In 1979, I was teaching nude photography at Parsons school in New York. I needed models for the course – and one day a 20-year-old called Madonna Ciccone showed up. She was just another citizen, a girl trying to make ends meet. She was quiet, taciturn. I'm not sure it was something she enjoyed. She did it for the money, in this case $30. She was relaxed, composed, did as asked. Some people are stiff, some are there to do a job, some give a little more. She was in the middle: she did what she was told but nothing extra.

I was into the body as a sculptural form in those days. I'd bend or twist models to see what I could create. It was experimental. She was very comfortable with her body, and with being shot by 15 students. When you're doing nudes, you have to have everything planned. The model has to be comfortable or you won't get a lot out of her. It was February and I had two heaters on to keep her warm.

In January 1985, I saw her on the cover of Interview magazine and thought: "Wow, I know her." Five months later, she was on the cover of Time magazine – as Madonna. I called the art director at Penthouse. One thing led to another and, eventually, Playboy published a series of photographs, in September 1985.

Shooting nudes is tricky. What are you trying to do? When is it a nude; when is it erotica? There is nothing erotic about these pictures. Erotica suggests sex; these pictures don't suggest sex. They are studies of the body – it's sculpture with a camera. Currently, I'm doing a series of nudes of women over 50. I want to show that their bodies are beautiful.

If I had my time again, I would have done more with her, changed the lighting, with maybe a bit more on her face, or tried a longer lens. I'd love to shoot her again in exactly the same positions – to see how she has evolved.

Curriculum vitae

Born: Prague, 1946.

Studied: "Fort Monmonth, New Jersey. I was drafted, then went to army photo lab school."

Inspiration: "Bill Brandt did great nudes. ­Peter Lindberg does great work, too."

High point: "Now – but also when my first book came out in 1981."

Low point: "When you don't work or don't sell. When you have a show and nothing ­happens."

Dream subject: "I'd love to travel and photograph indigenous peoples."