Mixing Fused Silica into Zircon Face Coat: What a Published Foundry Trial Found
Zircon flour holds its place in the face coat for good reasons — it is thermally stable, highly refractory and produces reliable casting results. It is also one of the more expensive powders in the shell room. For a foundry pouring stainless steel and similar precision parts, the question is always the same: can we cut shell cost without paying for it in surface quality?
A published production study asked that question directly. It tested a primary slurry made from a blend of zircon flour and fused silica powder, and compared it against the standard 100% zircon face coat. This article summarises what the study found, where the savings came from, and where the limits sit.
- The blend. Primary slurries using 70% zircon + 30% fused silica, and 50% zircon + 50% fused silica, in a colloidal silica (silica sol) system.
- The result. The 50/50 blend reportedly cut the primary refractory powder cost by roughly 40%, with surface roughness that matched — and in the tested conditions exceeded — the 100% zircon face coat.
- The catch. Zircon and fused silica have very different densities, so the two powders had to be kept evenly mixed with a homogenising additive.
What the trial actually tested
The study ran in a silica-sol shell-making line and compared three primary-slurry recipes side by side:
| Recipe | Zircon flour | Fused silica powder | Role in the trial |
|---|---|---|---|
| A | 100% | 0% | Control — the standard face coat |
| B | 70% | 30% | First mixed slurry |
| C | 50% | 50% | Second mixed slurry — highest fused silica share |
The two mixed recipes were evaluated on real castings, not just in a lab cup, so the outcome reflects how the slurry behaves under production conditions.
Why fused silica was put into the slurry
Fused silica brings several properties that make it an interesting candidate for shell systems. It has high SiO2 purity, very low thermal expansion and strong resistance to thermal shock. At around 2.2 g/cm³ it is also far lighter than zircon, which reduces the refractory mass sitting in the slurry system.
The study also pointed to a second benefit: fused silica can improve shell collapsibility, which makes the spent shell easier to remove after casting. That matters where a foundry wants faster knockout without hurting dimensional stability or surface roughness.
For the broader picture on how fused silica behaves in a shell, see our note on the three questions shell rooms ask and the investment casting application page.
The real problem: two powders that don't weigh the same
The obvious challenge is density. The two powders are very different:
| Material | Approximate density | Practical effect in slurry |
|---|---|---|
| Zircon flour | ~4.5 g/cm³ | Heavier — tends to settle |
| Fused silica powder | ~2.2 g/cm³ | Lighter — half the mass for the same volume |
Left to themselves, the two powders can separate inside the slurry. The study addressed this with a homogenising additive that helped keep the zircon and fused silica evenly distributed, improving slurry stability, flowability, coating performance and dispersion.
What the production runs showed
The first run used a zircon + fused silica mixed slurry and reported good flowability and a smooth coating on the wax patterns. The slurry spread evenly with no obvious build-up, and the sanding layer stayed uniform after stuccoing. Castings from this run showed surface roughness and finish comparable to the standard 100% zircon face coat, along with stable dimensions and easier shell removal.
A second run pushed the fused silica share to 50%. This 50/50 primary slurry was tested on stainless steel elbow-flange castings and a pump-body casting. The slurry stayed workable after several days of slow stirring. After standing, it showed some sedimentation, but the zircon and fused silica did not visibly separate — which the study took as evidence that the homogenising additive was doing its job.
The finished castings passed dimensional inspection with good surface roughness. According to the study, the casting surface quality reached, and in the tested production conditions even exceeded, the level of the 100% zircon face coat.
How much did it save?
The headline number is cost. Working from the 50/50 zircon + fused silica system, the study reported that the cost of the primary refractory powder could be reduced by about 40%.
That number applies to the face-coat powder itself, not to the whole shell. And it is not a licence to swap half the zircon out of every recipe. Slurry formulation, silica-sol quality, powder particle size, the homogenising additive, mixer design, drying conditions and the casting alloy all change the final result. What the study demonstrates is that fused silica powder is a practical option for a foundry that wants to use less zircon while keeping casting quality.
Six checks before you try it
For a foundry considering a zircon + fused silica mixed slurry, the study points to a short list that should be reviewed carefully before any full replacement:
- Match the fused silica particle size to the slurry system.
- Control the density difference through formulation and mixing, not by guesswork.
- Use a suitable homogenising additive to stop the two powders separating.
- Check slurry viscosity and coating weight before going into production.
- Run a trial casting before a full production switch.
- Judge surface roughness, dimensional accuracy and shell-cleaning behaviour together — not one in isolation.
What this means when you buy fused silica
For investment casting, fused silica powder is more than a cost-saving filler. It can also contribute low thermal expansion, thermal-shock resistance and easier shell removal. Combined correctly with zircon flour, it gives a foundry a way to build a more cost-effective face-coat system without giving up surface finish.
Chemsize supplies fused silica powder, flour, sand and grains for investment casting, refractories and other industrial uses. Particle size, purity, chemistry and document support can be discussed against your slurry system and production requirements. For batch-level chemistry and particle-size data, start with our sample COA and the fused silica flour & powder range.
Reference
- Jing Zongliang, Wu Yaozu, Sun Xin. Application of the Zircon Powder, Fused Silica Powder Mixed Slurry to Investment Casting Production. Foundry, 2015, Vol. 64, No. 6, pp. 517–520.
This article is a technical review based on the published production study above and is provided for general reference. Actual slurry performance varies with zircon flour, fused silica particle size, the silica-sol system, additive choice, mixing equipment, drying conditions and the casting process.
Published by Chemsize · Lianyungang, China. Last reviewed September 2026.