< FINISHING INTRODUCTION
FETTLING
When a recently poured cast has cooled off sufficiently, the founder can dismantle any surrounding FLASK or shuttering used to contain the mould during casting, and then remove easily broken off investment or sand from the cast. If a mechanical aid such as a JACK (BREAKING) HAMMER or hand chisel is used to assist in this KNOCKOUT, great care is taken to avoid cutting into and damaging the underlying cast; with some castings (esp iron), mishandling during the knockout can lead to a brittle fracture of the design.
Once the flask has been cleared from the foundry floor, the founder is left with a cast and attached metal running system covered with spent refractory. If the wax was CORED (to make the cast hollow), there will also be the remains of an internal refractory mass, complete with core vents and retaining pins projecting through cast’s metal surface. The process of removing these now superfluous attachments, and the initial working of the cast, is known as FETTLING.
The runners, risers and pouring cup are normally cut away first, though some may be temporarily left on for use as pick-up points for hooks, chains and vice grips. Any residual refractory in the vicinity of the cut is first removed, this prevents excessive dust exhaust. Most founders remove attachments by FRICTION CUTTING, (using an aluminium oxide cut off disc mounted on a portable angle grinder), though band saws and PLASMA CUTTERS may sometimes be used as an alternative. The attachment is cut off, leaving a short stub of metal close to the surface of the cast. This is done carefully, without damaging any part of the artist’s work. The remaining stub or SIGHTING permits fine finishing to be carried out a later, more convenient point.
If a core mass has been used to create a cast hollow, this material must now be completely removed. Core refractory (be it sand, plaster and grog, or ceramic), can be ‘cooked’ into a very hard, tenacious substance, so a complete removal of the core mass can be a very time consuming operation. Dislodging spent core from difficult to reach areas can be aided by working thin chisels and other dislodging tools through any core pin and patch holes already in the cast. WATER JET BLASTING may also be used to washout core debris.
It is important that all the core mass is removed from the interior of the cast to prevent a long term degeneration of the cast’s structure and minimise the weight of the finished sculpture. The hygroscopic potential of both PLASTER & GROG, and sand core refractory makes these materials especially prone to the absorbtion of moisture. Waterlogged core contents can over time expand to split the cast, or else react with the cast alloy to effectively rot the sculpture’s fabric from the inside out.
Any residual refractory still adhering to the cast after knocking-out is dislodged by either manual, mechanical or chemical means. Methods of fine refractory removal are diverse, and can include hand chiselling, wire brushing, pickling in a weak acid solutions, wet and dry shot blasting, ultrasonic washing, vibration and attrition by abrasive media – all of which can be used, either individually or in combination, to clean the cast.
The degree of difficulty encountered in removing residual mould material very much depends upon the type of refractory, how well it was fired, and the surface textures on the underlying cast. As a rule, CERAMIC SHELL investment easily breaks away from smooth surfaced casts, requiring little if any further treatment other than a gentle rub over with a fine abrasive cloth; this is possible because the cast contracts slightly on cooling, effectively breaking up the rigid ceramic coating which cannot gain a purchase on smooth cast surfaces. The same (ceramic) refractory will often adhere tenaciously to heavily textured or undercut cast surfaces, and probably require repeated treatments to fully dislodge the investment. In this instance, shell removal is made difficult and time consuming because the textured surface of the design acts as a key, into which the outer investment locks fast as the cast contracts.
Before progressing to fine finishing, the fettled cast may be given a light surface treatment such as a SHOT-BLASTING, this provides the founder with a clear view of the cast’s surface condition. Despite some artists’ concerns that shot or bead blasting removes significant amounts of surface detail to affect the quality of their work, the air pressure levels and blasting media used in art foundry workshops usually only removes surface material from the cast measurable in microns (1µm = 0.001mm). Unless seriously mishandled, a finished cast which has been shot-blasted should show no discernable reduction in surface quality when compared to a cast that has been prepared by other means. Exceptions to this general rule include the use of shot blast media on antique or restoration projects, or on very thin walled casts which might be inadvertently ‘holed’. Some flat designs (thin plaques, bas relief, and sheet metals, etc), can suffer a ‘stretching’ effect if heavily blasted, distorting them from a flat plane. As a rule though, shot and bead-blasting (glass bead peening) are perfectly legitimate and safe workshop procedures for use on new casts.
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