< AUTOGRAPH BRONZES
FINE FINISHING
Although some casts are finished by the artist away from the foundry, the vast majority are taken to completion ‘in house’. The fettled cast is passed on to a fine finisher, whose job it is to make good surface defects and any remove evidence of assembly attachments, including runner sightings and the residual holes left in the cast by removed core pins and vents. Fine finishing in art foundries is done by either a CHASER (small scale sculpture) or a CHASER-FITTER (larger scaled sculptures, which are often made up of several smaller sections).
Finishing procedures can vary from foundry to foundry, but before work commences the new cast is often given a cold PATINA treatment of dissolved POTASSIUM SULPHIDE (also known as LIVER OF SULPHUR) in solution. This compound deposits an even brown colouring on most copper casting alloys. Just as a wax is coloured with dye to help SIGHT a wax pattern, this preliminary brown patina allows the chaser to view fine detail across the cast’s surface. This initial patina does not adversely affect the eventual finish of the cast, and it is easily removed later if desired. The brown or PENNY BRONZE patina imparted by potassium sulphide also makes an excellent base for overlaying later patina applications, and is frequently left on the cast to later act as a GROUND.
If the artist’s original MASTER PATTERN is available to the chaser, the metal cast can be compared and checked for any discrepancies which may have arisen between wax working and fettling. This process of cross-checking ideally continues throughout fine finishing, emphasising the importance of [the artist] creating a good quality master pattern in the studio. The chaser, who may not have previously viewed the work, can then execute a true and accurate likeness of the original design.
Any sighting stub on the cast’s surface (from removed runners, risers, drains and other attachments), is blended back to continue the local surface detail across this disfigured area. The chaser uses a combination of mechanical and manual tools, including air driven die grinders, power chisels and ‘chisel like’ MATTING TOOLS which are fashioned from tempered tool grade or silver steels – all of these tools can be used to recreate surface textures on a cast. Many of the air and other power tools used for chasing have interchangeable cutting faces to accommodate variations in the cast’s surface texture.
Holes left in the metal cast from fettled core pins and support bars are also made good. This is usually done after the runner stubs are chased back – leaving any hole into the cast open as long as possible allows any residual core (dislodged by the impact of matting tools) to easily evacuate it’s interior. Traditionally, holes were plugged by DRILLING and TAPPING the aperture with a coarse THREAD, before selecting a suitable section of scrap runner which could be turned down and cut with a matching thread (using a DIE CUTTING tool). The ‘head’ of the threaded plug could be slotted, then screwed down into the tapped hole in the cast. The slotted end would be cut off just above the surface of the work, and the projecting stub matted down with hammer driven matting tools. In this way a metal section of exactly the same alloy as the cast could be used for plugging, ensuring an excellent ‘colour’ match. Any larger holes could be repaired by recessing their edges and forcing in a slightly oversize patch. The locked in patch could be further secured with screws, which were then set into the cast from the inside. These methods of MAKING GOOD are now rarely used in modern art foundries, having been largely superseded by gas and arc welding processes.
In addition to core pin holes and runner stubs, cast works occasionally show evidence of damage incurred as a result of casting faults. Though undesirable, and preferably avoided, casting faults are an intermittent fact of life for the art founder, who rarely, if ever enjoys the luxury of being able to make a speculative ‘test cast’ to fine tune the production process. Instead the founder has to rely on long experience to avoid excessive casting faults.
To what extent a faulty cast can be acceptably repaired is a rather subjective issue, dependent upon many factors including the complexity of the design, the ease by which the cast alloy can be worked and made good, the individual skills level of the chaser who has to repair the work and finally but not least, the opinion of the artist who’s work is affected. For certain key engineering components, where structural integrity is a critical issue, rejection rates for casts can sometimes exceed 90%. In contrast, the total rejection of an sculptural cast is a rather unusual event. Primarily this difference is due to continuous structural integrity being less of an issue in sculpture casts (this simply means that unplanned welds and patches are generally acceptable in an artwork). Normally a fine art cast will only be rejected out of hand when it is assessed to be more expensive to repair than recast.
The following pages briefly describes some of the typical casting faults that can arise, together with their usual causes and suggested remedies.
CASTING FAULTS 1 >
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