Here is a discussion concerning home made retardants... interesting.
"Glycerin is gooey and permanently water-soluble. It is sometimes used in watercolors as a binder and drying retardant. I'm inclined to think that in large enough proportions glycerin would leave acrylic prone to resolubility, but a few drops in a large mass of paint shouldn't cause too much problem.
Propylene glycol is what is usually used in commercial acrylic drying retarder. I've never heard of glycerin being specified for this purpose."