Almost every matting agent, anti-settling additive and free-flow aid in a modern coating plant is some form of silicon dioxide. Yet "silica" covers two materials that behave very differently: crystalline silica, the mineral in quartz sand, and amorphous silica, the engineered powder that goes into your can. For formulators and buyers the distinction is not academic. It decides how transparent a matte clear looks, how the powder is classified on the safety data sheet, and which numbers on a datasheet actually matter.
What "amorphous" actually means
Both forms are built from the same unit: a silicon atom bonded to four oxygen atoms in a tetrahedron. The difference is how those tetrahedra connect.
- Crystalline silica (quartz, cristobalite, tridymite) stacks the tetrahedra in a repeating lattice with long-range order. X-ray diffraction shows sharp, characteristic peaks.
- Amorphous silica links the same tetrahedra in a random three-dimensional network with no long-range order — closer to a frozen liquid than a crystal. X-ray diffraction shows only a broad, featureless hump.
Because there is no lattice to cleave along, amorphous silica can be built as porous particles with very high internal surface area. That open, disordered structure is exactly what makes it useful as a functional additive — and what separates it from ground quartz used as a filler.
Natural vs. synthetic amorphous silica
Amorphous silica occurs in nature (diatomaceous earth, opal, volcanic glass, rice-husk ash), but natural sources often contain or convert into crystalline phases, especially when calcined. For coatings, the material of choice is synthetic amorphous silica (SAS): made from a chemical process, controlled batch by batch, and free of detectable crystalline silica.
SAS comes in four main families, each produced by a different route:
| Type | How it is made | Structure | Typical coating use |
|---|---|---|---|
| Precipitated silica | Sodium silicate + acid under controlled pH and temperature; filtered, washed, dried and milled | Aggregated particles, broad pore range | Matting agents, anti-settling, reinforcement |
| Silica gel | Sodium silicate + acid forming a continuous gel, then washed, aged and milled | Rigid particles with a defined internal pore system | Matting agents (especially high-clarity), desiccants |
| Fumed (pyrogenic) silica | Flame hydrolysis of silicon tetrachloride in a hydrogen–oxygen flame | Non-porous nano-sized primary particles fused into chain-like aggregates | Thixotropy, rheology control, anti-settling |
| Colloidal silica (silica sol) | Growth of discrete particles in a stabilised aqueous dispersion | Dense, spherical nanoparticles in liquid | Hardness, scratch and block resistance, binders |
All four are amorphous. What differs is particle size, porosity and surface chemistry — and those are what you are really buying.
Why coatings rely on amorphous silica
1. Transparency in clear films
Amorphous silica has a refractive index of about 1.45–1.46, close to that of common acrylic, polyurethane and UV-curable binders (roughly 1.47–1.50). When the particle and the cured resin bend light almost equally, the particle scatters little light inside the film, so a matte clear stays clear. Quartz sits higher, around 1.54–1.55, which is one reason ground crystalline fillers turn clear coats cloudy. On dark solid wood this difference is the line between "water-white matte" and "milky matte."
2. Porosity drives matting efficiency
A matting agent works by creating micron-scale roughness at the very top of the dried film. Porous amorphous particles are light (low tapped density) and voluminous, so each gram supplies many particles and a lot of surface texture. That is why coating-grade silicas are specified by pore volume (typically 1.6–1.8 mL/g for matting grades) and BET surface area (often 250–350 m²/g), not just by particle size. More pore volume means more matting per gram — and more binder absorbed, which is the viscosity trade-off covered in our matting-agent selection guide.
3. A reactive, tunable surface
The surface of amorphous silica is covered with silanol (Si–OH) groups. They make the particle hydrophilic and allow it to form hydrogen-bonded networks — useful for fumed-silica thixotropy, but also the root cause of hard sediment when bare particles pack together in low-viscosity waterborne paints. Because silanols are reactive, the surface can be modified with wax or organic treatments to soften settling, improve touch and reduce water pickup at high humidity. We explain the mechanism in eliminating hard sediment in waterborne matte coatings.
4. Chemical inertness and purity
High-purity SAS (typically ≥ 99% SiO₂ on a dry basis) is chemically inert in coating formulations, colourless and stable to UV and heat. It does not yellow, react with the binder or catalyse side reactions — a quiet but important property in clear and light-coloured finishes.
Safety and regulatory status: why "amorphous" matters on the SDS
The health picture of the two forms is very different, and this is the question procurement and EHS teams ask most often.
- Respirable crystalline silica is a recognised occupational hazard. It can cause silicosis, and IARC classifies it as carcinogenic to humans (Group 1). In the United States, OSHA limits respirable crystalline silica to 50 µg/m³ as an 8-hour time-weighted average.
- Synthetic amorphous silica is treated differently. IARC places amorphous silica in Group 3 (not classifiable as to carcinogenicity in humans), and SAS is generally not classified as hazardous under GHS/EU CLP. Inhaled amorphous particles are cleared from the lungs and dissolve slowly rather than persisting like crystalline particles.
"Not classified" is not the same as "no precautions." Any fine powder is a nuisance dust: use local exhaust ventilation when charging dispersers, wear suitable dust masks and eye protection, and follow your local occupational exposure limits for inhalable and respirable dust. Always work from the supplier's current SDS for the specific grade.
When you qualify a supplier, ask for evidence that the grade is free of crystalline silica — typically an X-ray diffraction result showing no crystalline peaks above the detection limit. Synthetic amorphous silicas are commonly registered under CAS numbers such as 7631-86-9 or 112926-00-8 (silica gel and precipitated silica, crystalline-free); fumed silica is often listed as 112945-52-5. The CAS number tells you the family; the XRD result tells you the batch is clean.
How to read an amorphous silica datasheet
Once the material is confirmed amorphous, these are the parameters that predict performance in a coating:
| Parameter | What it tells you | Why it matters in the can |
|---|---|---|
| Particle size (d50) and distribution | Median size and width of the particle population | Matting strength, clarity, touch; a coarse tail causes seeds and haze |
| Pore volume (mL/g) | Internal void volume of the particle | Matting efficiency per gram vs. viscosity build |
| BET surface area (m²/g) | Total accessible surface | Binder and additive adsorption, surface activity |
| Oil absorption | Liquid uptake under a standard test | A practical proxy for porosity and rheology impact |
| Surface treatment | Untreated, wax-modified or organically modified | Settling behaviour, hand-feel, compatibility with water or solvent |
| Loss on drying / pH | Moisture content and surface acidity | Storage stability and compatibility with pH-sensitive systems |
| Crystalline silica content | XRD verification | Classification, worker safety, customer compliance |
Different suppliers publish different subsets of these numbers, which makes like-for-like comparison harder than it should be. We look at that gap in what the datasheet doesn't tell you about your benchmark matting agent.
Where Censil grades fit
The Censil matting series is high-purity synthetic amorphous silica (CAS 112926-00-8, no crystalline silica), engineered specifically for waterborne wood and plastic coatings. All grades share a 1.6–1.8 mL/g pore volume and 250–280 m²/g surface area, with tight particle-size control from 4.8 to 9.0 μm and wax-modified surfaces for soft settling and a smooth touch. For water-white clears on dark wood, Censil660 is the lead grade; for universal semi-matte to matte, Censil361C; for low-gloss tactile finishes, Censil1650A. The full grade table maps each one to its imported benchmark.
Key takeaways
- Amorphous and crystalline silica share a chemical formula but not a structure — and that structure drives both performance and safety classification.
- Synthetic amorphous silica (precipitated, gel, fumed, colloidal) is the standard in coatings because it is porous, transparent in clear films, inert and tunable at the surface.
- Refractive-index matching with the binder is why amorphous silica keeps matte clears transparent.
- Respirable crystalline silica is a Group 1 carcinogen; synthetic amorphous silica is IARC Group 3 and generally not classified as hazardous — but always handle it as a fine dust.
- Ask suppliers for XRD evidence of crystalline-free material, then compare grades on particle size, pore volume, surface area and surface treatment.