Professor of art history at Amsterdam University, Ernst van de Wetering, explains to Kam Patel his enduring fascination with Rembrandt, about whom he has just written a book

In the autumn of 1885 Vincent van Gogh, visiting the newly opened Rijksmuseum in Amsterdam with a friend, came upon Rembrandt's The Jewish Bride. He was entranced. After sitting and admiring the work for a long time he told his companion: "Would you believe it - and I honestly mean what I say - I should be happy to give ten years of my life if I could go on sitting here in front of this picture for a fortnight, with only a crust of bread."

In relating this tale in his new book Rembrandt: The Painter At Work, Ernst van de Wetering, professor of art history at Amsterdam University, reminds us of the fascination Rembrandt's work holds for many people. For Van de Wetering, Rembrandt was very much a "painter's painter". He writes: "With many artists the act of painting can easily be followed from brush stroke to brush stroke, but in Rembrandt's late work it appears to be the result of unfathomable processes." Having spent nearly 30 years researching Rembrandt's work - his use of materials, studio practice and ideas about painting, much of it made possible by science - Van de Wetering can stake a stronger claim than most to making sense of those "unfathomable processes".

Only fragments remain of the artist's own comments about his work and ideas. Nevertheless, through secondary sources, especially Rembrandt's pupils, and scientific study of the paintings, Van de Wetering can offer convincing new dimensions to the artist more than 300 years after his death. As he says: "What I am most proud of with this book is that I give Rembrandt a voice, I let him talk more indirectly than he ever did in life."

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Rembrandt, who died in 1669, is widely thought of as a dramatist, an artist keen on getting people to "act" in a strong way through, for instance, powerful facial expression and the creation of movement between characters and space. From his painstaking research Van de Wetering concludes that for Rembrandt form was as important as content. Improvement and invention were central concerns throughout his life and he was praised for novel approaches to dealing with light, structure and composition. "Once you know his work intimately, there is no question that this was one of his main concerns. And that is completely different from this pious, romantic view of Rembrandt that people have. Of course he was a dramatist, he was after effect, just like you could say film directors are. But at the same time he was a painter thinking deeply about painting," says Van de Wetering.

Rembrandt was famous worldwide during his lifetime, but he had his critics. They took issue with his representations of "ugliness" rather than "beauty", and his reluctance to conform to the world view of the classicists. "He did not obey the rules of art, beauty, proportion and so on I he did not idealise and that annoyed some people. Of course he was not the first to do so - Caravaggio did it before. Rembrandt built on his heritage. But he was not a strict Caravaggist, he developed the ideas further and combined them with other ideas of narrative and style in painting."

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There is a tendency to think of the artist as "rebel" as a phenomenon that emerged in the 19th century. But Van de Wetering says that the image of the "crazy", unadaptable "bohemian" artist, has a far longer lineage. He notes that in 1604 one commentator said: "The more they paint the more they are untamed." And in an anonymous pamphlet from 1662, arguing that people ought to dress more in accordance with their social status, the author acknowledges that he is at a loss with painters: "I these fellows are so different (in social status) and many are also crazy."

Van de Wetering says: "Many people today tend to think of artists in the 16th and 17th centuries as craftsmen, painting like others would make a wagon or a wheel or a shoe. But the artist already was seen even then as outrageous and outside the community."

Certainly Rembrandt seems to have led a very colourful life. He lived lavishly for a time on the back of his success and seems to have been socially ambitious, as a young man at least, buying an expensive house and filling it with a huge collection of art and ethnographic objects. At auctions he always directly made the highest bid for the work of great artists: "He wanted to show that great art is precious. It is also told that he believed great art should be well paid and it was probably the way he thought about his own work too." He asked for, and got, a lot of money for his works.

Some of his most powerful works are based on religious subjects, but it is not known whether he was religious. Van de Wetering believes he was not: "The Blinding of Samson, for instance, can be seen on one level as an illustration of a violent biblical story. But in the 16th and 17th centuries the story of Samson betrayed by Delilah was also regarded as a warning of women's power to corrupt men. So, suddenly, it does not have anything to do with the Bible anymore."

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There is less uncertainty about Samson when the aims of Rembrandt, the master technician, are considered. Rembrandt's primary purpose here is to organise light with paint and space by the complex grouping of light and dark colours and subtle changes in tonal strength. The result is a powerfully three-dimensional effect conjured from the confines of the flat plane. "He was highly praised by his younger contemporaries for developing this way of working. People talked and wrote about it a lot," says Van de Wetering, whose one regret about his book is that it does not have a chapter on light. "It would have been a massive chapter so I am going to write a second volume dealing largely with how he dealt with the behaviour of light," he says laughing.

One way in which Rembrandt sought to enhance the effect of light was to use paint in relief. Many, in his own lifetime and since, have been intrigued by Rembrandt's rough manner of painting, with very heavy paint structures and wide variations in the degree of finish. It had, says Van de Wetering, much to do with helping to create an illusion of reality: the structure of the paint surface was constructed in such a way as to suggest textures of the real world. A picture displaying a wealth of such surface effect is Musical Allegory (1662). The red leather shoe of the seated woman is smooth and gleaming while the gold brocade hem of her skirt, lying over the shoe, has ridges of yellow and white paint on a thick, lumpy yellow foundation.

Another device for suggesting movement in space was the putting of light into "atmospheric perspective". Van de Wetering says: "Whenever I go to see The Night Watch at the Rijksmuseum, and look from the painting to the surroundings, I have great fun in seeing that the painting is more real than reality." This has been achieved through "enormous and exact control of slow changes in tone but also by using a notion current at the time that the 'thickness' of the air changes the colour of things over short distances. They strengthened this effect which in reality does not exist. The Night Watch looks stronger than reality without it being really obvious and that is another one of the tricks he used," he says.

For Van de Wetering the study of Rembrandt was an "accident", although he grew up with the artist, his father having loved his work. "But, you know, when you have the chance to have hundreds of Rembrandts in your hands and to look over the shoulder of a great artist, and see how he is doing it, that is really addictive and I am still addicted to that. When you have worked for 30 years on a subject you suddenly start to understand things you never understood before."

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The passage of centuries has made his job difficult but science and technology have come to his aid, with ghostly images of what lies beneath the paint surface: "The moment you get the first X-rays of a painting you get very excited because suddenly you start to get hold of the process instead of just the surface. They tell us not just about the changes he made, that is nice, but even better is that you start to see Rembrandt thinking about process. You learn to appreciate his hand and gesture in working himself to a conclusion. And that is wonderful, really fantastic."