Records survive that show that such organic additives as blood, cheese, egg, and dung were used in mortars in past centuries. Restorers face the question of whether to replace old mortars with mortars containing similar organic components or with those containing technologically advanced synthetic components. Preliminary research was conducted on the various uses of organic and synthetic admixtures. Simultaneously, weathering and exposure tests were conducted on a variety of organic-based mortars. The compiled results, and factors for and against the use of the two types of additives, are set forth in this paper. See also AATA 19-1792.