The Collaborative International Dictionary of English v.0.48:

Ink \Ink\, n. [OE. enke, inke, OF. enque, F. encre, L. encaustum
   the purple red ink with which the Roman emperors signed their
   edicts, Gr. ?, fr. ? burnt in, encaustic, fr. ? to burn in.
   See Encaustic, Caustic.]
   1. A fluid, or a viscous material or preparation of various
      kinds (commonly black or colored), used in writing or
      printing.
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            Make there a prick with ink.          --Chaucer.
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            Deformed monsters, foul and black as ink. --Spenser.
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   2. A pigment. See India ink, under India.
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   Note: Ordinarily, black ink is made from nutgalls and a
         solution of some salt of iron, and consists essentially
         of a tannate or gallate of iron; sometimes indigo
         sulphate, or other coloring matter, is added. Other
         black inks contain potassium chromate, and extract of
         logwood, salts of vanadium, etc. Blue ink is usually a
         solution of Prussian blue. Red ink was formerly made
         from carmine (cochineal), Brazil wood, etc., but
         potassium eosin is now used. Also red, blue, violet,
         and yellow inks are largely made from aniline dyes.
         Indelible ink is usually a weak solution of silver
         nitrate, but carbon in the form of lampblack or India
         ink, salts of molybdenum, vanadium, etc., are also
         used. Sympathetic inks may be made of milk, salts of
         cobalt, etc. See Sympathetic ink (below).
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   Copying ink, a peculiar ink used for writings of which
      copies by impression are to be taken.

   Ink bag (Zool.), an ink sac.

   Ink berry. (Bot.)
      (a) A shrub of the Holly family (Ilex glabra), found in
          sandy grounds along the coast from New England to
          Florida, and producing a small black berry.
      (b) The West Indian indigo berry. See Indigo.

   Ink plant (Bot.), a New Zealand shrub (Coriaria
      thymifolia), the berries of which yield a juice which
      forms an ink.

   Ink powder, a powder from which ink is made by solution.

   Ink sac (Zool.), an organ, found in most cephalopods,
      containing an inky fluid which can be ejected from a duct
      opening at the base of the siphon. The fluid serves to
      cloud the water, and enable these animals to escape from
      their enemies. See Illust. of Dibranchiata.

   Printer's ink, or Printing ink. See under Printing.

   Sympathetic ink, a writing fluid of such a nature that what
      is written remains invisible till the action of a reagent
      on the characters makes it visible.
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