Five Joints, One Locket: Sequencing Solders and Saving the Hinge
You are building a sterling silver locket: a 32 mm body from 1.0 mm sheet, a hinge made from 1.6 mm outside-diameter tube with a steel pin, a bezel for a 14 × 10 mm oval cabochon, and a bale to take a 2 mm chain. The bench holds hard, medium and easy silver solder (flowing roughly at 780–800 °C, 740–760 °C and 650–700 °C), a butane-air torch, a charcoal block, borax and a fluoride-containing flux. On the first attempt the bezel, soldered on last with easy solder, dropped off while the bale was being attached, and the hinge knuckles softened and went oval, so the steel pin no longer fits. There is one spare hinge tube left. The customer has paid a deposit, the stone is not yet set, and the delivery date is fixed because the locket is a christening gift.
1. Set out a soldering sequence for the locket, naming the solder grade for each stage and justifying each choice by function, not only by temperature.
Read the worked answer
The governing principle is function first, temperature second: the joint that carries the most load or needs reaming gets the hardest solder; the joint that is easiest to make last and carries least load gets the softest. Then check that each stage is at least ~40–50 °C cooler than the previous one.
| Stage | Joint | Solder | Reason |
|---|---|---|---|
| 1 | Body seam / body closed up | Hard (780–800 °C) | Deep, structural, must survive all later heating |
| 2 | Hinge knuckles to body | Hard | Takes wear, must be reamed and hold the pin, must not soften again |
| 3 | Bale | Medium (740–760 °C) | Second-highest duty: it hangs the whole piece from a chain, so it should not be the softest joint, but it is small and easy to localise heat on |
| 4 | Bezel | Easy (650–700 °C) | Lowest stress, must be fitted to the actual stone after all metalwork, and easy solder lets it be closed with the shortest, coolest heat |
So the previous failure is a planning failure: easy solder was used on the bezel but the bale was then soldered at a temperature that reflowed it. Correct answer is to put the bezel last, or to put the bale at stage 1/2 with hard solder and the bezel at stage 4 with easy.
2. Explain why the bezel came away and why the hinge knuckles went oval, and state the physical measures you would use to prevent both.
Read the worked answer
Why the bezel came away. Easy solder's flow range sits below the temperature reached when the whole piece is brought up to solder the bale. The bezel joint was re-heated above its own flow point and simply ran. It was a reflow, not a failed joint.
Why the knuckles went oval. Sterling anneals around 600–650 °C, so every soldering operation softens the thin-wall tube. Soft 1.6 mm tube, heated unsupported, relaxes and creeps out of round; the steel pin was then fitted into a bore that had changed shape.
Measures.
- Localise heat: smallest suitable flame, play it on the joint, work on a charcoal block that reflects heat back, keep the rest of the piece away from the flame.
- Heat sinks and shields: water-soaked tissue or clay, metal blocks, or yellow ochre / a borax-graphite paste painted on finished joints as a stop-off.
- Support the bore: insert a close-fitting steel or charcoal pin, or binding wire, through the knuckles while soldering so the tube cannot collapse; then ream all knuckles to a matched bore with a broach after the last soldering operation and fit the pin last.
- Ream, don't force: never drive the pin into an oval knuckle.
- Order of work: all soldering, then pickle, then ream, then polish, then set the stone.
Safety on the same job: fluoride-containing flux gives off harmful fumes when overheated and is aggressive to skin and eyes - work with extraction, keep the flux off skin, never let it boil dry, use copper tongs in pickle (steel in pickle copper-plates the silver), and pickle before handling.
3. The customer asks whether the stone can be set next week. List the checks and operations you would complete before the setting bench sees the piece, with the reason for each.
Read the worked answer
Setting is the last operation, so everything that could damage the stone or disturb the setting must already be finished.
- Every joint tested. Flex the hinge, hang a weight on the bale, check the body seam under light pressure. A repair after setting means heat near the stone.
- Hinge reamed and pin fitted. Confirm the knuckles are round, the pin enters and withdraws freely, and the knuckle ends are dressed flush before the stone goes in.
- Bezel fitted to the actual stone, not the drawing. Measure the cabochon with calipers and compare with the bezel bore; a 14 × 10 mm nominal stone is often 0.2 mm different. If the bezel needs adjustment, that is a soldering job.
- Pickle and firestain removed, all emery stages complete, pre-polish done. Polishing mops and compound must not be used near a set cabochon.
- All soldering temperatures behind you. Once the stone is in, no torch near the piece.
- Document the stone: dimensions, weight, any existing faults, photographed. This is the protection for both parties.
- Confirm the setting method and the pressure plan with the setter - for a cabochon, a bezel that is pushed progressively with a polished pusher, not hammered.
Only then does the piece go to the setting bench, and the final polish is done around the stone, not over it.