I agree with Jim about the extra vents. Though I'd add that while I
put in several vents around the piece especially opposite the (5mm)
feeder sprue, my vents are tiny - made by pushing a piece of 0.8mm
wire through (a piece of stg earwire). Fumes escape but very little
metal goes up the vents. Very little clean-up afterwards.
Maybe you could try a really short feeder sprue, say no longer than
10mm. That's where the metal cools off a lot, going down the feeder
made of room temp clay.
If this is your first casting, why not do yourself a favour and try
easier shapes, to have some fun and to get some experience with the
Auckland NEW ZEALAND