Lost wax casting. A tecnique as old as sin. It has been used in various ways as long as metal has been cast. It is mosty used today by goldsmiths and other jewllery makers. But they tend to use special materials for molds, vacuum and centrifugal machinery for better results. The benefits are that you can make advanced 3d things with patterns on all sides and with finer detail. The downside is that once you crack upp the mold it is used and a new one has to be made. The molds can be heated for better flow of metal giving thinner things and more detail. They are also less forgiving when it comes to gas pockets and often needs centrifugal work and/or extra air channels for best result.
It all starts with a wax original. Sometimes I cast a rough shape to begin with, ohter times just start with a block. Then with a sharp instrument that is dipped every now and then in hot water carve the details. More time spent now means better result but there are lots of historic exaples of roughly cast originals that have adorned afterwards. Then you mix horse dung, sand and clay and build a little container around your item. Make sure you make funnels to ease the pouring. Two or more openings is also good to allow the air to escape. First slowly dry your molds. Then gradually heat them and finally put them in the owen making them as hot as the metal itself. It is important to have everything under control when the pouring is about to start. Make sure your mold is sturdy! One can also put the mold in sand in a bucket and then swing to make sure the metal is pressed out into the details.
With the mold made with just the right mix it can be opened quite easily, so that exessive force will not hurt the item whitin. Then some polish and you have a nice new pin like the picture in the middle.
I sometimes make rubber molds to cast wax models so that I can have all steps shown side by side without the toil of making everything from scratch again. But the first item is made more or less the same way as above. I sometimes aslo use a vacuum machine for massproduction but when it comes to medieval reproductions I always cast the thing with only period techniques first regardless of how many attemts it takes to prove to myself that it could be made this way. Then when I am satisfied with the result I can cheat if I want to make more of the same.