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I left some dirty dishes over a summer internship. Now, cleaning up, I have found that some of the silverware has developed a rough surface. After sitting overnight in a soapy bath, it persists, although can be scrubbed off.

Here's a fork, which appears to be stamped stainless steel, showing the rough texture: fork

I don't have my microscope, and this camera image (try expanding it) is about as good as my eyes resolve.

My question: what is this?

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    Chemical pitting of electroplate from high pH dish detergent? Such a bloom might mostly be scrubbed off, but if you look closely... Might also just be an insoluble salt deposit, calcium phosphate or similar. If that's the case, it should wipe off with vinegar. From what I can see, it looks more like case 1 to me. – Wayfaring Stranger Aug 23 '15 at 22:59
  • Have you heard about the Philosopher's stone? You might have discovered the opposite ... – Dr. belisarius Aug 23 '15 at 23:03
  • @WayfaringStranger I scrubbed it off, and the surface finish is completely unharmed. So, deposition seems likely. However, I have used this silverware here for a year previously, without mineral deposits. – imallett Aug 23 '15 at 23:06
  • Maybe someone switched dish soap on you, or the water has changed. More calcium, or other divalent metals, in the water leads to quicker soap scum build up. pH change in water will also affect solubility of soap and ions. Occasionally municipal water systems put out non-standard water; either intentionally or by accident. – Wayfaring Stranger Aug 24 '15 at 13:37
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I have seen similar deposited after stainless cutlery and plates were soaked in hard water from bores and from areas with hard water. Its just a collection of salts that like making a home on metal surfaces. You can remove it without scrubbing by adding some citric acid powder or a little white vinegar to some warm wash water. Denture cleaning tablets have a similar effect in cleaning off this buildup.

The citric acid powder is a really good way of cleaning up the built up scale in kettles and water heating appliances. It makes my glass electric kettle shine like new.

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