Water content of the ink can be up to 10%. Besides solvents and the dye itself, the ink may contain additives (e.g. nonylphenylpolyglycol ether, alkylpoly-glycol ether, fatty acid polyglycol ester, or fatty alcohol ethoxalates) and preservatives (e.g. 2-Phenylphenol and its sodium salt, 6-acetoxy-2,4-dimethyl-m-dioxane). Permanent markers are porous pens that can write on surfaces such as glass, plastic, wood, metal, and stone. The marks made by such pens are however, not permanent on some plastics like Teflon, polypropylene etc., and can be erased easily. The ink is generally resistant to rubbing and water, and can last for many years. Depending on the surface and the marker used, however, the marks can often be removed with either vigorous scrubbing or chemicals such as acetone. They are highly regarded by colorists and artists around the world. Highlighters are a form of marker used to highlight and cover over existing writing while still leaving the writing readable. They are generally produced in neon colours to allow for colour coding, as well as attract buyers to them. A whiteboard marker, or a dry-erase marker in some locations, uses an erasable ink, made to be used on a slick (or matte-finished), non-porous writing surface, for temporary writing with overhead projectors, whiteboards, and the like. They are designed so that the user is able to easily erase the marks using either a damp cloth, tissue, handkerchief, baby wipe, or other easily cleaned or disposable items. Generally, people use fabrics to do so, but others use items like paper, clothing items, some even use their bare hands to wipe it clear. The erasable ink does not contain the toxic chemical compounds xylene and/or toluene as have been used in permanent markers, being less of a risk to being used as a recreational drug. Wet-wipe markers are another version that are used on overhead projectors, signboards, whiteboards, and other non-porous surfaces. Special "security" markers, with fluorescent but otherwise invisible inks, are used for marking valuables in case of burglary. The owner of a stolen, but recovered item can be determined by using ultraviolet light to make the writing visible. Marker pens with election ink (an indelible dye and often a photosensitive agent such as silver nitrate) used to mark the finger, and especially the cuticle, of voters in elections in order to prevent electoral fraud such as double voting.