An insulated glass unit consists of two or more (usually no more than three) panes of glass held apart at a fixed distance by metal or composite spacers. Once the glass is connected to the spacer bar, butyl, or a similar sealant, is injected around the spacer bar for a seal. Most often, when moisture develops between two panes of glass, it is because this seal has failed. Desiccant (small absorbent pebbles) are inserted in and around the spacer bar to absorb moisture in the air trapped between the glass layers. Almost any type of glass and coating can be used to manufacture insulated glass units, and art or leaded glass can be incorporated. Often, insulated glass units are filled with inert gasses such as argon or krypton, which resist heat transfer to a greater degree than non-gas-filled insulated glass units.