The objective of the cleaning restoration of The Baha'i House of Worship, a national historic landmark located adjacent to Lake Michigan in Wilmette, Illinois, was to restore the aesthetic beauty of the building and physical integrity of the surface and substrate of the masonry facade by removing all harmful and unsightly deposits and stains without adversely affecting the surface and substrate of the masonry facade by mechanical or chemical action such as abrasion, etching or addition of a harmful residual material such as a soluble salt. It was also necessary to correct flaws in the normal functioning surface and subsurface systems of the facade such as vapor transmission that is blocked by an impermeable surface coating. It was imperative that the cleaned ornamental concrete surfaces were harmonious in color, texture and tone with the original qualities of the masonry as determined by historic documentation such as photographs and by comparison with relatively clean and unweathered sections of the building. It was also important that the actual intervention be the minimum necessary to achieve the objective (ie. using the gentlest possible methods).