Leather is a natural product which tends to burn during engraving or cutting; it also produces a particular smell. Furthermore, the cutting edges can turn dark and stain. Many types of leather can easily be cleaned. However, it is important to test the cleaning method at a discreet place. First off, try to remove the smoke residue with a dry cloth. If that doesn't remove the residue, try cleaning the leather with a damp cloth or an alcohol-based cleaner.