What are the materials needed to refill ink in HP printers?

I am looking for things needed to be able to refill my cartridge in my printer. Of course i already know what type of ink I will use. I was just wondering what chemical to put in the lid where ink comes out so the dried inks can be wiped out. When I was in INKTEC, I saw the sales lady put on some kind of white liquid in the lid of my cartridge and presto the black inks that are dried are now dissolved. I really need to know what chemical is that.. is it water? alcohol?What are the materials needed to refill ink in HP printers?
I don't about how Inktec does it, but I just use a lukewarm water and dip the cartridge's head into it. In some places the water is in a small tub with ultrasonic switch that when turned on, causes the hardened ink to dissolve more easily.