Three Questions Shell Rooms Ask About Fused Silica

When a foundry considers introducing fused silica into an investment casting shell system, three questions usually come up:

1. How much can we use with zircon flour?
2. If knockout becomes easier, will the green shell be too weak to handle?
3. If permeability increases, will molten metal leak through the shell?

All three are reasonable questions. The key is to separate what happens during shell building, firing, pouring and cooling.

In this article, "fused silica" means fused silica flour or stucco used as a refractory—not the colloidal silica binder.

For the broader context, see the investment casting application page, the fused silica flour and powder range, and the sand and stucco range.

1. How much fused silica can we mix with zircon flour?

There is no universal percentage.

The appropriate ratio depends on:

• Alloy and pouring temperature
• Casting size, wall thickness and metal head
• Primary and backup coat design
• Binder type and solids loading
• Flour and stucco particle sizes
• Number of shell layers
• Dewaxing and firing cycle
• Required hot strength and knockout behaviour

In many shell systems, zircon is retained in the primary coat because the face layer controls surface finish and metal–mould interaction. Fused silica is then introduced into intermediate or backup coats, where its low density, low thermal expansion and easier knockout can provide the greatest benefit.

This is usually safer than changing the face coat and the backup system at the same time.

Weight replacement is not volume replacement

Zircon has a true density of approximately 4.6 g/cm³, compared with about 2.2 g/cm³ for fused silica.

Therefore, replacing zircon with fused silica kilogram for kilogram changes the refractory volume significantly. As a rough volume calculation, removing 10 kg of zircon would require only about 4.8 kg of fused silica to occupy a similar solid volume.

However, this is only a volume balance—not a slurry formula.

Particle-size distribution, particle shape, surface area and binder demand will also affect:

• Slurry viscosity
• Slurry density
• Settling behaviour
• Drainage
• Coating weight
• Drying time
• Green strength

For this reason, a blend should be adjusted according to measured slurry behaviour rather than flour weight alone. There is also no universal percentage of wetting agent, suspension aid or other additive that works for every zircon–fused-silica blend.

A practical trial method

A controlled evaluation can follow four steps:

1. Keep the existing shell system as the control.
2. Introduce fused silica into one selected intermediate or backup layer.
3. Measure slurry viscosity, density, coating weight, drying behaviour and shell strength.
4. Pour comparable trial assemblies and examine surface condition, cracking, runout, inclusions, dimensional results and knockout effort.

If the first trial is successful, the fused silica content or number of fused silica layers can then be increased gradually.

Where face-coat chemistry needs special attention

For titanium and other highly reactive alloys, silica-containing face systems are generally avoided or carefully controlled because they can react with the molten metal and contribute to surface contamination and alpha-case formation.

High-manganese steels also require careful evaluation because manganese oxide can react with silica and form lower-melting manganese silicates.

These are metal–mould chemistry issues. They cannot be solved simply by reducing the fused silica percentage without reviewing the complete face-coat system.

2. If knockout is easier, will the shell break during handling?

Not necessarily. Green shell strength and knockout behaviour belong to different stages of the process.

During shell building

Before dewaxing and firing, the shell has not experienced the high temperatures required for fused silica to devitrify. Its handling strength is mainly controlled by:

• Binder condition
• Refractory packing
• Coating thickness
• Drainage
• Drying between coats
• Temperature, humidity and airflow
• Intercoat adhesion
• Shell geometry and handling practice

Changing from zircon to fused silica can still affect green strength because it changes slurry rheology and particle packing. It must therefore be tested, but easy knockout does not automatically mean a weak green shell.

If a shell cracks during dipping, drying or handling, the first question should be exactly when and where the crack appeared. Incomplete drying, excessive local coating thickness, poor drainage, weak intercoat bonding and difficult geometry are often more useful starting points than the name of the refractory.

During firing and pouring

Fused silica has very low thermal expansion, which helps reduce thermal stress during dewaxing, mould heating and temperature changes.

At sufficiently high temperatures and hold times, part of the fused silica may devitrify and form cristobalite. The amount formed depends on the complete firing and pouring history as well as impurities and other materials in the shell.

The shell must still have sufficient fired and hot strength to withstand handling, preheating and the pressure of the molten metal.

During cooling and knockout

After casting, the cristobalite transformation during cooling produces a significant dimensional change and promotes microcracking inside the ceramic structure.

This reduces the residual strength of the cooled shell, helping it break away from the casting during knockout.

That is why a properly designed shell can have both:

• Sufficient strength during building and pouring
• Lower residual strength after cooling

"Strong before pouring" and "easy to remove after casting" are not contradictory requirements.

3. If permeability and porosity increase, will metal leak out?

Gas permeability is not the same as an open crack.

A permeable ceramic shell contains a network of small, tortuous pores that allows displaced air and gases to escape during pouring. These pores do not normally create a direct opening large enough for molten metal to flow through.

Runout usually requires a structural defect such as:

• A crack formed during drying, dewaxing or firing
• Incomplete coating coverage
• A locally thin section
• Poor intercoat bonding
• Damage during handling
• Insufficient hot strength
• Excessive deformation or creep under metal pressure

Metal penetration is another related but different issue. It can be affected by face-coat integrity, pore size, metal pressure, pouring temperature, alloy wetting behaviour and metal–mould reaction.

Permeability must therefore be balanced with surface quality and shell strength, but normal gas permeability should not be confused with structural leakage.

It is also important not to assume that fused silica automatically produces higher permeability. Shell permeability is controlled by the complete construction, including:

• Flour particle-size distribution
• Stucco size and shape
• Slurry retention
• Layer thickness
• Number of coats
• Drying and firing conditions

For large, heavy or high-temperature castings, the shell should be checked for hot strength and creep resistance. A common approach is to retain a chemically compatible primary coat, introduce fused silica selectively into intermediate or backup layers, and reinforce the backup system where required.

The principle behind all three questions

A ceramic shell does not have only one type of "strength".

It has:

• Green strength during building and handling
• Fired strength after dewaxing and firing
• Hot strength during preheating and pouring
• Residual strength after casting and cooling

Fused silica can affect each stage differently.

The same distinction applies to porosity. Fine connected pores can provide useful gas permeability, while a crack or incomplete layer creates a runout path. They are not the same type of opening.

Once the process is divided into these stages, it becomes much easier to identify whether a problem comes from the raw material, slurry control, shell construction, drying, firing or casting conditions.

What your fused silica supplier should ask

Before recommending a grade or particle size, a supplier should understand:

1. What alloy is being poured?
2. What are the pouring and mould-preheat temperatures?
3. What is the maximum casting weight and metal head?
4. What refractory and binder are currently used in each layer?
5. How many coats are applied, and what stucco sizes are used?
6. What are the firing temperature and holding time?
7. At which stage and location does any cracking occur?
8. What result is the foundry trying to improve: cost, knockout, thermal shock, permeability or dimensional stability?

A TDS can confirm chemistry and particle size, but it cannot determine a complete shell formula by itself. The safest approach is to select an appropriate material, establish a measurable baseline and make controlled plant trials.

Fused silica for investment casting shells

Chemsize supplies fused silica flour, sand and stucco in different purity grades and particle-size distributions, with a batch COA for each shipment.

If you share your alloy, current shell construction, binder system, firing cycle and casting size, we can help select suitable fused silica grades and particle sizes for evaluation with your shell-room technician.

For batch-level chemistry and particle-size data, request our sample COA and technical data sheet.

The objective is not simply to replace one flour with another. It is to find a controlled starting point that can be measured, tested and improved.

Technical references

1. Breneman, R. C. and Halloran, J. W. — "Effect of Cristobalite on the Strength of Sintered Fused Silica Above and Below the Cristobalite Transformation" (Journal of the American Ceramic Society).
2. Bertão, I. et al. — "Influence of Quartz and Cristobalite on the Mechanical Behaviour of Ceramic Shell Materials for Investment Casting" (International Journal of Metalcasting).
3. Frueh, C. et al. — "The Effect of Silica-Containing Binders on the Titanium/Face Coat Reaction" (Metallurgical and Materials Transactions B).

These references are listed so engineers can verify the underlying mechanism. The present article is not peer-reviewed and should be read alongside the supplier’s technical data sheet.

Published by Chemsize · Lianyungang, China. Last reviewed September 2026.