Cézanne is not an artist who I’ve previously associated with portraiture. All the more reason, then, for me to visit the National Portrait Gallery’s exhibition dedicated to the portraits of the French, post-Impressionist painter, whose work paved the way for the likes of Matisse and Picasso.

What an interesting experience this was. By way of context, this is the only exhibition devoted to Cézanne’s portraits – until now, this aspect of his work has been largely overlooked, with his still lifes and landscapes believed more central to his art.

Sensibly, the exhibition groups the portraits by subject – for example, those of Cézanne’s Uncle Dominique are displayed together, as are a series of Hortense Fiquet, Cézanne’s wife. There are some stand-alone portraits, such as ‘The Artist’s Father, Reading’, below, which I loved for its subtle use of colour and the expression of fixed concentration upon Cézanne’s father’s face. It provides a tantalizing glimpse into Cézanne’s life and habits – in the painting, his father is reading a journal published by Cézanne’s close friend, Emile Zola, and you can see Cézanne’s studio in the background.

Cézanne, so I learned today, held very firm views about the practice of portraiture. To him, a portrait was a record of the thing recorded and he allowed no poses or acting by his sitters. Declining to follow the conventions of the time of making men appear important and women beautiful or seductive, he refused to paint his sitters in a flattering way.

It’s impossible not to be intrigued by the portraits of Hortense, especially when you learn about the couple’s complex relationship. They met when Cézanne was 30 and Hortense 19 and embarked upon a secret affair; Hortense came from a humble background and Cézanne feared losing his allowance from his wealthy father. It wasn’t until 1886, when the two married, that Hortense’s existence was openly acknowledged – and even then she was shabbily treated by both Cézanne’s friends and his family. She and Cézanne often lived apart, even when they were in the same city.

Yet Hortense had by far the biggest influence on Cézanne and he portrayed her more than anyone else. I particularly liked this 1877 portrait, known as ‘Madame Cézanne in an Armchair’ – powerful in its simplicity, it is the first major portrait of Hortense and is unusual because it shows her involved in an activity (sewing) rather than posing.

I was interested to observe how Cézanne’s technique changed as his portraits became more refined. Initially, he used the “manière couillarde”, or “crude and ballsy”, palette-knife technique. This is very evident in the early portraits, whose rough texture is highly visible. Gradually, though, you see the knife giving way to the brush.

A hard taskmaster, Cézanne – particularly in respect of his own work. Rarely satisfied with his portraits, he became notorious for destroying or not finishing them; even this wonderful portrait of art critic Gustave Geffroy fell by the wayside. If you look closely you’ll see that the face and hands remain unfinished.

It’s amazing how much you can learn from a portrait. About the sitter, but also about the painter – and about the fashions and politics of the time. Every portrait is a little piece of history in its own right. Full credit to the NPG for assembling such an absorbing and meaningful collection; I came away feeling educated, enlightened – and surprised.