Dear Alex,
The key to the product is what they say in the third line of the description: "Heats on contact with water". Chemical processes, like dissolving or adsorbing, can produce temperature changes because they absorb or release energy (heat) when the process takes place. Some compounds (like ammonium nitrate used in instant cool packs) need to absorb energy from the environment in order to dissolve in water (an endothermic process) and so they produce a drop in temperature. Other chemicals, like lye (sodium hydroxide) release heat when they dissolve (an exothermic process) and so give a rise in temperature. There are also chemicals that have crystal structures that allow them to absorb water into the molecular cage of the crystal - and these absorption processes can also be endo- or exo-thermic. To get an energy change that would give a temperature difference that you would be most likely to notice, you should probably look at the ingredients that are present in larger amounts - and on a cosmetic label this is the first few ingredients because they are listed in order of their amount. You should also probably be looking for an inorganic ingredient as these chemicals tend to have bigger energy changes (heats of enthalpy) when they interact with water than organic chemicals (and "organic" means a chemical based on carbon- this is a good explanation: Understand the Difference Between Organic and Inorganic ).
There are three mineral ingredients on the list - zeolite, kaolin, and talc - they are all inorganic compounds. Charcoal powder (pure carbon alone) is insoluble in water (and so interacting with the water very weakly) and so unlikely to give a temperature change when mixed with water. Dimethicone is a compound that it is based on silicon instead of carbon and although it is classed as an inorganic it is extremely insoluble in water and so also unlikely to give the temperature change. All the other ingredients are organic compounds unlikely to give the temperature change.
You can read more about zeolites here: What are zeolites? | How do zeolite catalysts work?
Kaolin is a specific type of clay. It can also go through the same changes as zeolites, absorbing and giving up water from its crystal structure but evidently less reversibly than zeolite. It is probably there to form the main bulk of the mask; some cosmetic chemist may want to comment on whether this guess on my part is correct.
Talc is much lower down on the list (lower amount) and although it can interact with water my impression from reading the literature is that the interaction is significantly lower than the interaction of zeolite and water.
So the most likely ingredient to give the temperature change - one that interacts strongly with water and gives a significant release of energy through either dissolving in water or adsorbing the water into its crystals - is zeolite. It is probably added to the product in a very dry state, and some of the other ingredients are probably helping to keep it dry by coating the particles and keeping water away from it (the dimethicone would be most efficient at this). The directions also say that the product will heat up as you massage it on your face - if the other ingredients are blocking the water from the zeolite it might take some mixing on your part to get the zeolite in contact the water from the wet skin.
I'm not a cosmetic chemist so I don't have in depth knowledge of how products like this are formulated but I have tried to take you through my chemical reasoning on your question and I think this is the likely answer.
Karen Wisniewski
