I wanted to try something new And also, yes, I feel like the point of adding coffee to a lotion is caffeine. I do have coffee infused olive oil that I like to use in soap for scent, but I know I can't hope for any other benefit in this form.Why not stick with making anhydrous balms? You could infuse some coffee grounds into a nice oil for a few days, melt in some beeswax, and call it a day.
If you are really after the caffeine, then yes you'd have to make a lotion with water in it, because caffeine is water soluble. But you can't make a lotion with an infusion of coffee for the same reason you can't make a lotion with a tea. There's just no way for you to filter out all the organic particles even with the finest filter. That's why we use hydrosols instead of simply making a herbal tea. As soon as you introduce "bug food" into your lotion, no amount of preservative will save you. I'd strongly recommend buying a proper coffee extract.
I learned a lot reading this thread. Thank you to all those who patiently explained everything. I think that for beginners, it is hard to imagine that preservatives are actually there to preserve pure water and are not strong enough to preserve anything else. I guess as soon as water is emulsified with something else, it is not pure water anymore and then susceptible to mold and bacteria growth. So if we start with anything else than pure water...
I'll consider buying caffeine extract. But I will post-pone the project, I can't buy any new ingredients right now.