< SURFACE CONDITION
SURFACE PREPARATION
Before a patina compound is applied to the cast, it is essential to ensure that the metal surface is free of grease or loose contaminants. Cleaning brings the cast to a uniform surface condition, with chased and worked areas visually ‘knocked back’ so as not to stand out from relatively unworked areas (translucent patina deposits will in particular show up any variations between bright and dull surface workings).
To clean their castings, the majority of founders use a low pressure air or water borne BLAST SYSTEM to deliver a fine abrasive (aluminium oxide) grit. Low pressure blasting imparts a consistent, if dull, finish to a cast surface; whilst at the same time ‘opening up’ the metal’s surfaces at a microscopic level. This opening up or KEYING provides a highly receptive surface for most applied patina compounds. A blast treatment will also normally remove any trace of oil or grease which might act as a resist to a water soluble patina preparation. Care must be taken when handling a sculpture after blasting to prevent sebaceous oils from the skin transferring to the receptive surface of the cast. Solvents may sometimes be used as a method of degreasing metal casts, this method is especially useful if a semi-polished surface needs to be kept intact. Unlike a mechanical cleaning process, solvent cleaning will not even out any obvious variations in reflective and non-reflective workings across the cast’s surface.
Whilst a shot-blasted cast easily ‘takes on’ most patina solutions, the dull metal surface normally imparted by this cleaning process is not always the ideal platform on which to work a patina. Some patinas, especially the red and brown finishes produced by the use of ferric or sulphide compounds, are suited to multiple layer applications, with each successive treatment leaving a light deposit over a reflective or semi-reflective cast surface. This controlled approach to patination ultimately builds up to produce a deep, yet translucent colouring effect that allows light to be reflected back off the underlying metal surface of the cast and through the applied layers of patina. This effect is comparable in some respects to the ‘glazing on ground’ techniques occasionally employed by painters.
A ‘neutral’, shot blasted finish (which tends to absorb light), is clearly not conducive to the kind reflective surface needed to enhance a layered patina, so some degree of enhancement is carried out first. Under most circumstances, working a full mirror polish as a preparation for a patina would be considered both excessive and unduly expensive. A perfectly satisfactory sheen on the cast’s surface can usually be achieved by rather more economic means. Methods of highlighting a cast variously include, working the cast’s surface with a soft brass brush, or naylon (‘SCOTCH BRITE' TM) type pad. Similarly pumice pastes, grit impregnated rubber abrasive blocks, flap wheels, glass beads (peening) and any number of other related techniques will produce much the same result. Again, many founders will retain samples of patinations that have been applied to variously prepared metal surfaces.
PREPARING & APPLYING PATINATION CHEMICALS >
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Castings can be prepared for patina by shotblasting, pieces that do not fit in an enclosed cabinet can be manually cleaned. Blasting removes oxides and other residues that might hinder a patina from adhering, it also provides a clean and receptive surface to work on. (Photo: R. Moule). |
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The surface condition of the underlying metal frequently has a direct influence on any overlaid patina. In this example, the founder has burnished the cast to a SATIN finish. When a delicate patina is applied to the cast, the semi polished state of the cast allows light to be strongly reflected back to the viewer giving a quite different effect than if applied to a dull surfaced cast. |
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