LaserCraft Background process question

Greetings friends,

Some of you might be familiar with a company from the late 70’s & 80’s called LaserCraft who is now defunct and out of business. But I’m sure you have seen their beautiful etching artwork.

Their products had a certain background look to their designs that were engraved in Walnut.

My question:

Does the walnut wood give that look after it’s engraved or is that background part of a burn pattern within the file?

See photo below for example

Do any of you have any experience with creating such backgrounds?

Thank you in advance from the Great Pacific Northwest