The term "extruded bronze" and the product have become so widely used that the average architect may have come to think that in using it he is getting a form of bronze which is simply cheaper than the cast article. As a matter of actual chemical analysis, it is not bronze at all, but brass containing lead; it is heated in billets to a cherry read and forced by hydraulic pressure through a steel die in almost any profile imaginable. The content of extruded brass (more properly so termed from the alloy make-up) usually consists of from 54% to 57% copper, 2% to 2 1/2% lead and the remainder zinc. When the bronze foundries agree to match their cast work in color with extruded, they often find it more difficult to do than to promise; when they do succeed it is because the alloy used in the casting coincides with the extruded product, omitting the 7% or more of tin usual in bronze.
It is only fair to "extruded bronze," and to the architect and client as well, that the physical properties of the material be recognized and realized. since the alloy is not bronze it cannot be expected to weather like bronze, and if it tarnishes like old cast brass automobile trimmings or plumbing fittings, the reason is evident. The work may be expertly lacquered by a spraying device, and for exterior exposure such finish may last for two years, but if at the end of that time, or even before, the finish becomes a swarthy black either in spots or all over, the architect's reputation ought not to suffer because he thought he was getting something "just as good" as cast bronze. Ijt may be said in behalf of extruded sections that their exterior comers are sharper than could be obtained in cast work, that lines are apt to be perfectly true and without flaws, and that for certain work which must necessarily be erected economically, the product serves a useful purpose. Certainly it is apt to be superior to cast iron or copper for shop fronts with small-scale pilasters and like architectural features. It should never be used for column shafts, because there is no entasis possible, but if extruded work be necessary to come within a budget allowance, in place of columns there should be designed square piers, engaged piers or pilasters. The practice of building up pilasters, panels, entablatures, etc., of stock profiles, fitting them together and planting ornament on them like so many wood members, may serve its purpose, just as a Ford car may fulfill certain missions as well as a Lincoln automobile. The makers of "extruded bronze" do not pretend it is cast bronze, and neither should the bronze foundries or the architect. The client should have a right to know what it is he is paying for and what he may expect. If he is satisfied with a Ford chassis, body and top, embellished by Lincoln upholstery and radiator cap, all well and good.
The designer has certain limitations in designing work which is to be extruded. This steel die through which the heated billet is forced is 8 inches in diameter, consequently any profile which he wants to have in a single piece must fit within a circle 6 inches in diameter, 1 inch all around the outside being necessary for the strength of the die. Also, in solid sections where there are grooves these cannot be greater in depth thank their width, or the strength of the die will be too weakened. Profiles which transgress both of these limitations of the die can be overcome if the section can be extruded in more than one member. An advantage in extruded sections is that for many members it is possible to form dovetailed profiles which make an excellent fitting. Plain facias are not affected by the above die limitations, since they can be make up of sheets which come in 48-inch widths. These may be anything in thickness from 2 inches down. Tubing is much employed in door sections and varies from 12 to 18 gauge. The content for sheets and tubing generally 85 parts of copper to 15 of zinc, while if used with extruded members (in order to match the color of cast bronze), the copper is reduced to 60 and the zinc increased to 40%.
Extruded brass does not tarnish with the same rapidity as cast brass, probably because its density in being forced through the die is less porous than would be the case in a casting. One of the problems attendant upon extrusion is that as the metal comes from the die it writhes in a trough, and on cooling must be straightened by hand hammering after it has cooled. Unless this is most carefully done the edges and surfaces will be warped and untrue. After erection it would seem that the best method for caring for it would be to have it naturally finished without lacquer, and to stress the necessity of employing only intelligent help to clean it regularly with a dry cloth moistened with oil, the same as for cast bronze.