I googled this problem. They advised many methods involving acids. I saw some less obtrusive methods including abrasive powders, limeaway and whitening toothpaste. That make sense. Thoroughly rinsing and drying the specimen is recommended in all cases. Any use of acids should be done in a well-ventilated areas with proper skin and eye protection. The limeaway and iron out products also produce dangerous fumes. I am a safety first guy so I would try using the whitening toothpaste before any of the acids(oxalic, HCL, phosphoric) were all mentioned.