A caster pulls a silver flask out of the kiln, pours the tree, and the castings come out black and sooty at the sprue attachment. The wax vendor blames the burnout schedule, the burnout schedule blames the investment, and the brand's production manager wants to know who reruns the 60 pieces. Carbon residue in a lost-wax flask is almost always a burnout problem, not a wax problem. The ramp rate, the hold minutes, and the flask temperature at pour are three numbers that decide whether the tree comes out clean or black.
Why burnout schedule is the carbon source
Wax and 3D-printed resin vaporize at different temperatures. Wax burns off cleanly around 300 to 600 C; resin needs a slower ramp above 300 C or it leaves a carbon skeleton that the metal cannot displace. If the kiln ramps too fast, the wax boils before it vents, leaves a carbon film on the investment wall, and that film turns into sooty black at the sprue joint. The lost-wax casting reference on Wikipedia covers the chemistry; the bench numbers below are what we hold on our own flasks.
Ramp, hold, and peak temperature per alloy
For 925 silver trees we run a two-stage ramp: 250 C hold for 30 minutes to melt wax out the sprue base, then 5 C per minute up to 730 C (1345 F) and a 60-minute soak. For 14K gold we push the peak to 760 C and hold 90 minutes. For brass and bronze we drop the peak to 700 C because the higher zinc content picks up sulfur from the kiln atmosphere. Our earlier write-up on investment powder water ratio and vacuum covers the mix side; this is the kiln side.
Flask temperature at pour
Pulling a flask straight from a 730 C kiln and pouring silver at 900 C is too hot a delta; the metal runs too thin and causes casting porosity at the thinnest details. We pull the flask and let it drop to 550 C (1020 F) for silver and 600 C (1110 F) for 14K gold before the pour. That 5 to 8 minute cooling window is where most mistakes happen: casters get impatient and pour hot. We use an infrared pyrometer held 30 cm from the flask bottom to confirm the number before we lift the crucible.
Bench numbers we hold
Wax tree pre-heat: 250 C, 30-minute hold. Ramp: 5 C/min. Peak: 730 C silver, 760 C gold, 700 C brass. Soak: 60 minutes silver, 90 gold, 45 brass. Flask at pour: 550 C silver, 600 C gold. The connection between sprue design and tree layout is that a well-vented tree lets wax leave cleanly, which shortens the burnout by 15 minutes. Our note on casting grain and alloy consistency also links burnout temperature to surface finish; too hot a peak frosts the silver.
Resin vs wax burnout differences
3D-printed resin (Castable, Freed, and similar) does not melt and run out the sprue the way wax does. It decomposes in place, which is why we run a slower 2 C/min ramp between 300 and 500 C on resin trees, and we add a 20-minute extra hold at 500 C. Resin also leaves a thin ash that the air vent pulls out during burnout; if the kiln vent is blocked, the ash redeposits on the flask wall and shows up as a rough surface. We clean the kiln vent after every five resin runs. Wax trees do not need this slow ramp, and mixing a wax tree with a resin tree in the same kiln cycle is the most common cause of mixed surface quality on a single rack.
FAQ: Casting burnout and flask temperature
Can I shorten the burnout cycle to save time?
By 10 to 15 minutes on small flasks, yes. Below 45 minutes total hold you start leaving carbon. We never run a 2-inch silver flask under 90 minutes total cycle.
Why does my silver come out frosty?
Peak temperature too high. Drop the 730 C hold to 700 C and make sure the flask has a 50 mm air gap from the kiln walls.
Does flask temperature differ for gold vs silver?
Yes. Silver pours from 550 C flasks, 14K yellow gold from 600 C, white gold from 620 C. Pouring gold from a cold flask causes cold shuts.
What if I overshoot the hold and the flask sits at 730 C for two hours?
The investment starts to lose strength and the flask may crack during pour. Let it cool to pour temperature and inspect the flask wall for hairline cracks before you lift the crucible.
Send us your wax trees for a logged burnout and pour. We tag every flask with ramp rate, peak, hold minutes, and pour temperature, and we photograph the first shakeout. Small-batch OEM custom manufacturing starts at 30 pieces per design; send the trees 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.
