The red was red iron oxide the yellow was from salt and the black is from the sawdust. The pot is buried in the layer of saw dust and that part absorbs the carbon. if you look you can see some popcorn tins. A piece of pottery was placed in them and the bottom of the tin was covered with sawdust then a lid was placed over the top. This caused the carbon to bond to the clay body. These pots that are fired in the tin will be all black.
