Original: Early carburization used a direct application of charcoal packed onto the metal ( initially referred to as case hardening or Kolsterising ) , but modern techniques apply carbon-bearing gases or plasmas ( such as carbon dioxide or methane ) .
Simplified: In earlier times one used a direct application of charcoal packed onto the metal for carburization , but modern techniques apply carbon-bearing gases or plasmas ( such as carbon dioxide or methane ) .