Silver Plating Problem: Low Brightness, Burning

A. Hi,
If cutting the bath then adding back the essentials significantly improves things, that is strong evidence that the bath is contaminated and that the cutting reduced the concentration of the contamination. However, I do not have enough experience in this to offer good guesses of what the contaminant is.
Luck & Regards,


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

A. It would be nice to save this big old $25,000 workhorse.

Here's a few more ideas:

1) What have you been using to treat your carbonates? If you use nitrates, a 5 year build up might be the problem. I know of no way to remove nitrates other than dilution, with subsequent build up of the other ingredients. You might try some Hull cell panels, using some dilution and additions.

If you use hydroxides, accurately test the pH. Use closely bracketing pH standards. I found an interesting page in an old British analysis book, concerning the Hull cell testing of conventional silver cyanide solutions that were similar to your formulation. The make-up pH was 11.5. At a pH of 13, the Hull cell showed rough, burnt deposits, over most of the panel. I know of no way of lowering the pH without killing everybody. Maybe someone else could give us a safe way of doing this. Maybe some sort of simple electrodialysis membrane could be used safely. To see if the pH is the problem, you might adjust a sample to pH 11.5, UNDER A FUME HOOD, with some sort of compatible acid - if there is any. Heat the solution up a little and stir for a few hours, in order to drive off the dissolved HCN. Cool and perform Hull cell tests under the hood.

2) I've suspected all along that your problem is organic breakdown products from 5 years of brightener additions and from such things as rack coatings. The dark color seems to be the key. I never believed in the "carbon dust" theory. I assume, since you are measuring your "carbon treatment" in hours, that you are either packing a filter with carbon or using carbon cartridges. This may not be enough. As I understand it, carbon filters only pick up short-chain organics and leave the longer-chain varieties behind. These build up and can eventually ruin the deposit, in various ways. You may need more power in your organics removal.

The only way to fix your bath may be a traditional, hot, peroxide, carbon treatment. Carbon treating precious metals baths is only done as a last resort, because about 10% of the silver or gold gets hung up with the carbon and must be sent out for refining. Try it on, say, 1000 mL samples in the lab, first. Make the first trial a little strong, just to see if it works. You might:
A) Heat 1000 mL of analyzed solution to 120 deg F.
B) With stirring, add about 3-5 mL of 30% (100 volume) hydrogen peroxide. Check the temperature.
C) Maintain temperature at 120 and stir for 1 hr.
D) Add about 2.5 grams of powdered activated carbon.
E) Stir and maintain temp. for 1 hour, or more.
F) With stirring, add about a gram of filter aid. Diatomaceous earth works OK.
G) Cool and filter well
H) Analyze and adjust silver and cyanide. Analyze carbonates and compare. Analyze brighteners, if you can.
I) Run Hull cell panels and adjust brightener(s).
J) If this works, repeat with more or less temperatures, concentrations, and times, if you wish. The more heat and peroxide, the more cyanide will break down to carbonates. The more carbon you use, the more silver it will adsorb.
If this works and, if you decide to go full scale, you have to do it right. You HAVE to do it in a separate treatment tank. The treatment tank should be located away from the plating tank, since even traces of the easily airborne carbon dust in the plating tank can cause roughness problems, sometimes long term. After treating, the filter must remove all traces of carbon before the solution is returned to the plating tank. To make filtering easier, let everything settle overnight and then start filtering near the top. Make sure everything is ready before you start.

According to one of my books, the chilling out of sodium carbonate, at 3 °C, won't lower it below 8 oz/gal. You're only at 8.5 now - why go to the expense?. Did you prove this in the Hull cell? If you can afford it (and, find it), barium cyanide is the best way to remove carbonates. Maybe you'll find it's time to retire the old gal.

? Steve,

I have a couple of questions. What are you using for anodes? Are you using steel anode baskets? Are you using a silver strike before this bath? any possibility of dragging in contamination from your strike bath?

A. The pH is certainly high enough now, but was it ever lower than 11? If it was lower and you used steel anode baskets then possibly the black color is ferricyanide.

I also agree there is no safe way to lower the pH.

As for selenium, it is a common brightener for silver is effective at concentrations of less than 5 ppm. Your supplier should be able to do a selenium analysis and give you a recommendation if it is too high or too low.

A. Hello.
I have not read all the answers in here, but the concentration of carbonate is way too high. You have to freeze the carbs out of the bath, lowering the temperature to below 40 °F. The carbs will then crystallize and you can filter the liquids out. Adjust the cyanide concentration and you are way to go again :)

A. DEAR SIR,

I HAVE GONE THROUGH THE TOTAL CONVERSATION, AND COME TO CONCLUSION THAT THIS IS NOT ISSUE OF ANY pH, BUT THIS IS PROBLEM OF HIGH ORGANIC CONTAMINATION IN SILVER BATH.

PLEASE DO HEAVY ACTIVATED CHARCOAL HOT TREATMENT AT 90 °C WITH MIN. OF 30-50 GM/LTR STIR WELL FOR 12 HOURS.
KEEP FOR SETTLING 24-48 HOURS. FILTER THE SOLUTION, MAINTAIN THE CARBONATE CONTENT BELLOW 40 G/L. ADD BEST QUALITY OF SILVER BRIGHTENERS A&B AGAIN AS MAKE-UP A- 25 ML AND B- 15 ML. MIX IT FOR 12 HOURS AND TAKE HULL CELL FOR 1 AMP WITH GOOD MIXING, YOU WILL GET 100% CLEAR & BRILLIANT BRIGHT & WHITE SILVER PLATING END TO END.

TRY THIS AND REPLY.

A. We have seen the silver cyanide solution turn dark orange if the solution is contaminated with nickel. The nickel contamination will also tone down the brilliance of the silver plated deposit.

A. We have been using a 5-gal silver cyanide tank with Silverex III brighteners for 30 years and have had the solution turn dark several times when the pH dropped below 12. We remedied this each time with treating a portion of the tank with barium hydroxide to drop some of the carbonate buildup and raise the PH at the same time. The Ba(OH)2 must be kept tightly sealed in storage not to clump into a rock. I used an old titanium tank heater and a 3-gal stainless steel pot to carefully mash the Ba(OH)2 crystals to get them to dissolve and let sit overnight. Pour the clear top liquid back into the tank and filter the rest before adding. Rinse the BaCO3 cake in the filer with DI water spray bottle and let sit a day. Destroy cyanide contaminated BaCO3 by adding it to a pot with a gallon of tap water and add chlorine bleach ⇦ bleach/sodium hypochlorite in bulk on eBay or Amazon [affil links] under a fume hood until a green, blue or brown color (of metallic ferrocyanides) and dispose of BaCO3 as solid waste after filtering again.