[Speedslug]

I think it was anise white that shows soot in even the smallest amounts if the fuel oxygen mix is even a little off.

I have heard of folks keeping a piece of that glass on the bench just to help reset the mix whenever they light up the torch.

Learning the fine points of where the sweet spots of heat control and complete combustion are is a skill that takes a great deal of time at the torch and there isn't much I know of that will reduce the time investment.