Can a painting get moldy?
 

I have several paintings that are painted in oil on paper and cardboard that were primed with shellac (one side of the paper only). The paintings were done in one session, alla prima, with little to no medium (walnut oil). I believe the paint I was using was M. Graham oil paints and I didn't use any turpentine. The paintings were done at least 6 years ago and they have never been varnished. I had them stacked on top of one another on a shelf. I looked at them recently and noticed a waxy looking film on most of the paintings, in spots, not over the entire painting. It only seems to have happened on the paintings that I did on shellacked paper and cardboard, my canvases don't have this problem. The waxy/dull looking stuff seems to rub away with just my finger. I'm not sure what I did wrong, does anyone know? I would appreciate any advice you may have on the matter.

Without further information, this is my opinion.

Shellac has a tendency to bloom (to turn cloudy) when exposed to humidity. You can see this on a coffee table when it has been shellacked and you place a glass of ice water on it. When you lift a glass there is a white ring left on the table. My understanding is that water is trapped in the shellac and can be easily removed by wiping it down with denatured alcohol. As the alcohol evaporates and removes the water with it. I have no idea how this would help you with works on paper. Unless you're willing to pay a conservator or to experiment with one that you don't particularly care for, I would proceed very cautiously. Good luck.

Thank you Clayton! I'll take a Q-tip dipped in alcohol and test it out on a painting that I don't care much about. I was thinking that because I was stacking the paintings that maybe the non-shellacked side of the paintings were reacting with the painted side, but I don't know. Is there a better way to store oil paintings done on paper?

The old guys called it Glassine Paper. I would ask a conservator but I would think that an inert, acid-free, fairly slick paper would do as a barrier sheet. These sheets are commercially available to industry much cheaper than from art supply stores. A little Internet searching should turn up something. Good luck.

It's still called Glassine - you can buy it by the roll. I use it as my first protective layer when packing a painting for shipping.

Use shellac that has been dewaxed, and avoid spirits that may have water content to avoid the risk of bloom.