I have been having an absolute nightmare recently with the sawdust. All the pots I was getting out of the fire were very dark... and mainly brown. I am ok with brown, but I know that people generally prefer more contrast and colour. it makes for a more dramatic vessel. The issue with switching sawdust is that each batch is different - more or less coarse or fine, more or less wet or dry. Each burns differently, and so each needs to be controlled in a different way to get colour. Colour is all about oxygen. The basic principle is that if you feed a fire all the oxygen it needs, then the pot will basically remain white. If you turn off the oxygen to a fire, creating a "reduction" atmosphere, the flames go looking for oxygen elsewhere, which includes in the surface of the pot itself. As oxygen is taken out, carbon enters. The amount of the carbon which enters broadly determines the colour. Obviously, you can't see whats happening in a fire (without dramatically affecting the oxygen levels!)... so its all intuition and guesswork..... and experience. It would be easier if the sawdust was exactly consistent.... over time you could scientifically tabulate the results. But you can't because the sawdust, and the weather change constantly. The easiest way I can describe the difficulty is if I asked you to cook a precious cake - and you had to decide exactly when it was done (to around 10 seconds) - but without being able to see it to check and without knowing the temperature of your oven. You see, its BLIMMIN DIFFICULT!
So when you get good results after a load of bad results, you might see why you feel like dancing and crying with relief. Its mad, but pure alchemy when it works.