Try using cornstarch, boil off excess water, leave ink out in air to evaporate(stir occasionally), extender base will thicken and make more transparent, pva glue( dries flexible, acid free), magnesium carbonate, molasses, ground chalk,

http://www.ehow.com/how_6659090_make-ink-thicker.html

http://www.gigposters.com/forums/screenprinting/51522-thickening-ink.html

http://www.barenforum.org/archives/vol14/v14_1343.txt

http://www.briarpress.org/20882

http://www.bombingscience.com/graffitiforum/archive/index.php/t-71.html

ink