What is a sprue in jewellery casting?
By Jared James · Last updated 21 May 2026
Definition
In jewellery casting, a sprue is a thin rod or channel of wax or metal that connects the model piece to the main body of the casting system, providing a pathway for molten metal to flow into the mould cavity. When the casting is removed from the investment mould after cooling, a small stub of solid metal remains attached at the sprue entry point. The jeweller clips and files this away during the finishing process, leaving a clean surface on the piece. The word is borrowed from the wider metalcasting and plastics industries where the same feature exists.
Frequently asked questions
- Why is a sprue necessary in casting?
- Molten metal needs a direct path to flow from the crucible into the mould cavity. The sprue provides that entry channel, and it also gives the metal somewhere to go as it contracts slightly during cooling, which helps prevent porosity in the finished piece.
- What happens to the sprue after casting?
- The metal sprue is cut away from the finished piece with snips or a saw, and the small nub left behind is filed and polished smooth. The removed sprue metal is usually saved and recycled into future casting.
- Is the sprue visible on a finished piece of jewellery?
- No. The jeweller removes it during finishing and polishes the area where it was attached. A well-finished casting shows no trace of the sprue entry point.
Designing a ring
Talk through sprue with us
Tell us what you have in mind, even if it is only a budget and a piece of jewellery type, and we will help you weigh up the options. We reply to every enquiry, usually within one business day.
Contact the studio