The letter threatening the doorman was written with one of the six fountain pens seized. A piece of the signature has been cut out and set running alongside a sample of every ink.
Each dye climbs to its own height. The guilty ink has exactly the same bands as the letter: not one more, not one fewer.
A black ink is not black: it is a blend of dyes that the paper pulls apart as the solvent climbs through it. Each dye stops at its own height, always the same one. There is the sample from the letter and the six references; only one of them has the same set of bands.
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