Instead of using chalk (as on chalkboards) to make the markings, a special whiteboard erase marker containing removable ink is used (see non-permanent marker). Whiteboards are normally constructed melamine resin, but more expensive models can be made from porcelain enameled steel. Steel whiteboards are magnetic, so one can use magnets to affix notes or a magnetic eraser to them. Porcelain boards are more durable than painted steel and standard melamin boards. Melamine resin and painted steel will fade to a slight gray color over time, as ink of the dry markers enters into the pores of the material. Porcelain, or vitreous enamel, is practically impermeable for dry marker ink and will remain white throughout the lifespan of the whiteboard. Both porcelain and melamine exhibit ghosting. Ghosting occurs as a result of the solvents (usually alcohol) in dry markers that effectively make the board cleaner where there has been writing. This remains visible after the ink is wiped off, but ghosting disappears after a short time. The term whiteboard is also used metaphorically to refer to features of computer software applications that simulate whiteboards. Virtual whiteboards allow one or more people to write or draw images on a simulated canvas. This is a common feature of many virtual meeting, collaboration, and instant messaging applications.