The are burned pre seal on bamboo. For the outline, I use a fine tip on my adjustable tip wood burner. Heat goes up to 105, I do a high medium (like 90)
The medium heat still gives you the dark deep burn, while keeping a crispy line. To high of heat and you get almost like a bleeding appearance. Then afterwards, I go back on high heat with my regular old faithful - The Hobby Lobby Burner
To finish it off, after burning I use a mineral oil rub