< PREPARING & APPLYING PATINATION COMPOUNDS
COLD PATINATION
Cold patination involves the application of chemicals to an unheated metal design. The chemical is usually prepared either in the form of a paste or solution, either of which can applied to the surface of the cast in a variety of ways, such as directly brushing on or by poultice application. After a variable period of time (from a matter of seconds to many months according to the chemical applied), a patina colouring develops on the treated cast’s surface. In some cases the applied chemical compound can be left permanently on the cast. However, it is more usual to wash or brush away any excess development prior to the application of a protective wax or lacquer coating.
A potassium sulphide (LIVER OF SULPHUR) solution is the most common of the cold patinas applied to sculptural casts. Potassium sulphide is normally supplied in the form of a dry compact lump, which is broken down and dissolved in tepid water. A brushed on solution of this chemical reacts rapidly on most cast copper alloys to produce a brown colour on contact. Many brasses are resistant to this (and other) cold patinations, in which case the potassium sulphide solution may be better applied over an earlier deposited hot torch copper nitrate ‘ground’.
The painted on potassium sulphide solution is allowed to dry slightly on the cast surface, after which any patchy areas are dispersed by vigorously brushing with a soft brass brush (brushing is omitted if the solution is applied over a copper nitrate ground). This method produces a classic PENNY BRONZE colouring, traditionally used as a base patina over which further treatments can be later applied – though the coating is just as effective as a finish in it’s own right if sealed with a wax protective coating. Potassium sulphide solutions adhere well to degreased cast surfaces, polished or otherwise. However, many other cold patination compounds benefit from application to a heavily keyed (blasted) metal surface (see also SUPPLEMENT).
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