Jack Sprat and the Law of Fat Over Lean

Every oil painter learns the foundational rule of the studio early on: fat over lean. It is a basic rule of chemical survival for a painting. If you do not follow it, your canvas will eventually crack, wrinkle, and fall apart. While thinking about this mechanical process recently, it struck me that the entire balancing act is perfectly explained by the old nursery rhyme about Jack Sprat and his wife. Between the two of them, they represent the perfect studio setup.

Let’s look at Jack Sprat first. He could eat no fat. In terms of paint construction, Jack is your underpainting. He represents the lean layer, which is paint thinned down heavily with turpentine or mineral spirits, containing almost no extra oil. Like Jack, this layer is thin, sparse, and dries very quickly because the solvent evaporates into the air. This leaves behind a flat, matte, and rather brittle layer of pigment. This is where you map out the drawing and establish the tonal values. Because this layer dries so fast and has no real flexibility, it belongs right at the bottom, directly on the gesso ground, where it cannot cause any structural trouble.

Then we have Mrs. Sprat, who could eat no lean. She represents the fat layers that must follow. This is the rich, buttery paint mixed with linseed oil, stand oil, or medium like Liquin. Mrs. Sprat takes her time. She does not dry by evaporation; she cures slowly by absorbing oxygen from the air over weeks or even months. This process leaves her layer flexible, rich, and full of depth. Because she remains pliable for such a long time, she needs to sit on top of a completely stable foundation that will not move underneath her.

The real trouble begins when you reverse this natural order and try to put lean over fat. If you slap a fast-drying, turpentine-heavy Jack Sprat layer over a slow-drying, oil-rich Mrs. Sprat layer, you are inviting a disaster on the canvas. Jack will flash-dry within hours, forming a hard, unforgiving crust on the surface. Meanwhile, underneath that crust, Mrs. Sprat is still moving and curing. As she slowly shifts, the brittle top layer cannot stretch to accommodate the movement. The result is a total structural failure. The top layer will crack, wrinkle, and lift away.

When you respect the rule of fat oner lean and keep Jack at the bottom and his wife on top, the layers cure in perfect harmony. The flexible top layer easily accommodates the solid foundation underneath. The surface remains stable, and the paint film stays intact. It turns out the old nursery rhyme is not just a quirky story about a couple sharing a meal; it is actually a very sound lesson in archival oil painting technique that ensures a picture will endure for centuries.