This is a build-up of waste material, removed during the cutting process, which sticks to the underside of the profile. This is preventable by correctly setting the cutting process variables.
The residue remaining in the pipe after prepared opium has been smoked.
The residue of resolidified melt on the lower edge of the laser cut. Using pulsed laser cutting and increased oxygen pressures can reduce dross. Cuts with higher dross are often the result of using poor quality steel, the existence of surface rust, or incorrect process parameters.