Can I use the ink from the Photosmart inkjet cartridges for my dip calligraphy pens?
Yes, inkjet ink works with brushes. It's thinner than the usual ink for dip pens, but give it a try. I haven't broken open cartridges to get it, but used the ink from a refill kit. Because they're pure, primary colours, you can theoretically mix any other colour from them, just as the printer does. You can certainly get strong, saturated shades.