I do not have a black light - I suppose I really should get one!
A simple blacklight (longwave uv) will not separate early Ysart stuff from later as it will show (for example) all Ysart / Vasart / Strathearn items as a Green reaction. As yet, I have not been able to confirm the views of some folk that different shades of Green uv reaction can determine different periods of manufacture or different makers.
A Green longwave uv reaction can also be seen for lots of other makers' items (using a soda glass batch). But if an item does
not show as Green under blacklight, then it is a safe bet that it was not made by any of the Ysart men.
With a shortwave uv light there is a Blue reaction for all later Vasart and Strathearn. The shortwave reaction is Grey for earlier Vasart (Ysart Bros) and art glass / weights made by the Ysarts at Moncrieff.