Leather is an organic material. It tends to burn during engraving or cutting and produces a particular smell. In addition, the cutting edges can turn dark and stain. Most types of leather can easily be cleaned, but it is a good standard practice to test the cleaning method on a discreet place. Start by using a dry cloth to remove the smoke residue. If that doesn't remove the residue, try cleaning the leather with a damp cloth or an alcohol-based cleaner.