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Enamelling common silver and gold alloys, Page 2 DEPLETION ENRICHMENT OF THE SURFACE. To produce by oxidation of surface copper & subsequent removal by acid digestion of those firescale oxides a substantial layer of pure silver (or gold) on the surface of the alloy, to which the first layer of enamel will be fused. METHOD: 1/ Engrave or otherwise work any sub-enamel areas if required. 2/ Heat the piece to be enamelled to dull redness and allow to cool until no radiance is visible. It will probably now show the dark firescale referred to previously. 3/ Place in an appropriate acid pickle (10% nitric or sulphuric acid, Sparex No 2, or vinegar and common salt) in a covered heat-and-acid-proof container and allow the piece to digest until all of the firescale has been dissolved. The work should appear dull & white (or yellow in the case of gold alloys). 4/ Remove the work from the acid using acid-proof tongs, rinse under running water, wash with water and detergent then clean with a glass brush or equivalent (eg. domestic fibreglass insulation, used with rubber gloves) under running water. Rinse again and dry. (Burnishing at this stage increases the surface density of the casting and can be used in a pinch to minimise porosity). 5/ Repeat these operations until no further oxidation of the surface occurs in the annealing stage (generally 2 to 4 times). Scrub with cloudy ammonia or dilute sodium bicarbonate solution to neutralise any residual acid, rinse under running water and dry. Depletion enrichment is critical to maximising the clarity and brilliance of transparent enamels on sterling silver and similar alloys. It is not generally possible to salvage a piece with a poor appearance caused by inadequate enrichment, other than by grinding off all the enamel and starting the process again from scratch, or dissolving the enamel by chemical process. Various literature suggests the use of Hydrofluoric acid - a dangerous suggestion unless the individual is familiar with the use of HF, has a "poisons" licence, and ready access to the appropriate facilities and equipment necessary for the safe handling of this extremely reactive, hazardous reagent. Don't overheat the casting either during enrichment or when firing the enamel; although the solidus point (melting-point is the equivalent in a pure metal) of sterling silver is 893 deg C its degradation point, (the temperature at which the alloy begins to "deteriorate" or " hot shorten") is about 770 deg C. Many gold alloys have melting points too low to allow safe vitreous enamelling with standard jewellery enamels. To provide a safety margin, select enamels with maturation points in the range 700 deg C to 760/770 deg C (soft to medium). Enamels do not have single well-defined melting points; their viscosity decreases exponentially with increases in temperature. What might be for convenience called the maturation temperature is reached when the enamel has been heated to redness and its viscosity is sufficiently reduced to allow the surface of the enamel to flow out and become shiny and smooth. The maturation temperatures of most contemporary jewellery enamels fall in the range 700 degrees Celsius to 820 degrees Celsius; it is generally accepted that within that range, enamels with maturation points in the area from 700 to 730 degrees Celsius are called `Soft' or `Low-firing'; in the area from 730 to 770 degrees Celsius they are called `Medium' or `Medium firing' and in the area from 770 to about 820 degrees Celsius they are called `Hard' or `High-firing' enamels. Check the progress of the firing process visually. The average thermocouple is measuring only the temperature of its own position in the kiln (and that only to an accuracy of about +/- 50 degrees C). Given the time lag involved in heating work from room temperature to the enamel's maturation temperatures there's no close correlation with the actual temperature of all (or any) part/s of the work. |
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