I am no coin collector, but doesn’t the cleaning part ruin their value?
Yeah let the patina stay right there.
It can, if done incorrectly. With Roman coins, however, it is to be expected. Most I have received are covered in centuries of dirt and other encrustations. The goal is to clean the dirt and crust off, while maintaining the patina of the coin itself.
This means days, sometimes weeks of soaking in distilled water, olive oil, and for really tough crust, vinegar, and quite a bit of scrubbing with a toothbrush. If you take it too far, exposing and in some cases, even 'polishing' the coin to a point, would ruin the value. Just removing the crust and such, however, leaves a clearly old coin with a classic patina covering the bronze, which many of these are.