Re: black stuff appears on metal when fire.

The candle flame is a result of the chemical reaction called oxidation 
that occurs during the burning process.  In fact, the "burning process" 
actually is oxidation.  As the candle wax heats up and becomes a vapor, it 
combines with oxygen in the air.  This process is called oxidation and it 
releases heat which is used to continue the process of burning candle 
wax.  But, because heated gases expand quickly away from the source of 
heat, they also cool down rather quickly also (sometimes before all of the 
"fuel" - vaporized candle wax - has oxidized).  This partially oxidized 
candle wax will then "condense", or stick to, a cooler object that happens 
to be nearby - like the knife you mentioned.

The "black stuff" you see is just a part of the candle wax that was not 
completely burned.  It would probably condense on any material placed 
directly above the flame that would not burn itself.  A metal object, like 
a knife, is a very good object to use because the melting point of most 
metals is higher than the temperature that a candle can achieve.  Also, a 
knife will stay relatively cooler than the candle flame for a long time 
because metals are usually very good at conducting heat away from the 
flame.  That is why you should not touch the metal blade of the knife 
directly when you do this experiment.  The metal blade will get very hot, 
even at some distance from the source of heat.