Embroidery milk : locut. Prison ink (thieves' slang).
History: In prisons, when the lazagner writes a letter for a comrade, he says that he uses the milk to embroider (to embroider) to tenderize the one to whom one writes. The word Ink is here a figure, because often it is the milk that is used. In prisons, we know that all letters from prisoners addressed to relatives or friends go through the registry. The clerk or director reads the letter and if it does not contain anything contrary to the rules, he marks it with the sign "V". The greatest concern of prisoners is to avoid this embarrassing formality, especially if the letter is addressed to an accomplice. So they use the milk to write between the lines written in ink. For that you need skimmed milk and unglazed paper, because the writing would be oily, shiny and the deception would be apparent. To make the writing appear, it suffices to strike the letter strongly with a slipper full of dust; the dust sticks to the characters which become legible. In the past in prisons they used onions, but the trick was discovered, they are no longer sold in the canteens, while there is milk.