I work with RTV rubbers of various hardness, Bondo auto body putty, modelers wax , modelers clay, plaster and wood or combinations there of.
For example the J.I Case mascot, ( book ends ) is one part of a fence finial.
I had the original finial, which to make a copy made a box, half filled with modelers clay - pushed the finial into the clay to the parting line then spraying some mold release onto the finial. Before pouring in your RTV make some dibits into your clay which will become registration marks when you pour the second half.
Now that I have the two halves poured and cured in RTV, I now make wax copies of the finial which get set into plaster. This is the lost wax method of casting.
The fish down rigger mold was purchased from eBay, which I spent some time filling in the imperfections and polishing, then making sure the edge's would release from the sand when removed with out pulling the sand away from the cope and drag box. When using an item such as described you lose approximately 1% of the originals size.
Making a lead fishing weight does not have to be that precise.
The McCulloch logo is from an old power saw fuel tank, cut away, filled with bondo on the backside before using it as a pattern directly into the casting sand - I use Petrobond which is an oil based sand. Notice the fine detail.
Those nicks in my pattern should have been filled with auto body filler and sanded out before casting this is aluminum.
FYI aluminum has many alloys, my favorite's for casting are diesel and automotive pistons, hydraulic pumps- this is Deltallloy, next in line automotive cylinder heads - blocks and rims - high in silicon. Marine aluminum from outboard engines is another good one - high in copper content.
I also use a commercial degassing agent which comes in tablets, proportioned to 100 lb pours. Since I my largest crucible holds 20 lbs, I break the tablets to suite the pour at hand.
The finished RTV mold of the fence finial with a wax copy ready for the plaster. The tapered wood attachment once set in plaster is easy to remove - this becomes my pour hole or sprue
The big J.I. Case eagle was taken directly from the firebox door of a Case Steam Tractor. With this RTV mold I'll either use plaster or concrete to make my positive mold - for my sand pattern.
Once I have a good copy of the large eagle in aluminum this will get cleaned, polished for my permanent mold.
Hope this give you some direction, there are plenty of home foundry sites on the internet.