At lunchtimes, I go to the British Museum to draw. It’s never not full of school children, performing their dutiful rites in the Egyptian rooms – cataloguing mummification urns and cat gods – or sitting on the floor in the Anglo-Saxon galleries, listening to stories of Sutton Hoo. There are tourists too, thousands of them, there for the Rosetta Stone and the Elgin Marbles. My favourite subjects, the Cycladic sculptures, are dotted along a corridor between the entrance shop and the café. I don’t mind: no one expects to find the British Museum empty any more; it’s an effective modern machine, not a sleepy repository of treasures. But I do think I’ve as much a right to be there as anyone else: it’s not I who get in their way, it’s they who get in mine; standing in front of Lely’s Venus to take pictures while I pause, mid stroke, and try to hold the image in mind until they move on. Some days you can’t catch more than a few seconds’ continual study, so I go to the quieter rooms – Ancient Crete, say – and look at things there.

It’s not only art students who sketch in museums and galleries, though I did lots of it when I was one. Drawing is a form of knowing; the best way, I think, of seeing. At the very least it shows how little you really see when you’re not drawing. The work isn’t in the hand so much as the mind – it’s not dexterity you gain especially, nor do you always get better at making things on paper look as they really are, but you start to alter the way you see them in the first place. You learn to flatten and expand space, to fix things in the mind’s eye; not just to arrest them, like a camera, but to hold the aspects in relation to one another as you move between and over them. The hand is just the extension of the mind, concentrated through the eye, hovering, taut and drawn out; and you try to keep it there, poised, as you work and rework what you see. You stop hearing the voices around you or noticing the people walking back and forth. Time passes in an instant.

It’s not essential to copy great works of art, but it is unlike other forms of drawing. You get to know the objects inside out and to see them as the artist did – fields of dimensions and decisions and measurements. It makes you a better art historian and a better artist, and it’s a form of respect, to know them so intimately. Most galleries are very good about sketching – I’ve seen artists with their oil paints out in front of Rubens’ Samson and Delilah – and it’s in their interest. Artists have always stolen from other artists.

But the gallery conditions that enable the unfolding of stools for hour-long residencies are no longer those of major London exhibitions, which have grown ever more popular, crowded and crushed. Most are timed, for a start – you must move briskly to prevent congestion – and now the V&A has forbidden sketching, along with photography, at its new exhibitions about Botticelli and the history of underwear, just as it did for its David Bowie one a few years ago. (What counts as sketching anyway? Are notebooks confiscated? Do doodles count?)

The idea is to keep people moving in synchronised channels – exhibition spaces as crowd control – and anyone who went to the Rembrandt show at the National Gallery last year knows how desperate it can be when that goes wrong: pictures obscured, crowds five people deep in places; the experience like wading through a crowd at Glastonbury, desperate just to make it to the other side. It’s annoying that we can’t sketch knickers at the V&A, but more annoying that footfall takes precedence over engagement. It would be useful if galleries, instead of banning drawing, thought about ways to make their popular exhibitions accessible to those who, for whatever reason, like to go slowly as they look.

Lead image: used under Creative Commons licence (CC BY-SA 3.0)