How Oil Paint Dries
Updated: Dec 19, 2019
By Stephanie Hart
Learn how oil paint dries to finally understand the importance of painting Fat over Lean.
Oil paint dries through oxidation not evaporation. Water based paints such as watercolor and acrylic dry through evaporation, which is the process that comes to most peoples minds when they think about how paint dries. The water in watercolor or acrylic paint evaporates into the air as it dries. As oil paint oxidizes it is absorbing oxygen from the air and is actually expanding as it dries. Oxidation is a longer process than evaporation and an oil painting is still drying even after it feels dry. With oil paint you need to think differently about how it dries from all of the other evaporative paints you have used.
Oil paint hardens, it does not dry. I use the term drying since that is how most people describe the process.
All of this is very important to understand if you want to paint with oils successfully. The oxidation process affects how the painter structures their painting and ties in directly with the Fat over Lean rule. If you follow the Fat over Lean rule you will prevent your painting from cracking. If you paint with oils in layers you need to think about the drying time for each layer or your painting may crack.
What Causes Cracking?
If the bottom layer of paint has a longer oxidation time than the top layer, the top layer will expand and harden first. When the bottom layer finally gets around to oxidizing and expanding it will stretch and crack the hardened layer above.
Always use Fat paint over Lean paint to avoid Cracks
Fat Paint - more oil than pigment
Lean Paint - more pigment than oil
A fat (oily) paint will absorb oxygen slower than a lean layer of paint. When you paint lean layers first they will dry faster giving the fat layers above more time to absorb oxygen and expand.
My Fat over Lean Recipe
Start with paint thinned by a solvent (turpentine)
With each layer add less and less solvent (turpentine)
Until you reach a pure paint layer
Now you can start adding oil, begin with a small amount and increase with each additional layer
But why is the solvent lean and the oil fat, they both let paint flow smoothly?
The solvent unlike oil will evaporate quickly allowing the paint to oxidize quickly. Solvents thin the paint and makes it dry faster so this is why it is considered ‘lean’ paint. Also the solvent loosens the pigment making it transparent, which is a great way to start a painting. You will still be able to see sketches through the paint and can you quickly block in color that will dry quickly.
The oil makes the paint fat because it will absorb oxygen slower and it will take more oxygen to dry making it Fat! The oil will also begin to make the paint more and more transparent and it will flow nicely so you can get clear details, which is very difficult to do when you are using just paint straight from the tube, which is often too pasty for details.
I really hope this helps you to understand more about oil paint the absolute best artists medium in the world. I know that at first the rules can seem a bit daunting, but it is definitely worth the effort and soon they will become second nature as you fall in love with oil painting. It is the most versatile, luminous, long lasting, and beautiful paint you will ever use.