When was it that people lost the formula for (and ability to make) Greek Fire?

History of Greek Fire.

The particular information from the site:

"There are however two known incidents of the Byzantines using this weapon. In 678 they utterly destroyed a Muslim fleet (it is believed over 30,000 men were lost) and also in 717-718, when Caliph Suleiman attacked Constantinople. Most of the Muslim fleet was once again destroyed by Greek Fire, and the Caliph was ultimately forced to flee. As there is virtually no documentation of its usage after this time by the Byzantines, it is generally believed (partially due to the poor performance of the Byzantine fleets after this date) that it was during this era that the secrets of creating Greek Fire were lost."

Thanks, sonnet26.