It got worse. ‘It’s been written on both sides. This is a problem for the ESDA machine because one side is imprinted on the other.’

I could feel my optimism evaporating. I pointed out that the third sheet of the letter had nothing written on the back, but by then the detective had discovered the final obstacle. ‘This paper is sort of plastic-coated, and we will have to be very careful with that. The ink hasn’t penetrated the paper very much. It’s just stayed on top and dried, not permeated in, so we’ll be very lucky to find anything.’

I showed him the smudge the graphologist had made. She had wanted to check whether the letter was written with a rollerball-type pen, and before I could stop her, she had licked her finger and wiped it over a word. The ink had followed her finger across the paper in a little blue smudge.

‘Yes, this is worrying me,’ said the detective. ‘As soon as it touches anything damp, that ink’s going to run. If we put it into the humidifier tray, with this ink and this paper, we’re going to create a lot of damage. The ink’s going to just slide off the paper. That will be the end of everything.’

I felt terribly protective towards the letter. When the graphologist had smudged the word, I had been annoyed; it had caused me something like a physical jolt, an irritation unconnected to the fact that she was possibly polluting the document by adding her DNA along with her saliva to it. I was not going to risk putting the letter in a tray of liquid and being left with a white sheet and a bowl of blue water. And somehow, the fact that the letter was holding on to its mysteries once again confirmed its status, even in the eyes of this clever professional with his serried banks of technical equipment.

We decided to check the letter on the ACO tester anyhow, since that caused no permanent physical damage. The detective placed the flattened sheets one by one under the filter and twiddled the dials until the ink disappeared from the paper and he was left with anything else there might be. ‘There’s a crossing out, a bit of white-out.’ He confirmed the graphologist’s comment that the letter-writer had used a line guide, for every line was exactly on top of the one on the reverse side of the sheet. There were no other indentations on the sheet, so he assured me that the ESDA would not have yielded any more information even if we had been able to use it. It seemed plausible because any indentations would surely have been revealed under the fluorescent light too. He agreed that the pages were pulled from a pad, that there were little roundels at the top of the sheet, where the page had been torn off, and that the page had probably been ripped from left to right.