Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Table II. Antique Baths Bright Yellow Highlights, Orange-Brown Smut Gold as KAu(CN)2 (g/L) Silver as potassium silver cyanide (g/L) Sodium hydroxide (g/L) Sodium carbonate (g/L) Ammonium carbonate (g/L) Sodium cyanide (g/L) Temperature (O Agitation F) Current density (A/ft2 ) 6 — 15 30 — 4 160-180 None 30-40 Green Highlights, Green-Black Smut 2 0.3 — — 38 15 70-90 None 10 3. Current density. Too low a current density tends to favor the deposition of gold and causes the alloy to become richer. Too high a current density at first favors the alloy and pales out the color. Raising the current den- sity further causes the development of pink, orange, or red tones. 4. Free cyanide. Solutions containing copper are very sensitive to changes in the free cyanide content. Low cyanide causes an increase in the pink and red shades, and high cyanide significantly increases the yellow by hold- ing back the copper. 5.Temperature. The effect is similar to current density. Low temperatures favor the gold yellows, and higher temperatures favor the alloy colors. Temperatures over 160O F should be avoided, except in the case of rose golds, because of the rapid breakdown of cyanide and the darkening of the color. 6. pH. It is rarely necessary to adjust the pH of a gold or gold alloy bath. They are usually buffered between pH 10 and 11. Only pink, rose, or red golds are favored by higher readings. BARREL PLATING (CLASSES A AND B) Gold as KAu(CN2 ), 0.4 g/L Free cyanide as NaCN, 30 g/L Disodium phosphate, 23 g/L Temperature, 100-120°F Anodes, stainless steel (1:1 or better) In typical jewelry barrel plating about 6 V is necessary. A decorative finish of 0.000002 in. is deposited in 3 to 4 min. If the parts are small and densely packed in the barrel it may be necessary to plate up to 8 min to get even coverage. The above formulation may be altered to achieve various colors. Champagne or light Hamilton colors may be achieved by adding 1.5 to 3 g/L of nickel. Lowering the tem- perature will also produce a lighter and more uniform color. ANTIQUE GOLDS (CLASSES A AND B) The art of the 19th-century platers was to produce a finish that looked as if it had been mercury gilt over silver or copper (vermeil) and buried or weathered by the ele- ments for a century or so. Each master plater developed an antique finish that was his trademark. The basic modern method to achieve this effect is as follows: 1. "Burn on" a dark smutty finish. 214

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