< SURFACE CONDITION
SURFACE PREPARATION
Before a chemical patina is applied to a 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 grit (aluminium oxide/glass bead etc). 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 now 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.
Under most circumstances, working the cast to a full mirror polish in 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™) 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.
PREPARING & APPLYING PATINATION CHEMICALS >
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