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

Sas