When Joseph Mallord William Turner died in 1851, he left behind enough masterpieces to put 90% of the world’s museums to shame—hundreds of oil paintings, thousands of watercolors, and tens of thousands of drawings, studies, and cartoons. What Turner chose to do with these works was nearly as astonishing as the works themselves: He bequeathed the majority to the British nation, no strings attached. Today, most of the contents of the so-called Turner Bequest can be viewed, free of charge, at the Tate Britain in London.

Turner’s sketches aren’t the most acclaimed part of his artistic output, but they might be the richest. As part of his bequest, he left Britain with 282 of his sketchbooks, spanning six full decades of his miraculous career. Because of the sheer number of works included therein—tens of thousands—it’s nearly impossible to make generalizations about their quality, subject matter, and style. Still, it can’t be denied that they represent an invaluable account of Turner’s growth as an artist and a considerable artistic achievement in their own right. So what exactly do those sketches reveal about their creator?

1. His sketches were more than practice

For most artists, a sketch is a rough draft for a later, better work. Not Turner. If anything, his paintings are attempts (not always successful) to evoke the energy and immediacy of his initial takes. Compare his watercolor sketches of wave-tossed ships (above, c. 1845) with any of his oil paintings of the same subject. The paintings may be more “finished,” but the sketches are electric.

2. Travel revitalized him

To some, Turner is the quintessential English painter. Yet he spent a considerable chunk of his adult life exploring continental Europe. As his fame grew, he visited Paris, Pisa, Amsterdam, and Rome, filling whole books at a time with sketches of ruins and cathedrals. His panoramic views of Rome (1819) can rival any of his early paintings, though his sketches of Pisa (1828) get the angle of the Leaning Tower hilariously wrong.

Turner sketched England as well; his impressions of the little coastal town of Folkestone (c. 1845), in fact, are easily some of his most ravishing works. But, as Turner scholar Andrew Wilton has pointed out, images of his immediate surroundings—i.e., working-class London at the height of the Industrial Era—are conspicuously absent from his sketchbooks. Nature was always more intriguing.

3. He couldn’t do everything

In a typical Turner painting, the figures are either ant-size or (more likely) absent altogether. Even in a masterpiece like Rain, Steam and Speed (1844)—sometimes interpreted as an allegory of humanity’s ambiguous triumph over the natural world—actual human beings are barely discernible. Generations of art historians have interpreted this as Turner’s critique of the modern age, a sign that he was more interested in the sublime than in civilization.

Or maybe there’s a simpler explanation: Turner was bad at drawing people. Consider his sketches of “Scottish Figures” from 1801 or the nudes he drew while visiting Devonshire in 1812. The best that can be said about these early missteps is that they convinced Turner to play to his strengths—i.e., making awesome, overwhelming images of storms and skies. What can’t be said about them is that they are… good.

4. The sun was his forte

Turner was one of the few early 19th-century landscape artists to consistently feature the sun in their works. This isn’t as surprising as it might sound. We can’t stare at the sun for more than a few seconds without damaging our eyes, and there’s almost a contradiction in the notion of rendering the sun visible on a canvas. To paint the sun even halfway realistically, you’d need to be a master at using oils and watercolors to create the illusion of bright light. Needless to say, Turner was exactly this, but his sketchbooks prove he was also dedicated to getting it right. As late as the 1840s, he was still making diligent sketches of the sun’s reflection on the water, complete with annotations like, “warm,” “red gleam,” and “glow ends.”

5. He got more daring with age

At some point in his middle age, in the mid-1830s, something changed in Turner’s art. His colors intensified, and the careful lines of his early work gave way to expressive blots and smears. Suns, seas, clouds, and skies all seemed to bleed into each other; thick, muscular brushstrokes mimicked the fury of the storms they represented.

For the art historian Gerald Wilkinson, the turning point in Turner’s art was 1834, the year the Houses of Parliament burned down. Turner’s watercolor sketches of this infamous disaster must rank among his boldest works in any genre—his grays smolder, while his reds, oranges, and pinks seem to “mix” in the viewer’s eyes instead of on the page. Generations of radical artists, from Claude Monet to Mark Rothko, were inspired by these works—it wouldn’t be much of an exaggeration to say that modern art began the day Parliament caught fire.