When a cast ring comes back pitted, cracked or half-filled, the instinct is to blame the pour. In practice, the pour is usually the last step in a chain that broke hours earlier. Casting defects are predictable, and the factories with low reject rates treat them as an engineering problem: each defect has a known cause, and each cause has a control that sits upstream of the machine.
Gas porosity: the small round bubbles
Tiny, round, smooth-walled bubbles scattered through a casting are gas porosity. The gas comes from the investment shell itself - trapped moisture, binder outgassing, or resin that did not fully combust during burnout. The cure is upstream: a proper burnout schedule that ramps the flask to 600-700 degrees C and holds it long enough for the wax and residue to ash out completely before the hot flask comes to the casting bench. Rushing the hold to fit more flasks in a shift is the classic way to introduce porosity that only shows up after setting and polishing.
Shrinkage porosity: the irregular cavities
Shrinkage cavities look different - irregular, angular, clustered near the thickest part of a piece. They appear because metal shrinks as it solidifies, and if the thick section has no liquid metal feeding it from a nearby sprue, it pulls material from itself and leaves a void. The cure is sprue design: the thickest mass on the piece (a heavy gallery, a wide bezel, a knot in a bracelet) gets its own direct feed, and the main sprue stays liquid long enough to feed the part. A factory that has to weld up shrinkage pits at the bench after casting is compensating for a sprue layout it should have fixed in CAD.
Mis-runs and short feeds
A mis-run is a casting that stopped partway, leaving a sharp, cold flow front where metal solidified before filling the cavity. Causes are almost always one of three: the flask was too cold, the metal was too cool when poured, or the sprue was too thin to carry the fill. Vacuum and centrifugal casting reduce mis-runs dramatically compared to gravity pour, which is why any serious jewelry line invests in vacuum-assist rather than relying on a crucible and a tilt.
Hot tears and cracking
Hot tears are irregular cracks that form while the metal is still at a high-temperature, brittle stage, usually at a corner where two sections cool at different rates. A sharp internal angle on a CAD file is a textbook hot-tear starter. Designers are asked to round internal corners to at least 0.3 mm radius, and flasks are allowed to cool slowly in insulation rather than being quenched for complex pieces.
Investment inclusion and metal fins
Rough, sand-grain-like particles embedded in the casting surface are investment inclusions. They happen when the investment shell cracks during burnout or when the flask was not vibrated sufficiently while being poured, leaving voids in the mold wall that metal rushes into. Thin, delicate fins of metal sticking off a casting - metal flare - are the same problem seen from the other side: the investment cracked, and metal seeped into the crack. Both are signs of under-poured investment, weak shell thickness, or an over-aggressive burnout ramp.
Metal temperature and alloy composition
Overheated metal absorbs gas from the atmosphere and releases it as it cools, which is the root cause of much gas porosity. A controlled pour holds gold or silver just above its melting point, not ten minutes past it. Alloy composition matters too: sterling silver with slightly too much copper is more prone to hot shortness, the brittle cracking that appears at high temperature. Reliable factories buy already-alloyed grain from a refiner rather than mixing their own, because consistent alloy means consistent defect rates, and because the refiner's assay paperwork simplifies hallmarking.
How a factory catches defects early
The cheapest place to reject a bad casting is the moment it comes out of the flask, before any bench time is spent. Inspection at that stage looks for visible porosity, complete fill, flash, and dimensional accuracy against the CAD. High-jewelry and piece-priced work gets X-ray or micro-CT scans for internal porosity that the eye cannot see. Rejecting at the cast stage costs the price of one sprue; shipping a porous piece to a retail customer costs a warranty claim, a photograph, and a return shipment.
What to ask on your next RFQ
Ask a factory what their reject rate at the cast stage is, and which defects they cover under replacement. A shop that gives you a single flat quality line is not running a controlled process. A shop that gives you a percentage, names the burnout hold temperature, and walks you through their sprue review step is running the kind of line that reorders well. See our facility page for the equipment and inspection gates behind our own numbers.
The 92.5% silver composition and properties of sterling silver are described in the Wikipedia entry on sterling silver.
Frequently Asked Questions
How do I verify a factory before placing a bulk order?
Request a production video, business license, export records and a paid sample run. A factory that accepts a 50-piece test order is usually a real manufacturer.
What payment terms do you offer?
30% T/T deposit with the balance against the bill of lading. This protects both sides on first orders and can move to net terms after a track record.
What is your normal reject rate?
3–5% under 10x loupe inspection for cast and set pieces. Defective units are replaced free in the next shipment.
Where is the factory located?
Wuzhou, Guangxi, China—the core lab-grown and moissanite manufacturing cluster. Shipments go via Guangzhou or Shenzhen ports.
