The Preparation of Paraffin is an item that can be obtained from the Artificer of Bucharest. It can do one of two things: Either increase the speed and altitude greatly of any Rozière Balloon, or destroy the balloon completely.
When arriving in Budapest aboard the Orient Express, Passepartout will witness a man being killed on the platform of the train station. If Passepartout tries to save him, he will be given a Black Control Cube. This can either be sold to the Sisters of Didacus, or brought to Bucharest.
In Bucharest, Passepartout may get the chance to visit the Artificer's Guild Outpost. The Artificer there, a lady named Steinberg, will pretend to know nothing of the cube when prompted. However, if Passepartout continues to needle her she will eventually give in, stating that she does know the significance of the cube. Thanking Passepartout for his service in bringing her the cube (which belonged to her late friend in Budapest), she will give him a Preparation of Paraffin in exchange for the cube.
If Passepartout visits Bucharest and its Artificer's Guild without first meeting the man in Budapest, he may still be given the Preparation of Paraffin. However, this preparation is fake and instead of increasing the speed of the Rozière Balloon will instead destroy it, forcing Passepartout to take another journey.