Also, many opalescents become more milky the longer you fire them (still talking about very moderate fires here), so for maximum control of the intensity of the opalescence, the fires have to be very careful and checked often.
The technique of Nouveau Grisaille developed by Andreu VilasÃs takes maximum profit of this effect, with opalescence control and thickness of layer to achieve different shades and volumes. See wondeful works here:
http://focgallery.com/SXXI.aspx
The only opalescent I personally know that becomes transparent with overfiring (probably because I haven't tried overfiring any other) is Schauer 64. But the opaqueness (this one is almost opaque white, but with a porcelain translucency) in this one comes back if it is fired again, very forgiving.
