typically made up of 85% copper, with the balance of the alloy made up of of tin, zinc and lead in more or less even proportions (hence the 85/5/5/5 reference). Gunmetals are excellent general purpose casting alloys, LG2 is the preferred version for commercial sand founding, LG3 for art founding. LG3 is composed of 7% tin, 4% zinc and 2% lead the balance being the copper parent metal and trace elements (nearest US equivalents: 844 & 836/C83500 & C83600 alloys).
The leaded alloy is preferred to unleaded grades (‘G’ series alloys), this is due to the lead bearing material’s superior machining qualities. For art foundry use, leaded gunmetal presents itself as a relatively SOFT alloy and this quality allows easy manipulation of the cast’s surfaces whilst chasing and finishing (some founders even add in extra quantities of lead to the charge in the furnace). The addition of lead does mean that gunmetal does not weld as well as many of the other copper alloys and the ‘as cast’ surface can be rather dull in appearance. A good polish can be achieved on gunmetal, but sculptures which are to be very highly polished may be better formed from a BRONZE (copper/tin) alloy, PHOSPHOR BRONZE, or SILICON BRONZE; all of which encourage a brighter finish, though even these alloys are not without disadvantages (see below).
Though it’s lead component slightly reduces the weldability of leaded gunmetal (lead can increase the incidence of gas porosity in the weld), the material is still quite straightforward to fusion join. Phosphor copper filler rod (type C10 or C11, or filler compatible with C54400 alloy), flows well and has a slight degassing action, this can help minimise weld porosity. A small quantity of phosphor copper may also sometimes be added into a melting gunmetal charge, this helps improve flow and reduce the absorption of gases (note: silicon bronze should not be added into a gumetal pot).
Using phosphor copper filler wire produces a welded joint of considerable strength in gunmetals, though the quality of a newly deposited weld run can be further improved by lightly hammering (PEENING) the weld’s capping (top) run whilst still warm. There may be a slight (pinkish) colour variation in the weld in comparison to the surrounding parent gunmetal, any obvious difference is usually negligible after the application of a patina to the cast.
The welding of gunmetal casts is normally carried out using a TIG or oxy-gas process, though flux coated electrodes and wire fillers are available for MMA and MIG processes. A very limited range of preformed wrought sections are available in gunmetals from specialist stockholders, usually in LG2 & NES 830 grades. Wrought gunmetal (in common with many other wrought copper alloys), is normally sold in a variety of imperial sizes, these are often quoted for by weight rather than stock length; for example: 1inch (25.4mm) round bar @ £/$ per lb/ft (or per kg/m).
Both cast and wrought sculptures formed in gunmetals readily accept standard foundry patinations, this material is also suitable for surface finishing by other decorative processes, including gilding, electroplating, painting and so on.
COPPER SILICON (SILICON BRONZE) >
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