Fumed Silica in Cosmetic Formulation: Rheology, Mattifying & Grades

A foundation batch passed every lab panel for matte finish and viscosity. Then a 2,000 kg production lot read glossy on the gloss meter and thin on the viscometer. The formula had not changed; the raw material had. The new drum was a precipitated hydrated silica (CAS 10279-57-9), bought against a purchase spec that read only “Silica,” in place of the hydrophobic fumed grade the formula was built on. Both are legal to label “Silica.” They behave nothing alike in a formulation.

The short version: Fumed silica is synthetic amorphous silicon dioxide (CAS 112945-52-5; INCI “Silica”) used in cosmetics as a multifunctional formulation additive: a rheology modifier and thixotrope that thickens and suspends, an anti-caking and free-flow agent in powders, and a high-surface-area oil absorber that lowers film gloss (a physical matte and soft-focus optic). The INCI name “Silica” hides the surface chemistry that decides performance. Hydrophilic untreated grades disperse in the water phase; hydrophobic silane-treated grades disperse in oils and stabilize water-in-oil systems. Choose by the continuous phase first, then tune loading to the grade’s BET surface area, and write the CAS, the surface treatment, and the surface-area range into the purchase spec so a cheaper “Silica” cannot satisfy it.

What fumed silica is, and how it differs from other cosmetic silicas

Fumed silica is synthetic amorphous silicon dioxide, SiO2, molecular weight 60.08 g/mol (PubChem CID 24261). It is made pyrogenically, by flame hydrolysis, which fuses nanoscale primary particles into branched, chain-like aggregates that cluster into agglomerates. A formulator breaks those agglomerates down under shear during dispersion. That physical structure, not any melting or dissolving step, is what builds viscosity and takes up oil in a film. The fumed registry number is CAS 112945-52-5; the generic silicon dioxide entry is CAS 7631-86-9, and the cosmetic INCI name is simply “Silica.”

On a Certificate of Analysis (CoA) the material may appear as silicon dioxide, silica, silicic anhydride, or amorphous silica, all pointing to the same compound. None of those names tells you the process route or the surface treatment, and that is exactly the gap a substitution slips through. Read the CAS and the data sheet, not the trade name.

The physical constants tell you how the powder behaves in process. Fumed silica is an odorless white solid, insoluble in water, true density 2.2 g/cm3, melting point near 1,710 C (3,110 F) per PubChem. It survives any temperature a cosmetic process reaches and does not volatilize. It also arrives as a fluffy, very low-bulk-density powder, which drives the freight and dust handling covered below.

The functional effect comes from surface area rather than mass: most of a fumed silica particle is surface, so a small loading does the work of oil absorption and thickening. For the chemistry behind the powder, see the fumed silica production-process guide.

This is a different material from wet-process precipitated hydrated silica (CAS 10279-57-9, INCI “Hydrated Silica”), and from colloidal silica and concrete silica fume. The three are routinely confused on spec sheets; our comparison of fumed, colloidal, and silica-fume types sorts out which is which.

Precipitated silica is grown in solution and dried, leaving a porous, harder particle suited to abrasion and bulk liquid uptake. Fumed silica is built in a flame, leaving an open aggregate network suited to thickening and oil control in the formulation. For a cosmetic spec, the rule is to name the process, not just the element.

Hydrophilic or hydrophobic: which grade for which phase

The most common procurement error with silica is treating “Silica” as one SKU. The families read alike on a label and diverge sharply in oil and water.

Hydrophilic fumed silica carries reactive silanol (Si-OH) groups on its surface. Those groups hydrogen-bond to each other and to water, so the powder disperses into the water phase and builds a temporary three-dimensional network that thickens serums, gels, and emulsions.

Hydrophobic fumed silica treated with trimethoxycaprylyl-silane has those silanols capped by a longer-chain silane, so it disperses into oils, stabilizes water-in-oil systems, and structures anhydrous balms and sticks. Silica dimethyl silylate is capped with a dimethyl (DDS) treatment and is the common choice for oil and silicone phases, including formulas built around polydimethylsiloxane (PDMS) and other silicones.

The water-affinity mechanism is detailed in our note on fumed silica’s hydrophobic properties and water dynamics.

The trade-off is concrete and worth naming. Hydrophobic grades give cleaner water repellency and hold structure in oil-rich and anhydrous systems, but they resist wetting and can over-thicken if the let-down shear is too low. Hydrophilic grades disperse more easily into water yet adsorb ambient moisture in storage, which shifts the effective dose between the lab batch and the plant.

Surface area compounds both effects: a higher-area grade thickens more per gram and takes up more oil per gram, while raising viscosity build and dust generation. Pick by the continuous phase first, then tune loading and dispersion energy, and treat a grade swap as a reformulation rather than a like-for-like change.

What fumed silica does in a cosmetic formulation

The EU CosIng database lists Silica’s functions as absorbent, opacifying, anti-caking, bulking, viscosity-controlling, and abrasive. Each maps to a measurable formulation function, not a skin-treatment claim.

Rheology and suspension. This is the workhorse role. In a serum, gel, or emulsion, fumed silica builds shear-thinning, thixotropic structure that suspends pigments, exfoliant beads, and actives without a gum, then thins again under shear so the product pumps and spreads. The same network controls slump in sticks and balms. Our explainer on fumed silica rheology and viscosity control covers the dispersion mechanics.

Oil absorption and matte optic. The porous aggregate network takes up oils and emollients within the formulated film. Measured as the grade’s oil-absorption capacity, this lowers surface gloss and produces a matte finish as a physical surface optic rather than any action on skin. Dose low, typically a low single-digit percentage of the total formula, and confirm against your own gloss and sensory panel; over-dosing over-absorbs the oil phase and can read as a dry, draggy payoff at application.

Soft-focus light diffusion. Soft-focus is an optical effect, the “opacifying” function in action. The submicron aggregates scatter and diffuse visible light, softening specular glare on the applied film. It modifies finish rather than color, so treat it as a partner to pigments and confirm the effect under both daylight and indoor light, where diffusion reads differently.

Anti-caking and flow. A small loading coats larger pigment and filler particles, stops them bridging into lumps, and keeps pressed and loose powders dosing evenly through a filling line. The bulking function adds structure to powder formats without shifting shade.

In oral care, the harder precipitated grade serves as a mild dentifrice abrasive and thickener rather than fumed silica; the two are not interchangeable, which is one more reason to specify the process route.

How much to use, and how it is dispersed

Loading and dispersion decide whether a grade delivers what the data sheet promises. Fumed silica works at low addition levels, often a low single-digit percentage of the formula, and the right number depends on the surface area of the grade and the phase it has to thicken. Set the dose against a viscosity, gloss, and stability panel, not against a number copied from another formula.

Dispersion is where most batches go wrong. Add the powder slowly into the phase it will live in, give it enough high shear to break the agglomerates apart, and let viscosity build before judging the result. Under-dispersion reads as grit and uneven thickening; pushing shear too far in some systems can break the thixotropic network you built. A grade with the wrong surface treatment for the phase will refuse to wet in at all, which is the fastest way to catch a substitution at the mixer.

What to question on the Certificate of Analysis before you buy

A CoA for cosmetic silica needs more scrutiny than a single assay line. Six items decide whether a lot is fit for the formula.

1. Crystalline silica content. The cosmetic form is amorphous; crystalline quartz and cristobalite are the IARC Group 1 concern. Require a crystalline-content figure and the test method, and reject lots that report it as “not determined.” 2. CAS and surface treatment, not just “Silica.” Confirm whether the lot is hydrophilic fumed (CAS 112945-52-5), caprylyl-silane treated hydrophobic (CAS 68909-20-6), silica dimethyl silylate (CAS 68611-44-9), or precipitated hydrated silica (CAS 10279-57-9). This single check prevents the substitution that opens this guide. 3. BET surface area. It drives thickening efficiency and oil uptake, so put a target range on the Technical Data Sheet (TDS). Lot-to-lot drift in surface area shows up directly as a viscosity change in your batch. 4. Loss on drying. Hydrophilic fumed silica adsorbs atmospheric water; a high moisture reading shifts both the effective dose and the rheology tuned in the lab. 5. Heavy metals and microbiological limits. For lip and oral-care formats, require food-additive-grade documentation and heavy-metal caps, because the product can be ingested. 6. Tapped or bulk density. It governs dosing accuracy, dust generation at weigh-up, and how much freight volume a tonne consumes.

Lock these into the purchase specification, not just the inbound QC check. A spec that names the CAS, the surface treatment, and the BET range cannot be quietly satisfied by a cheaper, compliant-looking silica. The substitution that opens this guide is rarely sabotage; it is a buyer or supplier reading “Silica” as one commodity and sourcing the lowest-priced line that matches the label. Writing the spec to the CAS, the process, and the surface-area range removes the ambiguity that makes the swap look compliant.

Regulatory and occupational-safety status

Silicon dioxide is cleared as a direct food additive at up to 2% by weight under 21 CFR 172.480, including its long-standing use as a food anticaking agent. That listing is the reference point for formats that cross the ingestion boundary, such as lipstick and toothpaste. Cosmetic silica is not pre-approved by FDA the way a color additive is, so the CoA and your own safety assessment carry the burden of proof; confirm status for your application and jurisdiction.

The carcinogenicity question turns on crystallinity, and the distinction matters for documentation. Crystalline silica is classified by IARC in Group 1; synthetic amorphous silica, the fumed and precipitated cosmetic forms, is not placed in that group. The aggregated GHS records on PubChem CID 24261 mix crystalline registrations into the same entry, which is why most amorphous registrants instead report not meeting GHS hazard criteria. This is a sourced classification point, not a finished-product safety claim.

The hazard to manage on the plant floor is respirable dust. The powder is irritating to eyes, skin, and the respiratory tract on contact, so follow the NIOSH Pocket Guide for amorphous silica at weigh-up and dispersion. Synthetic amorphous silica is a registered substance under EU REACH, and registrants largely classify it as not meeting GHS hazard criteria, but that classification does not waive dust controls on the floor. The current Safety Data Sheet for the grade you buy governs handling.

Packaging, handling, and bulk freight

Fumed silica’s low bulk density is the defining logistics fact. A tonne occupies far more volume than its mass suggests, so road and ocean freight are usually volume-limited, and a full container weighs out light. Budget for that when you compare landed cost across origins; a lower price per kilogram can lose to cube on the freight line. The hydrophobic, surface-treated grades are even lighter per unit volume, which widens the gap.

The material ships in multi-wall paper or poly-lined bags rather than drums, because the fluffy powder resists dense packing. Store it dry and sealed, since hydrophilic grades adsorb atmospheric moisture that later destabilizes viscosity. At weigh-up and dispersion the dust is the exposure to manage: use local exhaust, sealed transfer where the process allows, and the respiratory protection NIOSH recommends for amorphous silica.

Where each grade fits across beauty and personal care

Across the beauty and personal care portfolio, the grade follows the format and the continuous phase. Foundations, BB creams, and primers use hydrophilic or hydrophobic fumed silica for matte optic and soft-focus diffusion. Pressed and loose powders use it for anti-caking and even flow on the filling line. Serums and gels use it to suspend pigments and actives with shear-thinning structure.

Anhydrous balms, sticks, and water-in-oil emulsions call for a hydrophobic grade so the silica wets into the oil phase; silicone-continuous systems and color dispersions in emollient esters pair well with silica dimethyl silylate.

The selection logic stays consistent across every format: match the surface chemistry to the continuous phase, set the loading against a viscosity, gloss, and stability panel, and lock the BET range plus the crystalline-content method into the purchase spec. A formula built on the right grade fails quietly when a compliant-looking substitute slips through procurement, so the spec is the control point.

Grades and bulk sourcing from RawSource

RawSource supplies cosmetic-grade fumed silica from US stock for qualified bulk volumes: hydrophilic fumed silica (CAS 112945-52-5), the caprylyl-silane treated hydrophobic grade (CAS 68909-20-6), and silica dimethyl silylate (CAS 68611-44-9), each with CAS, CoA, and grade documentation so your purchase spec names the surface chemistry, the BET range, and the crystalline-content method instead of a bare “Silica.”

Send the target continuous phase, viscosity and gloss behavior, and the format, and request a sample to qualify the grade on your own system. RawSource serves beauty and personal care formulators in bag and bulk quantities; submit an RFQ for current availability and lead time.

Frequently asked questions

Should I use hydrophilic or hydrophobic fumed silica?

Choose by the continuous phase of the formula. Hydrophilic fumed silica (CAS 112945-52-5) carries an untreated silanol surface and disperses into water and polar phases, so it thickens and suspends in serums, gels, and the water phase of emulsions. Hydrophobic, silane-treated grades disperse into oils and silicones, stabilize water-in-oil systems, and structure anhydrous balms and sticks. A grade matched to the wrong phase will not wet in.

What is the difference between Silica, Silica Silylate, and Silica Dimethyl Silylate on an INCI list?

“Silica” is untreated (hydrophilic) silicon dioxide. Silane-treated hydrophobic grades carry a treated surface: the trimethoxycaprylyl-silane treated grade (CAS 68909-20-6) is capped with a longer-chain silane, while Silica Dimethyl Silylate (CAS 68611-44-9) is treated with dimethyldichlorosilane (DDS). The INCI name alone does not tell you the surface chemistry, so specify the CAS and the treatment on the purchase spec.

How does fumed silica give a matte and soft-focus finish?

Both are physical effects. The high-surface-area aggregate network takes up oils and emollients in the formulated film, which lowers surface gloss (the matte optic). Separately, the submicron aggregates scatter and diffuse visible light, softening specular glare (the soft-focus optic). Neither is an action on skin; both should be confirmed on your own gloss and appearance panel.

How does BET surface area change the formulation?

Surface area is the main performance lever. A higher-BET grade thickens more per gram and absorbs more oil per gram, but it also builds viscosity faster and generates more dust at weigh-up. Because performance scales with surface area, set a target BET range on the spec and treat a change in surface area as a reformulation, not a like-for-like swap.

What loading of fumed silica should I start with?

Fumed silica works at low addition levels, often a low single-digit percentage of the formula, with the exact figure set by the grade’s surface area and the phase being thickened. Disperse it slowly under adequate high shear, let the viscosity build, and dial the dose against a viscosity, gloss, and stability panel rather than copying a number from another formula.

Is cosmetic fumed silica the same as the crystalline silica IARC lists as a carcinogen?

No. Cosmetic fumed and precipitated silicas are synthetic amorphous silica, which is not placed in IARC Group 1; the Group 1 classification applies to crystalline silica (quartz and cristobalite). Require a crystalline-content figure and test method on the CoA to confirm the amorphous form, and handle the powder as a respirable dust under NIOSH guidance regardless of form.

Editorial note. This article is general technical guidance for cosmetic formulators, R&D, and procurement. Grade behavior, loading, rheology, gloss, and stability depend on your specific phase system, surface treatment, surface area, and process, and must be validated on your own system; the Certificate of Analysis governs the grade you buy. Synthetic amorphous silica is handled as a respirable dust that can irritate eyes, skin, and the respiratory tract, so review the current Safety Data Sheet (SDS) and use appropriate engineering controls and PPE before handling. Finished-cosmetic safety substantiation, INCI labeling, and regulatory compliance (for example FDA cosmetic requirements and EU Cosmetic Regulation 1223/2009) are the formulator’s responsibility, and food-additive status (21 CFR 172.480) must be confirmed for ingestible-adjacent formats. Nothing here is a cosmetic-efficacy, skin-health, or product-safety claim. RawSource makes no warranty, express or implied, and assumes no liability for use of this information.