| 摘要 |
<p>Envelope contains a water-soluble pigment mixed with a printing ink which is not water-based printed on the outside of the envelope-flap and/or seams, suitable to give indication of a possible unsuthorised opening using moisture. The print used may be a dotted pattern over part of the flap or over its entirety. A reversed printing technique may be used to apply a message or slogan. The water-soluble pigment consists pref. of 2 water-soluble colourless materials which react to form a dye upon exposure to moisture.</p> |
