Guys,
I do not do my own casting, but I do make the patterns. I get sand castings done to my wood/epoxy/JBweld patterns at 18th century Hardware, Derry, Pa. You can also use an original part for the pattern. You coat the original with a nice thick coat of 5 minute epoxy - about 1/32 inch thick on all surfaces. Use this as the pattern for sand casting. The epoxy gives the extra thickness for the casting shrinkage and the filing/polishing. After you are done, put the pattern in a can of acetone, the epoxy disappears leaving the original surface undamaged - including the old patina.
Always get 2 pieces cast at least, then you can use one for the pattern. The wood patterns have a short life.
Jim