The dome was last cleaned in the summer of 1998. Since 1959, the building has been cleaned about every ten years. The Legislative Building's exterior is made of sandstone, a relatively soft and porous material.
Water seeps into the stone, creating a nourishing environment for mold, moss, algae, and lichens.
Some of these organic materials are black or dark green and make the building look dirty. In addition, water also freezes and thaws in cracks, causing joints to separate. When water evaporates, it leaves damaging salt deposits behind. All of these conditions stress the composition of the stone, causing it to crack, flake, or otherwise disintegrate. A mild detergent with low water pressure and soft brushes is used to clean biological growth from the building.