Where weight and cost are major considerations, aluminum weighs about one third as much as bronze at about half the cost per pound. Large wall-mounted reliefs are often cast in aluminum since they require less structural support.
While it is softer than bronze, cast aluminum tends to be brittle. Thin sections could break under severe stress. It is also rather difficult to weld.
Paint is the usual surface treatment for aluminum castings, but highlights may be sanded and buffed to accent the surface texture. Anodizing can be done on some aluminum although cast aluminum is not the best for it. Outside, aluminum can form white oxides if not clear-coated for protection.
Another phenomenon to be aware of is electrolysis. Joined, dissimilar metals in the presence of an electrolyte such as salty water will act like a battery generating tiny amounts of electricity while slowly dissolving one of the two metals. For this reason stainless steel pins or bolts are often used to secure outside installations since stainless steel is particularly non-reactive.
Cast iron is even more economical than aluminum and may be the most historically correct in some applications. During the nineteenth century, finely detailed, intricate iron castings were as ubiquitous as plastic is today. Of late, iron has enjoyed a rebirth as a sculpture medium. It is superior to bronze in castibililty, since it runs further and picks up greater detail. In corrosion resistance it is a near equal. People often confuse iron and steel. Generally, steel flakes away as it rusts. Iron does not. However, iron does present design limitations. Like aluminum, it tends to be brittle in thin sections and does not weld especially well. Plan for mechanical joints and beefier section thickness especially if the piece may be subjected to any physical abuse.
The sculptor saves by thinking and planning ahead. Realize that the model may need to be cut into sections either for rubber molding or use as a pattern for direct sand molding. Facilitating this saves the foundry time and the artist money. To ensure the various processes continue smoothly, efficiently, and on time, the artist must provide the foundry with all necessary information about the final product at the beginning in the contract stage: wax inspection, metal alloy, base and mounting instructions, surface finish patination (refer to our color patina sheet), edition number, signature and positioning of work (where is the chin in relation to the big toe on the right foot? for example).