Below I summarize mechanisms, materials, process behavior, practical numbers, strengths/weaknesses and selection guidance.

Silica-sol shells (colloidal silica)

What it is and how it works
Silica-sol shells use a colloidal suspension of sub-micron silica particles as the binder. The first coats (very fine wash) use the colloid to carry ultrafine stucco that records detail; subsequent coats build thickness and are consolidated by drying and high-temperature firing (sintering) that produces dense, strong shells.

Key characteristics (practical):

• Surface fidelity: best available — as-cast Ra commonly ~0.6–3 µm with fine wash.
• Thermal stability / firing: shells can be consolidated at 600–1,000°C (shop practice varies with stucco). High-temperature firing increases shell strength and thermal shock resistance.
• Vacuum/inert compatibility:excellent — silica-sol shells are compatible with vacuum and inert-atmosphere pours and are the usual choice for titanium, nickel and cobalt superalloys.
• Permeability control: can be tuned by stucco grading and firing to give controlled venting for high-value, tight castings.
• Contamination sensitivity:high — colloid stability is upset by ionic contamination (salts, metal fines) and organics; slurry and plant cleanliness are critical.
• Typical first-coat stucco: sub-10 µm fused silica, zircon or zirconia for reactive interfaces.
• Typical use cases: aerospace turbine components, superalloys, vacuum-poured titanium, medical implants, precision small parts.

Process controls to watch

• colloid solids & zeta potential / pH; filtration to remove fines; careful drying schedules to avoid cracking; controlled high-temp sintering for final strength.
• Because silica-sol shells are often less forgiving to contamination, maintain dedicated mixing lines and rigorous QC (solids, viscosity, gel/dry time).

Strengths

• Highest surface and detail fidelity; best chemical/thermal resistance; suited to vacuum/inert pours.

Weaknesses

• Higher material and process cost; longer shell preparation and firing cycles; more sensitive to shop contamination.

Water-glass shells (sodium-silicate)

What it is and how it works
Water-glass shells use an aqueous sodium (or potassium) silicate solution as binder. Coats gel to a silica-like network by CO₂ gassing or chemical hardeners (acid salts), producing a rigid ceramic shell when combined with graded refractory stucco.

Key characteristics (practical):

• Surface fidelity: good for general engineering — as-cast Ra typically ~2.5–8 µm depending on wash and stucco.
• Firing: usually stabilized at ~400–700°C; shells are not sintered to the same extent as silica-sol systems.
• Vacuum compatibility:limited — not ideal for vacuum/inert pours or the most reactive alloys.
• Permeability / venting: generally good for steels/irons; permeability tends to be coarser than optimized silica-sol shells.
• Curing method:CO₂ gassing (rapid gelation) or acid hardeners — fast, robust set on the shop floor.
• Contamination sensitivity: moderate — ionic contamination affects setting and gel uniformity but water-glass is generally more tolerant than silica-sol.
• Typical first-coat stucco: fine fused silica; zircon can be used for improved surface protection.
• Typical use cases: valve bodies, pump housings, large steel/iron parts, marine hardware, general industrial castings.

Process controls to watch

• slurry specific gravity / alkalinity; CO₂ dose and coverage; staged drying to avoid shell cracking; control of water removal during dewax/burnout to prevent steam blistering.

Strengths

• Lower material cost; fast, controllable cure; robust for large shells and harsher shop environments.

Weaknesses

• Coarser finish and less suitable for vacuum/inert pours or reactive superalloys.

Hybrid shells (silica-sol or zircon inner coat + water-glass outer coats)

What it is and how it works
A common economic compromise: a premium inner coat (silica-sol or zircon/zirconia wash) is applied first to capture detail and create a chemically resistant barrier, then water-glass outer coats are built to give bulk strength at lower cost.

Key characteristics (practical):

• Surface fidelity & chemical barrier: inner silica-sol/zircon gives near-silica-sol surface quality and helps prevent metal-shell reactions at the metal interface.
• Cost & handling: outer water-glass coats reduce total silica-sol usage and make the shell more robust for handling and large sizes.
• Vacuum compatibility: improved vs pure water-glass (thanks to inner coat) but still not as ideal as full silica-sol shells — useful for many stainless and some nickel alloys if melting/pour atmospheres are controlled.
• Typical uses: valve bodies with high-quality wetted surfaces, medium-value turbine parts where some vacuum compatibility is needed, applications where cost vs performance must be balanced.

Process controls to watch

• compatibility between inner and outer coats (adhesion), differential drying stresses at the coat interface, and matching firing profiles so the inner coat achieves its purpose without the outer coats cracking.

Strengths

• Balanced cost/performance; captures much of silica-sol benefit at lower total cost.

Weaknesses

• Adds a layer of process complexity; hybrid shells require careful procedure and QC to avoid delamination or differential shrink stresses.

Practical parameter ranges & checkpoints

These are practical shop-level targets (approximate) to use as starting points for trials — each foundry will refine to its recipes.

• Silica-sol wash (first coat): very fine stucco (<10 µm); slurry solids tuned for film formation; final shell firing 600–1,000°C for consolidation.
• Water-glass wash (first coat): fine fused silica or zircon <10–20 µm; gel by CO₂ in seconds→minutes; final shell firing 400–700°C.
• Slurry control metrics to log: specific gravity / solids %, pH (silica-sol often slightly acidic to neutral; sodium-silicate strongly alkaline), viscosity, gel/dry time.
• Permeability check: airflow/pressure test on coupon shells before pour for critical parts.
• Green strength test: simple ring-crush or flex test on representative coupons prior to dewax to ensure handling robustness.
• Cleanliness: filtration of slurries and prevention of ionic contamination for silica-sol; strict housekeeping for both systems.

Selection guidance — how to choose the shell system

Use the following decision rules as a starting algorithm:

1. Alloy & melt atmosphere

• Titanium, Ni/Co superalloys, vacuum or inert pours → Silica-sol (or full silica-sol route).
• Carbon/low-alloy steels, ductile/gray iron, larger copper parts → Water-glass.
• Stainless steels, duplex, many pump/valve parts → Either; choose based on surface requirement and cost.

2. Surface finish & detail

• Highest detail / lowest Ra → Silica-sol.
• Moderate finish acceptable → Water-glass or hybrid if you need a good face finish but cheaper outer coats.

3. Part size & handling

• Very large shells / heavy handling → Water-glass (or hybrid with robust outer coats).
• Small precision parts → Silica-sol.

4. Budget / cycle time

• Tight cost and faster cure cycles → Water-glass.
• Total part value justifies longer cycle and higher materials cost → Silica-sol.

5. Risk / criticality

• Fatigue/pressure-tight/aerospace → prefer silica-sol + vacuum + HIP if needed.
• General industrial components → water-glass is often perfectly acceptable.

Common failure modes specific to shell chemistry

• Silica-sol: gel/sedimentation issues or poor shell strength from ionic contamination; slow drying can cause microcracking if oven profiles aren’t controlled.
• Water-glass: shell cracking from too-rapid drying or uneven CO₂ cure; metal washout can occur if first coat is weak or melt superheat is excessive.
• Hybrid shells: delamination at inner/outer interface if cure or drying profiles are mismatched; ensure adhesion and staged drying.

Practical takeaway

Pick silica-sol for the highest surface quality, vacuum compatibility and reactive/high-temperature alloys; pick water-glass for economical, robust shells on larger steel/iron/copper castings; and use a silica-sol/zircon inner coat with water-glass outer coats (hybrid) when you must balance surface chemistry and cost.