Pulling the plug and checking the flame color during the firing process to make sure I had a reduction atmosphere in order to get the glaze colors I wanted. Understanding the atmosphere inside of the kiln during the fire is essential, which is the regulation of oxygen allowed to enter through the venturi burners with the natural gas to produce an oxidation , neutral, or reduction atmosphere. For example, copper carbonate and tin oxide in a glaze which is fired in an oxidation atmosphere will turn a very light green color — but when fired in a reduction atmosphere will turn red or purple. Plus the atmosphere must be regulated at the appropriate time and temperature in the kiln.