Zinc oxide: the barrier powder that turns a balm into a drug product

Zinc oxide at 1 to 25 percent under the US skin protectant monograph, non nano and nano grades, particle sizes, dispersion method and the settling problem.

Zinc oxide is the only common balm ingredient that can change what a product legally is. Stir it into a balm, say what most people want to say about it, and in the United States you have made an over-the-counter drug. This page covers the grades, the numbers, the dispersion technique that decides whether a batch is smooth or gritty, and where the regulatory line sits.

• INCI: Zinc Oxide
• CI 77947
• US monograph 1-25%
• SG about 5.6
• EU nano: 1-100 nm

What you are actually buying

Zinc oxide is an inorganic white powder, ZnO, the only thing in an ordinary balm cupboard that is neither a fat nor a wax nor a plant extract. It does not melt at any temperature a workshop reaches. It is hard, highly refractive, amphoteric, and roughly six times denser than the oil around it. Every practical difficulty here follows from that last figure.

Three production routes are sold, and the route explains the impurity profile. The indirect or French process burns vaporised refined zinc metal in air and gives the high purity behind cosmetic and pharmaceutical grades. The direct or American process reduces ore with carbon and carries more of the ore with it, lead and cadmium included. Wet precipitation gives the fine, narrow distributions used for micronised grades. Anything sold as rubber, feed or paint grade has no cosmetic specification behind it.

Two named preparations are the reference points every commercial barrier product is judged against: zinc oxide ointment, 20 percent zinc oxide in a white petrolatum and wax base, and zinc oxide paste, historically Lassar's paste, at 25 percent zinc oxide, 25 percent starch and 50 percent white petrolatum. Calamine is not a separate mineral but zinc oxide with roughly half a percent of ferric oxide, an effect you can reproduce deliberately with iron oxides and mineral pigments.

The US skin protectant monograph, and the exact words it buys

In the United States, over-the-counter skin protectants are governed by 21 CFR part 347. Section 347.10 lists the permitted actives with their concentration ranges, and zinc oxide appears at 1 to 25 percent. Below 1 percent it is not an active; above 25 percent it is outside the monograph, which is why the widely sold 40 percent nappy creams are approved products rather than monograph products.

What the monograph gives in exchange is a small set of permitted claims, in section 347.50. The general skin protectant wording covers temporarily protecting and helping to relieve chapped or cracked skin, and helping to prevent and temporarily protect chafed, chapped, cracked or windburned skin. Products containing zinc oxide or calamine may additionally say they help treat and prevent nappy rash, and that they protect chafed skin due to nappy rash and help seal out wetness. Those sentences are the whole prize and the whole permission: a monograph product must not wander into curing, antibacterial or eczema language.

How the same powder is treated in the UK and EU

There is no monograph system in the United Kingdom or the European Union, so the same jar is sorted differently. Zinc oxide is a permitted cosmetic colourant, CI 77947, and separately a permitted UV filter at up to 25 percent in both conventional and nano form under Annex VI of Regulation (EC) No 1223/2009, with the standing condition that it is not to be used in applications that may lead to exposure of the end user's lungs by inhalation. The retained UK regulation carries the same annexes.

A barrier balm with zinc oxide in it stays a cosmetic in either market provided the claims stay within cleaning, perfuming, changing the appearance, protecting or keeping the skin in good condition. Say it treats nappy rash and the product becomes a medicine by presentation under the Human Medicines Regulations, which is a licensing question rather than a labelling one. A third route exists, since some barrier products acting by purely physical means on damaged skin are placed on the market as medical devices, but that is not the easy option it sounds like. The pre-market paperwork for the cosmetic route is in selling balms in the UK and EU.

Particle size, and what "non nano" actually certifies

Two numbers get confused constantly. The primary particle size is the size of the individual crystallites, from electron microscopy or inferred from surface area. The agglomerate size is what the powder in the bag consists of, from laser diffraction, and it is routinely one to two orders of magnitude larger because dry oxide particles stick together.

The European definition of a nanomaterial for cosmetics, in Article 2(1)(k) of Regulation 1223/2009, is an insoluble or biopersistent, intentionally manufactured material with one or more external dimensions, or an internal structure, on the scale from 1 to 100 nm. Note "or an internal structure": an agglomerate of 50 nm primaries measuring 3 micrometres by laser diffraction still has nanoscale internal structure. The 2022 Commission Recommendation on the definition adds a number-based threshold, counting a material as nano where at least half the particles by number fall in that range.

So "non nano" is a phrase suppliers apply with varying rigour. Some mean the primary particles are above 100 nm, which is the meaningful reading; some mean only that the agglomerates are, which is not. Ask for the primary particle size and the method in writing, because the consequence in the EU and UK is a nano suffix in the ingredient list and a notification obligation six months before the product goes on the market. That is a question for the supplier, not for the bench.

Coated, treated and uncoated grades

Bare zinc oxide is a polar, hydrophilic surface sitting in a non-polar oil. It is poorly wetted, which is the root of every clumping problem, and it is chemically reactive, which is the root of the thickening problem two sections down. Surface treatment addresses both. Silane coatings, most often triethoxycaprylylsilane, and silicone coatings of the dimethicone type replace the polar surface with an oleophilic one, so the powder wets into oil with far less work, disperses at lower shear and settles more slowly because it is no longer flocculating into clusters. Silica and alumina coatings do more for chemical passivation, damping the photocatalytic activity of the surface and slowing its reaction with acids.

At the same 20 percent loading and with the same spatula, an uncoated pigment grade gives a chalky paste with a settled layer, while a silane-coated grade of the same particle size gives a smoother paste that stays suspended longer and drags less. The trade-offs are cost and the fact that the EU nano entry permits only specific surface treatments, so a coated micronised grade has to be checked against the current annex rather than assumed compliant.

Dispersion: pre-wet, then shear

There is one technique and it is not negotiable. Never tip dry zinc oxide into a hot melt: the outer layer of each clump wets and you get a shell of oiled powder around a dry core that no stirring will open. Those are the white specks people find a week later.

1. Sieve the powder through a 250 micrometre mesh onto paper, which breaks the loose transport agglomerates.
2. Weigh a wetting oil at one and a half to two times the powder's oil absorption value. For a pigment grade at 20 g per 100 g, that is 30 to 40 g of oil per 100 g of zinc oxide, taken out of the formula rather than added to it.
3. Make the paste cold. Add the powder in three portions, working each in against the side of the bowl until there is no dry powder and no lumps. It should look like thin emulsion paint.
4. Apply shear. Ten to thirty seconds with a small stick blender, or a few minutes in a pestle and mortar, does more than any amount of stirring.
5. Add the paste to the melt below 70 C and keep it moving. Adding it hotter only gives the powder longer in a thin liquid.

Choice of wetting oil matters more than people expect. Castor oil is still the best available in a home workshop: its ricinoleic acid carries a hydroxyl group, which makes it unusually polar for a vegetable oil and gives it a real affinity for a mineral surface. The synthetic alternative is a benzoate ester, which wets mineral powder better than any triglyceride, as the polarity ranking in cosmetic esters shows. A light non-polar ester makes a paste that flocculates back together in the melt.

Settling, and the hard cake at the bottom of the jar

Zinc oxide sinks in a molten balm because it is five to six times denser than the fluid. How fast depends on the size of the settling unit, and the relationship is quadratic, which is why dispersion quality dominates.

Properly dispersed zinc oxide barely moves during a normal cool; poorly dispersed zinc oxide is on the bottom before the surface has skinned over. The effort belongs in the paste, not the stirring.

The second half of the fix is to shorten the liquid window. A balm has essentially no yield stress above its setting range and a large one below it, and the stress to be resisted is small: for a 20 micrometre particle it is a fraction of a pascal, which any set wax network beats by orders of magnitude. So the useful moves get the network to form early. Stir continuously through the setting range rather than pouring and walking away, pour at the cool end of the window given in pour temperatures, and add a structurant that gels above the wax crystallisation temperature: 1 to 3 percent hydrogenated castor oil from castor wax, 2 to 5 percent ozokerite, or 0.5 to 2 percent fumed silica, covered with the other particulates in starches and silica. Why a yield stress holds a particle up indefinitely is in rheology and yield stress, and the rescue for a batch already gone wrong is pigment sinking to the bottom.

Settling in a jar does not leave a loose sediment you can stir back in. Dense particles consolidate under their own weight into a cake a fingertip cannot lift, so the top of the jar is clear balm and the bottom is chalk. Remelting and re-dispersing is the only rescue, and only if the powder was properly wetted to begin with.

Zinc soaps: the thickening that shows up six weeks later

Zinc oxide is amphoteric. It reacts with carboxylic acids, and free fatty acids are carboxylic acids, so ZnO plus two molecules of fatty acid gives a zinc carboxylate plus a molecule of water. Zinc stearate and zinc oleate are themselves used as thickeners and as pressed powder binders, so the reaction product stiffens the product it forms in.

The signature is a batch that keeps hardening well past the 24 to 48 hours normal wax crystallisation takes: scoopable at day two, short and crumbly at week six, sometimes with a soapy slip on first contact. What separates it from ordinary post-cure hardening is that it does not plateau.

The free acid comes from unrefined butters, oils with a high acid value, deliberately added stearic acid, and slow hydrolysis over shelf life. So choose refined fats with a low acid value, keep free stearic acid out of a high zinc formula, prefer a coated grade, and keep water out, since the water the reaction liberates accelerates further hydrolysis. A room-temperature sample will not show this in time, so run a 40 C sample alongside as in accelerated ageing and check it against the ordinary causes in balm hardens over time.

The same basicity neutralises acidic actives. Salicylic acid, ascorbic acid and the alpha hydroxy acids are all wasted in its company. The non-acidic actives in menthol, camphor and actives are chemically fine alongside it, though stacking two monograph actives raises its own labelling problem.

Weighing it without breathing it

Zinc oxide on skin is unremarkable. Zinc oxide in the lungs is why every regulator that touches the material attaches an inhalation condition to it. The UK EH40 list gives zinc oxide an eight-hour workplace exposure limit of 5 mg per cubic metre of inhalable dust with a 10 mg short-term limit, and the ACGIH threshold limit value is 2 mg per cubic metre for the respirable fraction. Both are far below what a cloud of loose powder produces.

Use levels, and where zinc belongs

Every point of zinc oxide stiffens the balm and cuts the glide, because you are adding a hard, high surface area solid to a soft matrix. Above roughly 15 percent, expect to cut the wax by two or three points to keep it spreadable, and expect a matte, slightly draggy result however well you disperse it. A zinc-loaded balm is a barrier product, not a comfortable one. The worked formula is nappy rash balm.

Where it is the wrong material

Zinc oxide has three uses in a balm: opacity, physical bulk in the film, and a regulatory status you have chosen deliberately. If you want none of the three, leave it out, because everything else about it is a cost. It is the wrong material for sun protection made at home, because the percentage in the pot is not the protection on the skin and the SPF figure comes from a test on the finished product; the argument is in SPF in lip balm. It is the wrong material on a fresh tattoo, where an opaque paste hides the thing the artist has told the client to watch and where a cosmetic must not approach wound care language, as balms and tattoo aftercare and tattoo balm set out. And it is the wrong material wherever an acidic active is doing the work, or wherever you want a light, transparent balm.

The decision rule is short. If the answer to "why is this in here" is a claim you intend to make, price the regulatory route for that claim before you buy the powder. If the answer is "it seemed protective", make the plain occlusive balm instead: easier to make, easier to wash off, easier to see the skin through, and a cosmetic in every market.

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