I'm always dubious about adding a 'chemical ingredient' to acrylic paints because it will alter the manufacturer's 'recipe' for their paint, but a definitive answer is beyond my technical expertise. So I emailed the experts at Golden's Technical Support for an opinion on using glycerin as an extender. This is the response I got. You may be dubious because they sell extender products, but as with so many things artistic, the ultimate choice is always yours. Just ensure it's an informed choice.
"While I'm sure the artist is able to find references on the Internet to adding glycerin into paints as a retarding agent, our testing with its use in acrylics has found it sub-par to the more commonly used glycol-based retarders in today's market. The most common artist use of glycerin is in watercolor paints, as cited by both Mayer and Gottsegen in their books.
"The disadvantage of glycerin is that it will take a very long time to escape the paint film, especially thicker paint layers, and will allow the paint to remain tacky (not workable) for quite some time possibly for weeks or months. This, in turn, will act as a magnet for dust to collect and stick to the surface. Additionally, it's possible this slow drying time may also harm the film formation process of the acrylic, impacting long term painting stability.
"Subsequent paint films may partially reactivate the underlying paint causing unintended mixing of layers. This issue may be reduced if the paint films are thin washes and on an absorbent surface such as watercolor paper, but if one is using the glycerin as a retarder for acrylics, they would have far greater working time with the glycol retarders on a sealed support. In summary, using glycerin in acrylic paint films have the potential to cause harm to the painting.
"While we always encourage artists to try new materials, processes and techniques, it's important for them to do their homework and investigate and test on surfaces other than the work in progress."
-- Technical Support, Golden Artist Colors, Inc.