Early carburization used a direct application of charcoal packed onto the metal -LRB- initially referred to as case hardening or Kolsterising -RRB- , but modern techniques apply carbon-bearing gases or plasmas -LRB- such as carbon dioxide or methane -RRB- .

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 -LRB- such as carbon dioxide or methane -RRB- .
