Question: How to keep oil paints from drying on your palette?

I put my entire palette in the freezer between painting sessions. Most oil colors dry very slowly but several develop a film that must be removed when left out for only a few hours (burnt umber for example). Cooling down the paint greatly slows the rate of oxidation and preserves your palette when you are not painting.

Freezing your palette works because oil freezes at very low temperatures. In fact, the leftover paint doesn't actually freeze in the freezer because the freezing point of linseed oil, -20°C (-4°F), is lower than the setting of most domestic freezers, 0°C (32°F). You can even freeze dirty brushes if you don't like cleaning them.

I have heard that covering oil paints with water will slow-down the oxidation process as well, but this is not a good practice. Exposing oil paint to water weakens the paint's ability to adhere to other surfaces.