What type of surface is most commonly used for engraving pieces, such as the one above? why would the process be difficult?

In engraving, artists usually use a metal sheet as the surface for their working space. It is really difficult to etch, due to the amount of scraping required, so artists also appeal to their strength and control.  As you know, when working at such piece of art, artists are unable to make corrections, so their stubbornness and ability to control are the most helpful features.

Thom bell , bobby martin , and norman harris .