That some unburned carbon is present in a flame is shown by the fact that whenever a cold object is held in the flame, it becomes smoked or covered with soot. 伯莎M.克拉克.科学通论.
Sand should abound in the clay in a certain proportion, or be mixed therewith, otherwise the clay, whether burned or unburned, will crumble. 威廉·亨利·杜利特.世纪发明.
If the quantity of air is insufficient, the carbon particles remain unburned and form soot. 伯莎M.克拉克.科学通论.
If a cold object is held in the bright flame of an ordinary gas jet, it becomes covered with soot, or particles of unburned carbon. 伯莎M.克拉克.科学通论.