To start with, they tried tinkering with the mixture of ingredients in the ink. This worked, but only to a point. The maximum by which such tinkering could delay the colour change was half an hour. Given that strong sun creams and darkly pigmented skin delay burn times by up to five hours, the researchers realised that they needed a different approach if they were going to create a useful product. They therefore started experimenting with cheap, non-toxic UV-filtering films of various opacities, and found that the strongest of these could delay the sensor’s colour change by 9½ hours—nearly twice what was needed.