Maybe it’s just been the “apple barrel” cheap brand, but it seems that the lighter green varieties turn into a completely thin green liquid after a few years that smells vaguely like an alcohol/“chemical-ly.”
It’s specifically the greens that do this, so I’m wondering if there’s something in the green pigment that reacts with the binder after time. Most other cheap acrylics tend to dry out - reds more so. Is there some sort of slow chemical reaction happening with the greens instead?
Edit: I found a bottle of Orange just now that went off in the same way. Apple Barrel, matte, “Pumpkin Orange” 2047E.
It’s just a very striking way for paint to degrade. Drying out or just have the binder separate I get, but this one has to be involving some sort of fun chemistry.
I can’t say without knowing the specific ingredients and solvents, but just in general.
Different ingredients used for coloring dissolve differently in various solvents. Unfortunately, colors that are not from single ingredients sometimes have to be made from ingredients that don’t dissolve well in the same solvents. Also, different solvents and ingredients are less shelf stable than others or react more with oxygen or light.
Generally, if you want to keep pigments, or anything really, from decomposing, you need to store them in the dark or a totally opaque container and in a vacuum or in a very well sealed container, preferably topped off with an inert gas, or at least not oxygen which is very volatile.
The alcohol or chemical smells usually are the solvent evaporating. Things that harden in without special additives or things like UV do so by the solvents evaporating. The quicker the solvent evaporates, the quicker the paint sets. Alcohol evaporates much faster than water, for example.
Anyway, the containers that cheaper paints come in, generally are not 100% air tight or light blocking. (Remember, many thin plastics are air permeable, just barely, air particles are way smaller than water particles so something can be water right but not air tight.) Often I put my paints and similar things I dont use often in a vacuum bag that you cam pump the air out of and store them in a thick, dark box.