A plating house sends back a run of 120 sterling pendants pitted and grainy. The casting room swears the grain was fine, the pour was on target, and the burnout schedule matched last month's good batch. When the team pulls the last investment ring off the tree, the problem is obvious: the slurry was mixed thin, the vacuum pull was short, and the flask went into the kiln with two hours left on the hold instead of four. Investment powder is the cheapest material in the casting room and the one most often mixed by feel. It is also the reason a good wax tree turns into pitted, finned, or scumming metal.

What investment powder actually does in the flask

Traditional gypsum-bonded investment (the type used for silver and gold, not for platinum) is plaster of Paris fused with crystalline silica, usually cristobalite. The plaster sets around the wax and binds the silica together; the silica expands on heating to counteract the shrinkage of the metal as it cools. When you mix it too thin, you under-dose the silica, the mold wall is weak, and gas from the burning wax escapes through the wall as rough surface grain. When you mix it too thick, trapped air cannot escape, you get nodule-like pinhole investment bubbles on the casting, and the flask cracks under steam pressure during burnout.

The standard water-to-powder ratio for most fine-jewelry investments is 38 to 42 mL of water per 100 g of powder, measured by weight, not by bucket. A 1 kg mix uses roughly 400 mL of room-temperature water (20 to 25 C / 68 to 77 F). Warmer water shortens setting time by 30 to 40 seconds per 5 C, which is why summer flasks set faster and why casting rooms keep a water chiller on the slurry station. For the underlying chemistry of the process, see the Lost-wax casting reference on Wikipedia.

Mixing, vacuum, and flask vibration

Dry the powder into the water under a slow-speed mixer for 60 seconds before you turn the whisk up, then run 90 seconds at full speed. Pull the vacuum to 25 to 28 inches of mercury (85 to 95 kPa) for 90 seconds, tapping the mixing bowl on the bench as the slurry thickens. If the surface does not subside into a smooth, bubble-free dome, pull another 30 seconds. Vibration after pouring matters as much as the vacuum: a vibrating flask table running at 60 Hz for 90 seconds drives bubbles off the wax surface before the plaster sets. Skip vibration and the bubbles cling to the finest detail of a 3D-printed wax, which shows up as micro-pitting on the finished surface.

Why burnout schedules tie back to the mix

A 2-inch flask (50 mm) for a silver tree wants a 450 C (850 F) hold of 90 to 120 minutes, followed by a ramp to 730 C (1345 F) and a 30-minute soak. A thin mix produces more carbon residue because the silica wall cannot breathe, which is why short holds and thin mixes stack into the same pitting. If you keep getting casting porosity that polishing cannot fix, measure the slurry with a graduated cylinder for one full week before you change the alloy. Most alloy problems turn out to be water-weight problems. Our write-up on sprue and investment defects covers the failure patterns you see after shakeout, and the connection between 925 silver porosity and under-vacuumed investment is the same root cause.

Quick bench numbers to hold the line

We run a 39 mL water per 100 g powder mix on silver trees, 41 mL on gold, and pull vacuum for a full 120 seconds on any flask carrying openwork or filigree. Flask temperature at pour is 550 C for silver, 600 C for 14K yellow gold. Record the water weight, powder weight, and vacuum time on every flask tag. When a bad tree comes off the line, the tag tells you in ten seconds whether the slurry was the problem, no meetings required.

FAQ: Investment powder for jewelry casting

Can I reuse investment powder that has been sitting open?

No. Gypsum-bonded powder picks up atmospheric moisture within two weeks of opening, which shortens setting time and weakens the mold wall. Seal bags with a desiccant pack and rotate stock on a three-month cycle.

Why does my surface come out frosty even when the cast looks clean?

Frosting is usually a silica-ash reaction from too-high burnout temperature. Drop the peak hold by 20 C and make sure the flask air gap in the kiln is at least 50 mm on all sides.

How thick should the slurry look before pouring?

Like heavy cream: it ribbons off the spatula and settles back into the bowl in three to five seconds. If it sits on top like pancake batter, it is too thick; if it runs like milk, it is too thin.

Does vacuum time change with flask size?

Yes. A 3-inch flask (75 mm) carries more slurry and wants a full 120-second pull, while a 1.5-inch ring flask can run 75 seconds safely.

Bring your own waxes, CAD files, or reference designs to our casting bench. We run gypsum-bonded investment in 1 kg to 50 kg batches, tag every flask with water weight and vacuum time, and send pre-polish photos before we finish. Small-batch OEM custom manufacturing starts at 30 pieces per design; send the tree and we will quote the pour.

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.