Always begin working calypso in a reducing flame, after a core heat has been achieved, a more aggressive neutral flame can be used. Heating too aggressively with too much oxygen present will cause a film to develop on the surface at the edge of the puddle. For some seals one will need to use an aggressive flame.
To avoid buildup, while using an aggressive flame, switch back to a reducing flame and flash the area near the edge of the puddle keeping it hot but still cool enough to maintain structure. If any film does develop, a coating of soot followed by a slow heating until completely molten can totally reverse any surface issues.
If calypso is heated hot enough it will become completely translucent, a flame strike can bring the opacity back if desired.