The various types of nail varnish.

Practical and economical, traditional nail varnishes are those found almost everywhere, online, in stores, in pharmacies. Easy to apply and remove, they offer great versatility and allow for changes in colour and finish as desired. This type of varnish also has the advantage of drying relatively quickly in open air. The nail varnishes are typically applied after a clear base that improves the adhesion of the varnish and protects the nail from pigments, and before another clear varnish, also known as a "top coat", which ensures better durability. These varnishes are removed with a remover and usually start to chip 5 to 7 days after application.

Traditional nail varnishes come in various textures and finishes, offering a broad range of choices. Among them, cream varnishes are popular for their smooth and opaque finish, ideal for uniform coverage. Matte effect varnishes provide a more subtle and elegant result, without shine, while glitter varnishes add dimension and whimsy. For more visual effects, one can opt for a mirror varnish, which creates a highly shiny and reflective finish, or a metallic varnish. Velvet varnishes, though less common, offer a satin finish. In short, there is an endless array of traditional nail varnishes, catering to a multitude of occasions.

Traditional nail varnishes are a practical and accessible option for individuals seeking a manicure for a few days.

As their name suggests, semi-permanent nail varnishes stand out from traditional varnishes due to their extended wear of two to three weeks. Unlike regular varnishes that dry in open air, semi-permanent varnishes require exposure to UV or LED lamps. This exposure triggers a polymerisation reaction essential for hardening the varnish layer. The application of a semi-permanent varnish involves several steps: nail preparation with a degreaser to remove any oil, followed by the application of a base coat to ensure varnish adherence. The colour layer is then applied in several thin coats, each hardened under a UV or LED lamp. Finally, a top coat is applied to enhance the shine and ensure the longevity of the manicure.

The removal of a semi-permanent varnish is somewhat unique and cannot be performed with just any remover. A special remover, often acetone-based, is required. This is applied to a soaked cotton pad, which is placed on the nail and left to act for 10 to 15 minutes. Once the varnish has softened, it can be gently removed with a manicure stick, taking care not to scrape the nail too much to avoid damaging it. It is also possible to simply use a nail file to remove a semi-permanent varnish, but this method tends to weaken the nails if not performed by a professional in a salon.

Semi-permanent nail varnishes are a compelling option for individuals seeking a long-lasting and durable manicure.

Permanent nail varnishes provide an even more enduring and shock-resistant solution than semi-permanent varnishes, allowing for a finish that can last beyond three weeks, sometimes up to a month. The main temporal limit then becomes the regrowth of the nails, which can make the manicure look unsightly after a certain time. Permanent nail varnishes are applied in a similar way to semi-permanent varnishes and require polymerisation under UV or LED light. Although both types of varnish contain durable polymers, which are responsible for their longevity, permanent varnishes also use acrylic resins, allowing for the creation of a robust barrier. This resistance allows permanent varnishes to remain intact even in the presence of heat, humidity or cleaning products and not to show signs of wear for several weeks.

This robustness, however, comes at a cost: it is impossible to remove a permanent nail varnish without visiting a professional salon as no remover will completely eliminate this type of varnish. Only a nail file handled by a professional can remove permanent nail varnish. It is strongly advised against attempting to remove permanent nail varnish oneself as improper removal could damage the nails.

Permanent nail varnishes are favoured by those seeking a highly durable and long-lasting solution. However, their removal can be inconvenient as it requires professional assistance at a salon.

In the midst of a surge, so-called natural nail varnishes have been developed in response to allegations of the dangers of nail varnishes of the traditional kind. These varnishes can be identified by the label "n-free", where n denotes the number of controversial ingredients removed. For instance, "3-free" varnishes are formulated without toluene, dibutyl phthalate (DBT), or formaldehyde, while "5-free" varnishes also exclude camphor and xylene. Natural varnishes typically replace solvents with plant extracts, such as potato, cassava, corn, or sugarcane. "n-free" varnishes cater to an audience keen to avoid potentially harmful chemicals. Aside from their composition, their application, finish, and removal are similar to those of traditional varnishes.

Even though natural nail varnishes are generally safer than traditional ones, it's important to clarify that terms like "3-free", "5-free" or "7-free" do not adhere to a universally established standard. This was notably demonstrated in a 2018 study that examined the composition of 40 "n-free" nail varnishes from 12 different brands. The figure below illustrates the lack of uniformity in the formulas among these nail varnishes. The differences are particularly pronounced for varnishes that exclude more than 5 controversial substances.

Nail varnishes known as peel-off distinguish themselves from other varnishes by their simplified removal. Indeed, while they are applied in the same way as traditional nail varnishes, the peel-off varnishes are formulated to be peeled off the nail, hence their name. They contain specific polymers that, once applied and dried, form a flexible and durable film but flexible enough to be removed in one go. This property makes peel-off varnishes particularly appealing for those who wish to frequently change colour without having to use a remover and who want to preserve their nails as much as possible. However, they are much less durable than traditional varnishes.

Peel-off nail varnishes can simply be removed by lifting a corner of the varnish with a manicure stick or file and then gently pulling to peel off the film in one go. Kinder to the nails, this process also has the advantage of being very quick and offers great flexibility for changing manicures.

Peel-off nail varnishes are highly beneficial for those seeking a gentle, convenient, and temporary manicure.

Thermal nail varnishes, which are quite playful, have the unique characteristic of reacting to temperature changes and altering their colour in response to these shifts. This phenomenon is made possible by thermochromic pigments, compounds that change hue when subjected to heat or cold. Most thermal varnishes are semi-permanent varnishes. Therefore, they require exposure to a UV or LED lamp to harden and are not easily removed.

After applying a thermal nail varnish and allowing it to harden, you can enjoy immersing your hands in cold or warm water and observing the colour changes this produces. Depending on the varnish, it's possible to see a transition from pale pink to purple, from light blue to dark blue, or even from yellow to green. Some cover an even broader colour spectrum, ranging from white to dark grey.

By allowing the nail colour to change in response to environmental interactions, thermal nail varnishes provide an interactive and engaging experience.

Positioned between a treatment and a traditional nail varnish, the hardening nail varnish aims to strengthen and reinforce the nail. The use of this type of varnish is particularly beneficial between each application of varnish or in cases of brittle nails, soft or split nails. The properties of hardening nail varnishes come from the ingredients they contain, notably film-forming agents, such as nitrocellulose, which form a protective layer on the surface of the nail, thus limiting the evaporation of water from the nail plate. Other beneficial actives for nails can also be included in the formula of hardening nail varnishes, such as keratin, vegetable oils or vitamins (B5 or E for example). Most hardening nail varnishes are colourless, although there are also coloured ones. This may seem paradoxical as it is then necessary to use a remover to take it off, which can weaken the nail, the opposite of the protective effect sought.

Nail hardeners are supplementary nail varnishes that aid in strengthening fragile nails.

• CEBALLOS D. M. & al. Phthalate and Organophosphate Plasticizers in Nail Polish: Evaluation of Labels and Ingredients. Environmental Science & Technology (2018).

• DINANI N. & al. Nail cosmetics: a dermatological perspective. Clinical and Experimental Dermatology (2019).