[jaskent]

Is this experiment still available please (june 2005), link does not work

[Arkcon]

Lots of things from 2005 aren't available any more.  Briefly, Wikipedia suggests recrystallizing the black allotrope will produce one of the many red allotrope as monoclinic crystals.  Do you have the ability to try that?

[jaskent]

I am a chemistry teacher and have seen this a long time ago, but cannot remember best way to recrystalise the black to red.

[blueberry58]

On converting gray selenium to red I found this information is taken from A Facile Lab Scale Synthesis of Red Selenium
Jorg Ebels, Stephan Pirk and Rudolf Pietsching

100 mL of concentrated sulfuric acid are heated to 150°C. 1 g of grey selenium is added in
portions while stirring. Heating is continued for a short while, after which the solution shows
a dark green color. The mixture is poured on approximately 200 mL of crushed ice which
causes a red precipitate of selenium. Once the ice has melted completely, the mixture is
filtered and washed with water until the filtrate is neutral. For faster drying the raw product is
then washed with ethanol and later diethyl ether. The product is collected and dried in vacuum
without heating.

I hope works