A wholesale buyer sends over a CAD-approved ring, the wax room sprue-stems it onto a tree, the investing team vacuums it into a 3-inch flask, and the casting technician walks it into the kiln. Twenty minutes later the buyer is asking why two out of twelve pendants came back with a bubble on the top surface. In nine of ten cases the problem was not the alloy, the vacuum, or the water ratio. It was the burnout schedule: the ramp, the holds, and the soak temperature that decided whether the wax left cleanly or left carbon behind.
What a burnout schedule actually does
Lost-wax casting is not simply melting the wax out. The flask sits inside a gypsum-bonded investment that must cure, then be brought up to temperature slowly so the wax melts, drains, vaporizes, and oxidizes before the metal is poured. If you rush the ramp, trapped wax expands faster than the investment can vent, and you get cracked investment, flash, or a flask that pops open. If you overshoot the peak temperature, the investment breaks down, sulfur leaches back into the gold, and you get what casters call hot-short grain growth on the surface.
Our working schedule for a standard 3-inch flask tree of sterling and 14k yellow pieces looks like this: room temperature to 350°F (177°C) at 200°F per hour, hold 45 minutes to let wax drain through the sprue base; then ramp to 800°F (427°C) at 250°F per hour, hold 60 minutes to fully vaporize residual wax; then ramp to 1350°F (732°C) at 300°F per hour and soak 90 minutes for burn-out of carbon residue. Platinum work pushes the final soak to 1700°F (927°C) for 2 hours. These numbers are not magic; they are the curve that matches lost-wax casting best practice for gypsum investment.
Why the first hold at 350°F matters most
Most new casters want to jump straight to 1350°F. That is the mistake. Wax melts around 140 to 190°F and runs liquid long before the flask reaches any serious temperature. If the kiln climbs past 350°F before the wax has drained out through the sprue hole, the remaining wax boils, expands, and carbonizes into a black film that the 1350°F soak will not fully remove. That carbon shows up on the casting surface as a dark, rough fisheye skin that pickling will not lift, and it is the leading cause of the porosity buyers blame on the alloy. Pair this with investment powder water ratio vacuum and casting porosity causes remedies, and you can trace most first-run defects back to the first two hours of the furnace.
How schedule changes with flask size and metal
A 5-inch flask tree holding forty pendants needs a longer first hold, not a hotter peak. The wax load is heavier, and the center of the flask lags the outer wall by 80 to 120°F. We extend the 350°F hold to 90 minutes and the 800°F hold to 90 minutes on the larger flask. For platinum, the investment formula itself changes (refractory rather than gypsum bonded), and the burnout curve is different again. Buyers who send a CAD file should ask the factory which alloy and which flask size the tree will go into; the burnout answer is part of the quote, not a hidden step.
How to read a bad burnout from the casting
Pull the flask after pour. When you break it open, the investment should be pale off-white or light gray at the sprue end. If you see black soot on the inside wall, the wax did not leave cleanly. If the casting surface is matte and pitted near the sprue, you have carbon residue. If the investment is crumbled and pinkish at the top, the peak temperature ran too high and the investment is collapsing. These are the same tells we cross-check when a buyer asks us to compare vacuum vs centrifugal casting investment for a reorder.
FAQ: Jewelry burnout schedule
How long does a full burnout cycle take?
For a standard 3-inch flask in sterling or 14k gold, plan on 8 to 10 hours from cold start to pull. Platinum runs 12 to 14 hours. Shortening the cycle is the single most common cause of surface porosity.
Can I reuse a flask that did not fully burn out?
No. Once the wax has carbonized into the investment, reheating will not remove the carbon. The flask must be broken out, the metal cleaned, and a new tree invested. We flag any run where the burnout log shows a missed hold and re-cast rather than ship.
Why does my gold come out with a pink or purple cast after burnout?
That is sulfur attack from overheated gypsum investment, usually a peak above 1450°F (790°C) held too long. It is the same defect family described in casting burnout flask temperature hold. Lower the peak or shorten the soak.
Does 3D-printed resin wax change the schedule?
Yes. Resin-based burn-out wax needs a longer first hold at 350 to 500°F because resin outgasses more slowly than injection wax. We add 30 minutes to the first hold on any tree printed on our 3D printed wax resolution fit line.
If you are specing a custom run and want the burnout log attached to the quote, send us the alloy, piece count, and ring size range. Our custom manufacturing team will lay out the flask tree, confirm the schedule, and return the casting timeline before any wax is printed.
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.
