Which term names the piece of work a journeyman had to produce for the guild to judge before he could be admitted as a master?

Explanation

Admission to the highest rank of a craft normally required a demonstration piece that the trade's own officers examined for skill and finish, and only work judged good enough opened the way to an independent workshop. The description of a celebrated work of art fails because the word is used here in the sense the guilds gave it, a qualifying test piece, and the modern sense of an outstanding artistic achievement developed later.