A buyer pulls a newly cast silver band from its flask, brushes off investment, and sees three tiny dark pits on the inside of the shank. The instinct is to send the whole lot back. The bench jeweler looks at the shape of the pits and says which type they are, whether they are fixable, and whether the rest of the tree is affected. Casting porosity is not a single defect — it is three different failure modes with three different remedies. Here is how we read it.

The three types of porosity

Gas porosity shows up as round, smooth holes 0.1–0.5 mm across, scattered randomly on the piece. It comes from trapped air or released gas as the metal solidifies. Shrinkage porosity looks like irregular, angular voids near the thickest part of a casting — usually inside a heavy bezel or at the junction where a ring shank meets a shoulder. Microporosity is a fine, sponge-like texture under the surface, invisible until polishing opens it up. The physics are summarized in the porosity reference on Wikipedia. Porosity specific to 925 silver is detailed in 925 silver casting porosity defects.

What triggers each type

Gas porosity usually means the investment was not properly vented, the burnout schedule was too short, or the casting temperature was too high — gas comes out of solution as the metal cools. Shrinkage porosity means the sprue feeding the thick section froze before the part was full; the fix is a wider sprue and a feeder at the heavy point. Microporosity on silver often points to alloy inconsistency — reused grain mixed with fresh without a proper flux. We address alloy mixing in casting grain and alloy consistency, and the sprue layout that causes shrinkage is in casting sprues and investment defects.

How we fix it at the bench

Small gas pits on a flat surface are repaired with a small weld or a punch-and-burnish, then re-polished. We allow up to three repaired pits per ring before we reject the piece. Shrinkage porosity in a structural area (a prong base, a shank shoulder) is not repairable — the piece is remade because welding over a void weakens the joint and the surrounding metal is already compromised by the solidification pattern that created it. Microporosity that appears after polishing is treated by grinding back 0.1–0.2 mm and re-polishing; if it recurs on the second pass, the whole lot is recast rather than chased further.

How we keep it out of production

We X-ray spot-check 5 percent of every production tree, and 100 percent of gold pieces over 10 grams. Burnout schedules are logged per flask so a bad shift can be traced back to a single oven cycle. On reorders we freeze the sprue layout and the burnout curve from the approved first article, which drops porosity reject rate from a typical 5–8 percent to under 2 percent. The outgoing inspection that catches these pits is described in jewelry order QC checklist.

What X-ray tells us that a loupe cannot

A loupe sees the surface. An X-ray sees inside the wall. On a 10-gram gold signet, we X-ray because a shrinkage void inside a heavy shoulder will not show until the ring is worn and the shoulder splits. On silver we sample at 10 percent because the alloy is cheaper and the consequences lower, but on any piece over 15 grams we X-ray 100 percent. The most common X-ray finding is a small void at the junction where a prong meets the head — usually from an undersized sprue. We log the X-ray, mark the tree, and adjust the sprue on the next run. A void larger than 1 mm in a structural area is a reject, not a weld repair, because the surrounding metal is already weakened by the solidification pattern that created it.

Are porosity holes safe to ship?

Closed pits under 0.2 mm on a non-contact surface are acceptable. Open holes on a ring's inner comfort-fit are not, because they trap skin moisture. We reject those.

Can porosity be fixed by polishing alone?

No. Polishing opens pits, it does not close them. If a pit shows up after polish, the casting itself is defective.

Does plating hide porosity?

Rhodium or silver plating can cover a shallow pit for a few weeks, but the pit telegraphs through as plating wears. We do not plate over porosity.

What porosity rate is normal?

Under 3 percent for silver on a well-laid tree. Above 5 percent, we pause and review the burnout curve before continuing the run.

Send us a photo of a defective casting and we will identify the porosity type. HOLYCOME casts, weld-repairs, and re-polishes under one roof — ask about our custom manufacturing program and send your CAD plus target reject rate for a same-day feasibility read.

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.