Burnishing steel parts in Italy

Recently I was asked to produce steel parts ( Material : C 45) with burnishing from a client from Italy.
As per them " Burnishing" was effected by dipping the finished part in nitric acid for a while and then applying some branded coating which turned the finished parts black.
No dimensional change was observed.

Can anybody highlight this coating process what that client called it " BURNISHING "

Hmmmmmmm. That's interesting. Never heard of that before. What you describe does not match the normal description of burnishing, which is a lapping process to produce a smooth shinny surface. Out of curiosity, I looked in the dictionary and it does say " to make brown". Hmmmmmmm.