Sometimes a tinkerer needs to accomplish something that literature says requires special lab equipment…
There is very little information online about correcting silicon carbide (SiC) discolouration outside of an industrial laboratory. After some experimentation in our glass studio, we successfully cleared the yellow tint from SiC using a modified atmospheric annealing process. Here is how we did it:
The Problem: Lattice Defects from Overheating during Polishing Turn White Moissanite Yellow
Yellowing in SiC is typically caused when clear or ‘white’ Moissanite gets too warm during faceting or polishing. I believe this was a 6H or 4H stone. To clear these, the material must be heated to a range where the atoms can “reset,” but doing this in open air causes the surface to oxidize and char.
The Custom Setup
We bypassed the need for an industrial vacuum furnace by repurposing high-heat glassblowing tools and cannabis paraphrenalia to create a controlled inert environment:
- The Chamber: A high-purity quartz banger. Quartz was chosen for its ability to withstand extreme temperatures without cracking or reacting.
- The Atmosphere: We flowed high-purity Argon through a flow meter into the bottom neck of the banger. We kept the flow very low—just enough to maintain a positive-pressure “blanket” that displaced all oxygen.
- The Heat: An oxygen/propane glassblowing torch.
The Process
We took multiple tries at heating and cooling the stone. Three times of raising to glowing hot, briefly holding, and then cooling. There’s no way for us to know that those three heats were as good as one big heat – but this is simply what happened and worked in our little setup.
- Slow Warm-up: We brought the temperature up gradually to avoid thermal shock.
- Visual Heat Management: While we used a thermal camera and a thermocouple to monitor the start, our equipment maxed out at 1000°F. We pushed significantly hotter, using the bright incandescent glow of the crystal as our indicator that we had reached annealing temperatures.
- The Reducing Flame: To cool the piece, we used a large, bushy reducing flame. This provided a gentle thermal taper while adding an extra layer of protection against oxidation.
- Cycling: We repeated this heat-and-cool cycle three times.
The Result
By the end of the third cycle, the yellowing had completely dissipated. By maintaining a strict Argon environment and controlling the descent with a reducing flame, we achieved laboratory-grade clarity using studio equipment.
With precise atmospheric control and a good “feel” for heat, the yellowing of SiC can be fixed without industrial-scale machinery.