Bob made a gorgeous tart using blood oranges and lemons (see picture). The colour was quite lovely, a deep golden yellow which reminded me of Renaissance fresco paintings.

I started to search around on art-history sites and paint history websites for a description of that particular colour. In the process I found all sorts of interesting stuff about the history of yellow paint and the ingredients – rocks, minerals, plants, flowers, berries, clay and earth – which are used to make it. I read about a yellow called ‘gamboge’, and about umber and burnt sienna, about Naples Yellow and lead-tin yellow, about King’s Yellow, and about red and yellow ochre. I read about orpiment, a dangerous reddish-yellow which contained arsenic sulphide, and about the expensive saffron. I learned about apprentices whose task was to mix up colours from ingredients hard to obtain and infuriating to grind. By switching the search to ‘images’ I found cascades of glorious pictures of all these yellows, and instances of their appearance in paintings and art objects around the world. From which I concluded that Bob’s blood orange tart was perhaps the colour of orpiment.