The borostix 4" bars of white work really well, with essentially no boiling unless of course you blast it with a good-sized torch in a neutral to oxidizing flame. Over the years I've gotten used to working some colors about 5" - 6" back in the flame (I have a Carlisle CC torch with tanked oxy). the borostix 4" bars of white are not as dense as any of the other whites, so if you use this white to cover a dark color, more of the dark color will come through than if you use china white (Asian white) or Tag or Northstar's white, which are all denser.
The big marble used the borostix white to cover purple, and the smaller uses a denser white (Asian white or Star White (Northstar) to cover the same purple.