How does the hardening process happen?
Molecules change their position and form chains. The faster they move (become hardened, fixed) the more heat they produce (similar process occurs when you rub your hands - heat is produced). Gels include so called 'photo initiators' – chemical compounds that start the reaction of hardening when they are exposed to the ultraviolet light (UV-A).
Some gels include more of that compounds (usually they need less powerful light to be hardened). Other gels have less photo initiators and so they need stronger light (9 watt) to be hardened. They are also more elastic and don’t turn yellow when exposed to the ultraviolet light.
The thicker gel coat the more heat is produced. The same happens in case of acrylic if it is not properly mixed with liquid (mixture not well-proportioned, too wet), however it takes more time to harden acrylic and so the unpleasant feeling is less noticeable.
In case of gels it is possible to slow down the process by putting a hand under the lamp for 3 seconds and then removing it also for 3 second (repeat 2-3 times, then leave the hand exposed to the light). I ask my clients to put the hand ON the lamp after removing it as leaving a hand next to the lamp doesn’t slow down hardening because in this position nails are still exposed to UV rays.
t should be also mentioned that the thinner a nail is the more likely you are to feel the heat. For the best effect it is advisable to apply thinner coats of gel as in this case the gel contracts less, hardens better and is more durable.