Whilst it is possible to reuse old gold by melting it down, we absolutely do not recommend it. Gold is an alloy, this means that pure gold is mixed with other metals to make it strong, malleable and workable.

The re-melting process diminishes these alloys, causing increased porosity, brittleness, oxide inclusions and surface problems. The kind of solder used and it’s variable amount in the original piece, also causes problems as it mixes with the melted gold, resulting in a lack of durability, causing faults with re-casting and cracks when reworked.

Old gold is generally incompatible with modern alloys and techniques as it mostly contains nickel, now illegal to use in modern jewellery, as it causes skin problems and allergic reactions.

Even if you do have the correct alloy, it is unlikely to give a good casting result.
All this make it an impractical and often futile process. Re-using old gold is perceived to be a money saving exercise, however the resulting much reduced quality item at least equals and mostly exceeds the cost of making the piece from new gold.