Chemical Etch on Sterling Silver from Soap

Afternoon Steve,

Paul Fisher pointed out that your question about animal fat compounds hadn't been answered.

Perhaps I can help. I am supposed to be a professional cleaning consultant..... :)

I wonder if your cleaning operations are leaving scrap animal compounds on the surface as a residue? Others in the jewelry business find that they leave microcrystalline waxes on surfaces after they clean them in aqueous cleaning baths using ultrasonics.

In order to clean these chemicals with ultrasonics, one normally needs to melt or soften them. I don't know what compounds you are using, but common melting points for these chemicals are at least 160 oF which is a fairly hot temperature for an ultrasonic cleaning bath.

Do you think this could be the source of your problem? Do you know the melting points of your animal compounds (probably C18 alcohols)? At what temperature do you run your ultrasonic bath?

Please respond as you think I can help

John Durkee

Hello Steve,

I have faced similar issues of surface etch and haze with a couple of my customers. Once was in cleaning polished sterling silver parts, the other mirror finished zirconium parts. Both processes used ultrasonic cleaning.

So, I thought I'd share my experience with you in the hope that it will help you with your current challenge.

- The sterling silver problem situation:

The cleaning process consisted of one ultrasonic cleaning tank, and one heated (static) rinse tank. The problem was high rate of rejects and reworks due to damaged surface finish after cleaning.

Attempts to correct the situation by changing chemical brands, strength, concentration, heat and cycle time were all to no avail, In fact, the high concentration, harsh chemistry, combined with the  amplifying effects of long ultrasonic cycle time did nothing but accelerate and increase the surface etching. On the other hand, the haze remained as it was in the domain of carry-over from from the buffing compound residues.

- Here's how the issues were resolved:

Step 1: Pre-clean at 175-180ºF (higher if no ultrasonic used) with a brief ultrasonic coupled with oscillation (up+down movement). While the proper parts positionning, chemistriy and heat are instrumental to dissolve the fatty compound, oscillation proved helpful to "release" the contaminant off the surface. Also, this step helps prevent air entrapment as the parts in question have fine details like brazed appendices, tiny holes and knurled areas.

This tank is equipped with adequate filtration and recirculation system to handle both of types of contaminants present in suspension and in dissolution, and hence reduce the carry-over to a minimum into the precision cleaning step.

Step 2: Ultrasonic cleaning using specialized ultrasonic cleaner at 140ºF. Specialized ultrasonic detergents are more expensive than general purpose's, but proved less expensive to use. They are more reliable in getting a faster and repeatable sequence of cleaning operations (but this is beside the point for now)

Step 3: A heated, cascaded/counterflow city water rinse.

Step 4: DI water rinse

Step 5: Hot air drying

NOTE: In the zirconium experience, step 3 is skipped as the parts have plain smooth surface.

Hope this helps. I'll be happy to answer any questions you may have regarding this information

Clement Gemayel