Kaolin, bentonite and rhassoul in anhydrous products

Oil absorption values, particle sizes, use at 1 to 10 percent, the drag a clay adds to a balm, plus heavy metal specifications and the dust hazard when weighing.

A clay in an anhydrous product does three jobs: it soaks up free oil, it thickens the batch without raising its melting point, and it holds pigment in suspension while the balm sets. It does none of the things a wet clay mask does, because there is no water for it to do them in. This page gives the oil absorption values, the particle size that decides whether a lip balm feels gritty, the use levels that work, and the two safety questions a mined mineral genuinely raises.

• INCI: Kaolin
• Oil absorption 30-45 g/100 g
• Use 1-10%
• Grit above about 20 microns
• Does not swell in oil

Four different minerals, all sold as clay

"Cosmetic clay" is a trade category, not a mineral. The four materials that turn up in balm formulas belong to different structural families, and the differences that matter in water are not the differences that matter in oil.

Kaolinite is one silica sheet bonded to one alumina sheet, stacked tightly enough that water cannot get between the layers, which is why kaolin does not swell and has almost no cation exchange capacity. Bentonite and rhassoul are 2:1 smectites, three-sheet sandwiches with exchangeable cations between the layers, and that is where their swelling comes from. French green clay is mostly illite, whose interlayer potassium is not exchangeable, so it behaves closer to kaolin than its smectite content suggests.

Here is the part most clay descriptions skip. Swelling, cation exchange and adsorption of dissolved species are all water-phase behaviours, and none of them happen in an anhydrous balm. What survives into an oil system is a shorter list: how much oil the powder immobilises, how big the particles are, how abrasive they are, and what colour they carry. Every choice between clays in a balm comes off that list.

Oil absorption, and what the number is worth

Oil absorption is measured by rubbing oil into the powder with a spatula until a stiff, coherent paste just forms, then reporting grams of oil per 100 g of powder. The standard version of the method uses raw linseed oil on a glass plate. It is an endpoint judged by hand, so operators differ by 10 to 20 percent on the same sample, and the value shifts with the oil used. Supplier figures are comparative, not absolute.

Turn the number into formulation arithmetic and it becomes useful. Five percent kaolin at 40 g per 100 g ties up about 2 percent of the total formula as oil that is no longer free to spread, wet pigment or carry a fragrance. The same 5 percent of calcined kaolin ties up nearer 3.5 percent, and 0.5 percent of a porous silica ties up about 1 percent. That is why a formula that felt fine with kaolin turns chalky when someone substitutes an equal weight of a more absorbent powder. It is also why the comparison worth making is against starches and silicas rather than against another clay, which is set out in starches and silica.

Particle size and the grit threshold on lips

Skin starts to register discrete particles somewhere around 15 to 30 microns, and lips sit at the sensitive end of that band because the vermilion is thin, densely innervated and constantly rubbed against the other lip and the teeth. Take 20 microns as the working threshold: below it a well dispersed powder reads as texture, above it individual grains are felt. A water-washed cosmetic kaolin at a median of 0.5 to 5 microns is nowhere near that limit. A coarse airfloat grade, an unmilled rhassoul, or any clay sold by the sack for masks may well be.

Ask for a particle size distribution with a median and a top cut, not a mesh number. A "325 mesh" clay has passed a sieve with 44 micron apertures, which tells you the largest particle is under 44 microns and nothing about how much sits between 20 and 44. Ask for d50 and d97, or for the residue on a 45 micron sieve as a percentage.

If a finished balm feels gritty, work out which grit you have before changing the clay. Mineral grit stays hard when you press it between two fingers and does not disappear on the skin. Wax grit crushes and melts, and is diagnosed in gritty balm from hard wax. Fat crystal grain feels like fine sand that vanishes as it warms.

Thickening without hardening

A clay stiffens a balm by a different mechanism from a wax, and the distinction has practical consequences. Wax builds a crystal network as it cools, which raises the temperature at which the product loses structure. Clay particles form a loose network of their own and immobilise oil in the spaces between them, which raises low-shear viscosity and gives the product a yield stress, the stress it will take before it flows at all. The melting behaviour does not move at all, because nothing in the formula has changed phase.

So a clay-thickened balm feels firmer on the finger, resists slumping in the tin, and holds pigment in place, but it will still soften on a dashboard at the same temperature it always did. If your problem is a balm that collapses in warm weather, clay is the wrong lever and the right ones are in balm texture science. If your problem is a product that slumps or settles at rest, clay works, and the underlying property is explained in rheology and yield stress.

One genuine oil-phase structurant does come from this mineral family, and it is not plain bentonite. Organically modified clays, made by exchanging the interlayer cations for quaternary ammonium surfactants, do gel oil: disteardimonium hectorite and quaternium-18 bentonite are used at roughly 0.5 to 2 percent in lipstick and colour cosmetics, and they need a polar activator such as propylene carbonate, typically at 30 to 40 percent of the clay weight, to open the platelets. Untreated kaolin or bentonite will not do this at any sensible loading. Compare that with the wax networks described in oleogels and wax networks before deciding which route your formula needs.

Use levels from 1 to 10 percent

Read down the "what it costs" column rather than the middle one when you are deciding. Every gram of clay is a gram of formula that is not oil, wax or butter, and the effect on payoff compounds: the clay both displaces the emollient and absorbs some of what is left. In a lip product, keep to 1 to 3 percent unless matte is the whole point of the product, and expect complaints past 5. The knock-on effects on glide are catalogued in drag and poor glide.

Clay also settles. A 5 micron particle sinks slowly while the balm is liquid, and a slow-cooling tin gives it time, so pour closer to the setting point and stir until the batch thickens, following pour temperatures. A pale, chalky layer at the base of a set tin is the fault described in pigment sinking to the bottom.

What a clay does to colour and payoff

Payoff is the mass of product a stick or tin deposits per stroke, and clay reduces it twice over: the absorbed oil is no longer free to carry pigment onto the skin, and the stiffer film resists shear, so less transfers per pass. In a tinted balm that reads as a colour which looks strong in the pan and weak on the lip, and the usual reaction, adding more pigment, makes the product draggier still.

White clays also lighten and desaturate a tint. Kaolin behaves as a low-opacity white filler: at 3 percent it will visibly pastel a red iron oxide shade, and it flattens the sparkle in a mica-based tint. Rhassoul and French green push the shade brown or grey-green before they do anything else. Always match shades with the clay already in the formula, never by tinting a clay-free base and adding the clay afterwards. Pigment interactions in general are in colourants and tints, and the specific behaviour of the mineral pigments a clay sits alongside is in iron oxides and mineral pigments. If a balm has come out flatter than you wanted, work through matte instead of glossy balm.

Heavy metals and what the certificate must carry

Clays are mined, and mined minerals carry whatever the deposit carried. Lead, arsenic, cadmium, mercury, antimony and nickel are all found in clay deposits at trace levels, which is why this is the one balm ingredient class where a heavy metal specification is not optional.

The limits are set on the finished cosmetic, not the raw material. Health Canada's guidance on heavy metal impurities gives limits of 10 ppm lead, 3 ppm arsenic, 3 ppm cadmium, 3 ppm mercury and 5 ppm antimony as impurities in a finished cosmetic. The US Food and Drug Administration recommends a maximum of 10 ppm lead in cosmetic lip products and externally applied cosmetics. German authorities publish lower orientation values, including 2 ppm lead and 0.5 ppm arsenic, which are not legal limits but are what a European enforcement laboratory is likely to measure against. Under the EU and UK cosmetic regulations, substances on the prohibited list are tolerated only as technically unavoidable traces and only where the finished product remains safe.

Do the arithmetic before panicking about a raw material figure. A kaolin certified at 5 ppm lead, used at 5 percent, contributes 0.25 ppm to the finished balm. The same clay at a deodorant stick's 10 percent contributes 0.5 ppm. Both sit under every limit above, which is why the calculation matters more than the raw material figure does.

Microbial load in a mined mineral

Clay comes out of the ground with spore-forming bacteria and moulds in it, and unless it has been heat treated or irradiated it arrives with them. Cosmetic grades are commonly specified at no more than 1,000 colony forming units per gram of aerobic mesophilic bacteria, with yeasts and moulds lower, and with the four specified organisms absent. The international standard for microbiological limits in cosmetics sets no more than 100 CFU per gram for products for children under three, the eye area and mucous membranes, and no more than 1,000 CFU per gram for everything else.

An anhydrous product tolerates that load because nothing in a balm can grow. Bacterial and fungal growth needs free water, and a fat-only product has a water activity far below the level any organism can use. The organisms you add with the clay survive as spores and stay at the number you introduced. That is also the limit of the tolerance: a preservative would not help even if you added one, because preservatives partition into a water phase that does not exist, and any water that gets in later, from a wet spatula or a condensing lid, creates exactly the local pocket those spores need. The full argument is in do balms need preservatives.

Two things this page cannot decide for you. Whether your lip balm counts as a mucous membrane product for the stricter limit is a judgement your safety assessor makes, and assessors differ. And whether a given clay's bioburden is acceptable in your specific product depends on the whole formula and the packaging, not on the clay alone.

Crystalline silica, and weighing powder safely

Some clay deposits contain free quartz, and calcined grades can contain cristobalite formed during firing. Inhaled crystalline silica in the respirable fraction is classified by the International Agency for Research on Cancer as carcinogenic to humans, and long-term inhalation causes silicosis. That is a chronic occupational hazard rather than an acute one, but weighing powders every week for years, in a room chosen for having a sink rather than for its ventilation, is exactly that kind of exposure.

The workplace exposure limit for respirable crystalline silica in Great Britain is 0.1 mg per cubic metre averaged over eight hours, which is also the European binding limit; the US limit is half that. Nobody can measure that on a kitchen scale, so the response has to be procedural. Ask the supplier for the crystalline silica content and prefer grades that state it. Weigh at bench level with the container close to the scale rather than pouring from height. Keep the bag closed between scoops. Pre-disperse the clay into oil straight after weighing so the dusty step happens once. Wear a fitted disposable respirator rated for fine dust rather than a surgical mask. Never dry sweep and never blow the bench down with compressed air: wipe with a damp cloth, which is the same principle as the rest of workshop hygiene. Keep powders out of any space used for preparing food.

Where a clay genuinely earns its place

Deodorant sticks are the strongest case. A stick has to absorb sweat and sebum, glide without smearing and stay solid against skin at 33 C, and 5 to 10 percent kaolin does the first two without touching the third. In bicarbonate-free formulas the clay carries more of the work because there is no alkali doing it, usually alongside a starch and magnesium hydroxide: see solid deodorant stick and the reduced-irritation route in bicarbonate free deodorant balm.

Mattifying and cleansing products are the second case, at much lower loadings. One to three percent kaolin in a cleansing balm or a face balm dulls the shine of an oil-rich product without materially changing how it performs. The third case is suspension: in a tinted lip balm or a poured lipstick, 1 to 2 percent kaolin measurably slows pigment settling during the pour and the first minutes of cooling, which is worth more than it sounds when you are filling by hand and the last tube in the tray sets ten minutes after the first.

What clay cannot do in an anhydrous product

The adsorption chemistry that clays are sold on belongs to water. Cation exchange, binding of dissolved metals, the drawing action of a wet mask: all of it requires water to carry ions to the mineral surface and all of it stops in a balm. A clay in an anhydrous stick is an oil sponge and a rheology modifier, and that is the whole of it. It does not detoxify skin, draw out impurities, purify pores or pull anything through the stratum corneum, and claims of that kind are both unsupported and a regulatory problem, as set out in cosmetic vs drug claims.

The decision rule follows from that. If you want the product harder, use more or better wax. If you want a dry, powdery finish without mineral drag, use a starch. If you want strong absorbency at a fraction of a percent, use a porous silica. If you want to structure the oil phase itself, use an organoclay with its activator or a fine-crystal wax. Reach for kaolin when you specifically want matte, slip and suspension in one cheap, mild, white powder, keep it between 1 and 6 percent outside deodorants, and buy a grade with a particle size distribution on the certificate.

Reviewed and updated 6 September 2026. Spotted an error? Tell us and we will fix and log it.