< OPTIONS FOR RESTORATION
MECHANICAL & ABRASIVE
In some cases it can be appropriate to physically remove surface corrosion through the use of dental picks, air tools and similar mechanical devices. These operations can require a delicate touch, errors may result in even greater damage being suffered by the sculpture, especially if the underlying thickness of sound metal is accidentally penertrated. The use of ULTRASONIC CLEANING (high frequency sound waves), and similarly modern techniques may be preferable if the scale of the sculpture and nature of the damage allows for the use of this type of equipment. Hand held and motorised wire brushes of any description should never be contemplated as a cleaning method for sculpture of any quality.
Abrasive cleaning with ALUMINIUM OXIDE and similar shot blast media (including GLASS BEAD peening) is very much out of favour as a process for use on restorative projects. Whilst these techniques are considered suitable for use in the foundry, and may even be appropriate for use during the repair or preparation for re-patination of recent casts, they are definately not recommended for cleaning older or corroded casts unless the circumstances are exceptional. Low pressure blast cleaning using less agressive media such as walnut shell, or else water jetting (either with water alone or with the addition of baking soda or fine glass particles), is now considered a far more suitable use of blast clearing areas of corrosion. Even then extreme care must be taken when using any media in a pressurised blasting system.
CHEMICAL CLEANING
Like aggressive blast cleaning processes, the use of strong acid solutions (which are still sometimes used for post-cast cleaning in the foundry), is nowadays avoided as a method of cleaning restorative projects. Nitric, chloric and oxalic acid solutions have all been applied in the past to clean encrusted bronzes, though now there is little support for using any acid on cast sculptures, other than perhaps a weak CITRIC ACID treatment (<10% in solution + etching inhibitor), for cleaning low levels of encrustation. FORMIC ACID is considered a suitable electrolyte for electrolytic reduction cleaning.
Cuprous chloride, a major consituent in the corrosion of copper alloys, can be retarded through treatment in a SODIUM SESQUICARBONATE BATH. This chemical solution removes cuprous chloride and neutralises incidental acid contamination with repeated bath treatments and washing with purified water. This form of treatment often takes some months; requiring the sculpture to be fully immersed in the solution (thus making the treatment of large artworks difficult or inpractical). Sodium sesquicarbonate does however, have the advantage of leaving any existing patina more or less intact, though some darkening may be observed.
BENZOTRIAZOLE (see MAINTENANCE), is also used as a means of treating cuprous chloride damage, though unlike sodium sesquicarbonate, BTA does not remove cuprous chloride, but instead creates a barrier between the cuprous chloride deposit and the atmosphere, effectively preventing contact with airborne moisture which would continue corrosion of the sculpute’s underlying fabric.
ELECTROLYTIC CLEANING
Electrolytic cleaning is related to other elecro-processes such as anodising, electropolishing and electroplating etc. The electrolyte in this instance is typically a solution of either sodium carbonate (0.25-1%), sodium hydroxide or formic acid.
The sculpture is attached to the CATHOIDIC (-) terminal of the cell (the opposing terminal to the electrodepositing processes mentioned above), a carbon or stainless steel plate functions as the ANODIC (+) terminal. A low density DC current is then passed through the electrolyte from the cathode to the anode. One of the effects from passing a current through the electrolyte, is that cuprous chlorides detachs from the corroded sculpture and passes over to the anode. The length of time required for a elecrolytic cleaning treatment to complete varies from a few hours to many months, according to the scale of the subject work and the degree of damage sustained. Once again the requirement to immerse a damaged sculpture can limit the scale of artwork that can be treated by this method. The use of an electo-cleaning process will normally remove any remaining patina deposit evedent on the sculpture [refs].
FINISHING THE RESTORATION & PATINATION >
|
