Conservators frequently wash paper to remove dirt, stains, acids, and decomposition products. Washing an acid paper with the proper water significantly increases its life. The use of high-quality water, distilled or deionized, would seem to be advisable for this purpose. However, recent work at the Library of Congress shows that such extra-pure water, con-taining no calcium, may shorten the life of the paper when compared with an unwashed control or Washington, D.C. Municipal tap water, as measured by folding endurance and brightness tests carried out after accelerated aging in dry and humid ovens. Washington tap water has certain drawbacks because it contains chlorine which acts to oxidize cellulose, and iron and copper compounds which may act as oxidation catalysts for paper stored under humid conditions. Distilled or deionized water passed through a column of calcium carbonate chips becomes acceptable for paper washing, and can be used without shortening the life of the paper. This calcium modified water will, however, pick up copper compounds from copper pipes used to deliver it to washing sinks, and should be carried by inert piping.