The flask has been filled with a plaster like substance. Jewelers call this substance investment. We got the term from the Dentists who pioneered the modern lost wax investment casting method in New York City in 1928.

There are probably groups from Moscow, Beijing, and Mumbai, India who simultaneously pioneered this technique. I've just been educated by the New Yorkers and so tell the truth according to them. :-)

Before we poured in the plaster, we put four plastic grids onto the sides of the flask. This will help remove gases from the investment when it comes time to pour the molten metal.    The investment forms a hard shell around the wax preserving its shape right down to the scratches made by sandpaper.