The inks are water-soluble when in liquid form, but when they dry they can
form a hard crust that resists water.  The solvent provided by the ink
manufacturer is an acid which is why repeated uses destroys the printhead.

If 1 or 2 attempts at unclogging the printhead with the solvent fail your
only recourse is to physically replace the printhead itself.
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Birk Binnard
Peninsula Software
http://www.birkbinnard.com