Metal Finishing Guide Book


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Table II. Antique Baths 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 (OF) Agitation Current density (A/ft2) Bright Yellow Highlights, Orange-Brown Smut 6 — 15 30 — 4 160-180 None 30-40 Green Highlights, Green-Black Smut 2 0.3 — — 38 15 70-90 None 10 shown in Table I, will give a representative sample of colors. All of the "coloring" baths in Table I should use 316 stainless steel anodes. The ratio of anode to cathode area is best at 1:1 or 3:1. Very high ratios, when the tank is used as an anode, tend to give an uneven color and thickness of deposit, and the end pieces will frequently burn. No agitation should be used to ensure a uniform color. Sliding or tapping on the cathode bar will increase the deposition of gold and make each color richer but will quickly deplete gold and unbalance the bath. Gold and alloying metals should be added periodically, based on ampere-hour (A-hr) meter readings. The baths, with the exception of the white, green, and rose solutions, should operate at approximately 6% cathode current efficiency. Every 11 A-hr of operation 5 g of gold should be added, together with the proper amount of alloy. All operating conditions should be controlled as closely as possible. Any variation of the conditions will affect the cathode current efficiency of the gold or the alloy, or both. Changes in the amount of the metals deposited will change the color of the deposit. Other factors that will alter the color of the deposit are the following: 1. Surface finish. The surface finish of the basis metal will change the apparent color of the deposit. This is particularly noticeable when a single item has both bright and textured areas. Plated in the same bath, they will appear to be two different colors. 2. Color of basis metal. The color of the basis metal alters the color of the gold deposit by adding its color to the gold until the deposit is sufficiently thick to obscure the base. Most gold alloy deposits, if properly applied, will obscure the base after 2 millionths of an in. have been applied. Proprietary additives will allow the gold to obscure the base with as little as 1 ½ millionth of an in. to allow richer colors with the use of less gold. 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 density 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 holding back the copper. 286

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