After minutes or hours of preparation finding heavy scratches on your specimens can be infuriating. Chances are it is from contamination, the metallographer’s bane. It can come in a few forms from a number for sources. There are several things you can do to interfere with the activities of contaminants and not allow them to do as they please.
Bits of abrasives from the previous stage being stuck to the specimen surface, then coming off in the next stage is one common way polishing cloths get contaminated. These abrasives can also hitch a ride on the specimen holder to arrive in later stages. Cleaning specimens and the specimen holder in between steps can put an end to this. Wash thoroughly and rinse between every stage. Sometimes, manual cleaning after a step isn’t enough – especially if specimens have cracks and pores. Abrasives from previous steps can hide deep in them and come out later. The same thing can happen if there is a large shrinkage gap between your specimen and mounting media. In these cases, it is best to clean them with an ultrasonic cleaner filled with soapy water for about a minute.
Unclean hands or gloves are another way polishing surfaces become contaminated. Washing your hands in between steps is just as important as washing specimens and the specimen holder.
Another source of contamination can be material that is coming from the sample itself. In this case, no amount of cleaning can necessarily resolve the problem and we need to change the preparation approach. Switch to napless cloth and reduce pressures where possible. If contamination problems persist even with thorough cleaning, consider using only complementary rotation mode. Complementary rotation tends to encourage used abrasive and contaminants off the platen. Although keep in mind this rotation mode increases the amount of consumable used, and its less aggressive polishing action typically means longer polishing times.
Lastly, remember to always maintain a clean environment. Dirty machines and surfaces create many more contamination opportunities. Cleaning your grinder/polisher every day can eliminate many problems. These practices can help keep contamination under control. Although they may seem tedious, it is often far less bothersome and time consuming compared to re-preparation of scratched specimens!