Desktop inkjet printers, as used in offices and homes, tend to use aqueous ink based on a mixture of water, glycol and dyes or pigments. These inks are inexpensive to manufacture but difficult to control on the surface of media, often requiring specially coated media. HP inks contain sulfonated polyazo black dye (commonly used for dyeing leather), nitrates and other compounds. Aqueous inks are mainly used in printers with thermal inkjet heads, as these heads require water to perform the ink-expelling function.