< INTRODUCTION TO SURFACE FINISHING
POLISHING
Polishing imparts a refined LUSTRE to metallic surfaces, up to and including, a highly reflective MIRROR effect. A high mirror polish is achieved by either progressively working the cast with an increasingly fine abrasive material, or alternatively, through an electrochemical process (see ELECTROPOLISHING). By arresting the polishing process before a full mirror effect is reached, intermediate levels of finish can be obtained. Intermediate finishes can range from a rough SCURFED or BRUSHED appearance (as evident in David Smith’s notable ‘Cubi’ series of stainless steel sculptures of the early 1960’s), through to a more refined SATIN (often pronounced ‘satine’) or semi-polished effect. In all cases the natural colour of the parent metal has some considerable bearing on the visual impact of the work, so the selection of an appropriate alloy for the cast or fabrication is an issue which should not be overlooked (see METALS).
After being KNOCKED OUT of it’s refractory mould, the cast is CHASED in the usual fashion, making good any evidence of core pins and reuniting any separately cast sections. Cast artworks for polishing should be jointed and welded as little as possible, this minimises any discrepancies between the cast and filler alloy colourings, in some cases off-cut runners are retained and used to effect plug repairs in order to maintain a close colour match.
Simple, expansive cast surfaces may be initially machined with the aid of a rotary mounted grit impregnated disc, this is known as LINISHING. Linishing discs are usually graded within the range of a 30 (very coarse), to 240 (very fine) ALUMINIUM OXIDE grit. ZIRCON grits are preferred for working stainless steels and other hard materials. Applying TALLOW to the disc’s surface helps reduce wear and minimise clogging, especially when working soft metals such as aluminium – though care must be taken not to accidentally direct the exhaust on to any nearby work, as settled tallow will act as a resist to any later applied patination compound.
Residual tooling marks on the cast’s surface from initial machining are evened out using cylindrical FLAP WHEELS, sometimes also called LEAF MOPS, which are mounted on a straight DIE GRINDER tool. Like rotary linishing discs, flap wheels are impregnated with a range of abrasive grit grades. Even at this early stage, great care is taken to avoid any scoring of the cast’s surface, as the complete removal of blemishes can be extremely time consuming. Before fine polishing commences, the cast is carefully examined for pin holes or other minor defects, these may later become more prominent as the surface is gradually refined. Any noted defects are filled with weld and made good, though RE-TOUCHING is normally an ongoing process as further polishing reveals previously innocuous faults.
It is essential that the preliminary machining process evens out any undesirable deviations in the cast form. The process of fine polishing removes little in the way of metallic material, but does acutely reveal uneven surface undulations. For this reason alone, any master pattern constructed by the artist for a polished cast must itself be finished to a very high standard – minute defects present in the pattern’s surfaces or forms will inevitably be transferred directly to the metal cast. Any surface deficiencies in the cast will likely as not require a considerable amount of working to effect their removal.
After the basic surface preparation process is complete, fine polishing of the work’s surface can commence. Fine polishing is carried out the aid of a flexible fabric disc, usually constructed from CALICO, LINEN or else SISEL (used for coarse and satin finishes). The cloth disc is impregnated with an abrasive ALUMINA or IRON OXIDE grit, variously referred to as a polisher’s COMPOUND, SOAP or ROUGE.
For initial coarse workings, the operator uses CUTTING COMPOUND – an impregnated abrasive media containing PUMICE STONE or hard mineral. The cloth is securely attached to a high speed rotary tool via a POLISHING MANDREL, and the impregnated cloth worked over the surface of the cast. As the work progresses to finer grits, a fresh cloth is used for each grade of soap, thus avoiding cross contamination of abrasive media. Polishing continues in this manner until an agreeable degree of reflective lustre on the cast metal alloy is obtained. Any residual grease left on the surface of the cast by the applied rouge can be removed by dusting with a fine LIME POWDER, before wiping off with a soft duster. The use of white or methylated spirits is avoided because these solvents can leave a greasy deposit over the cast’s surface. A polish effect can also be achieved by the use of fine files, or a spoon like BURNISHING tool, both of which are manually worked over the cast’s surface.
Polishing tends to ‘close up’ a metal’s surface, effectively improving the resistance of the cast metal to corrosive action – not always an advantage, as selected patina and other finish effects applied to a polished surface can suffer from poor adhesion. Cast designs and sculptures intended for certain chemical patinations, especially those evolving a heavy oxide deposit, are often worked to a low level satin finish with SILCIONE CARBIDE wet and dry, or pumice pastes of various grit sizes, as this preparation provides a light key for the patina to adhere to. Alternatively, the cast may undergo surface conditioning by BEAD BLASTING, buffing with a soft brass brush or stainless steel/copper wire wool, and so on. The semi-polish effect produced by these various means will enhance the reflective quality of the metal, and can significantly improve an overlaid patina deposit. Pre-polishing for patination is particularly effective when the applied chemical solution deposits little more than a light translucent film across the cast’s surfaces (see PATINATION and SUPPLIMENT information).
The finest polished works are produced from master patterns that have been carefully designed with this finish effect in mind. Master patterns featuring simple, regular and smooth forms and surfaces are usually considered the most suitable for polishing. Forms displaying complex, heavily textured surfaces, particularly those exhibiting undercutting and deep creases are notoriously difficult, time consuming and subsequently expensive to bring to a mirror polish. Whilst complicated polishing operations are possible, these jobs may be charged at a rate several times that for taking the same work to a patinated finish. Isamu Noguchi (1904-1988), a former assistant to Constantin Brancusi (1876-1957) during the late 1920’s, provides an insight into the commitment required to polish work to a high standard when describing of the completion of a Brancusi sculpture (‘Fish’ 1926).
“The matter of finish was an obsession with him [Brancusi], to capture the prime essence of the material with no accidental blemish. When I saw him after the show he told me of the difficulty he had in getting a proper mirror surface on the disc under his fish” [ref 1 ]
Brancusi famously worked his own cast sculptures “since the moment the metal came out of the foundry”, and this is perhaps as good an example as any of the dedication demanded of the artist if they are to achieve a high quality polish finish on their work. Despite technical advances and mechanical alternatives (some of Brancusi’s work was machine polished in Germany, and it is is also quite possible that he also experimented with electroplating processes during the 1920’s and 30’s), hand polishing is ultimately often the only way for an artist to achieve the desired effect on their work [ref 2].
PROTECTING A POLISHED FINISH >
|
|
|
|
