Steel etching with Winsteard's reagent. It is a bit dangerous because if done wrong it forms explosive dust. It was also long and tedious because the liquid must be near boiling and stirring so it evaporated quickly and has to be topped off and brought back to temperature often. The etch itself requires a long temper of a quenched sample and has an iterative process of etching and back-polishing to gradually remove surface roughness but leave the slightly deeper grain boundaries.

It took several hours of preparation and several hours of active work per sample and even then had a 50/50 success rate. I was professionally trained by a third party who learned this process from the person who perfected it, George Vander Voort.