Metal finishing Q&As since 1989

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Diamonds acquire black haze from masking for rhodium plating

We use nitrocellulose paint for selective rhodium plating; suddenly on removing the masking in acetone [on eBay or Amazon] the diamonds acquired a haze resulting in them looking blackish and dull. The haze went away on wiping the removed diamond with cloth.
Question how does one solve this problem without damaging the ornament?

Khushroo Kotwal
- Mumbai, Maharashtra, INDIA
December 8, 2011

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Very simple answer: you boil your rhodium jewellery in sulfuric acid and hydrochloric acid mix. Note: rhodium dissolves in hot conc. H2SO4. So you first take a beaker [beakers on eBay or Amazon] of 500 ml, put approx. 50 ml to 75 ml H2SO4 into it and then add 50 ml of HCl slowly till bubbles starts coming. Then dip your article into it and heat it on hot-plate to warm the mix up to 10 to 15 min. Remove the article using tweezers and quickly dip it in ultrasonic pot which has basic surfactants so that acid base reaction takes place. Then dip article in clean water and gently give steam to it and see the result.
If you are not satisfied please tell me, I will give you another technique but in this case you have to again plate rhodium on article.

Bhupesh Mulik
- Mumbai
December 26, 2011

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Kotwal, It sounds to me like the problem is due to contaminated solvents and poor rinsing. I would suggest you use a sequential acetone [on eBay or Amazon] rinse, with increasingly pure solvent, as you would do with conventional counterflow rinsing. The final rinse could be in warm, pure, water, because acetone is miscible with water.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
December 28, 2011

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Thanks Trevor.

Dealing with rings probably implies very small quantities of acetone. But readers who might consider using acetone in other situations must be mindful of its flammability. Perhaps it's because of its ubiquity in nail polish remover, often watered down, but people tend to forget that acetone is highly flammable.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
December 28, 2011