The Church specifies . . . without getting too specific . . . that the liquid used must be “true” water. The older code of canon law used to include “natural,” as well, which is a dogmatic requirement (see Trent, Sess. VII), but that doesn’t get us much closer to concrete differentiations of liquids, which in doubtful cases remains a judgment call. But the question to ask is: would this ordinarily be thought of as water? Saliva doesn’t fit the bill. The old
Catholic Encyclopedia has a good initial taxonomy to consult:
The remote matter of baptism, then, is water, and this taken in its usual meaning. Theologians tell us consequently that what men would ordinarily declare water is valid baptismal material, whether it be water of the sea, or fountain, or well, or marsh; whether it be clear or turbid; fresh or salty; hot or cold; colored or uncolored. Water derived from melted ice, snow, or hail is also valid. If, however, ice, snow, or hail be not melted, they do not come under the designation water. Dew, sulfur or mineral water, and that which is derived from steam are also valid matter for this sacrament. As to a mixture of water and some other material, it is held as proper matter, provided the water certainly predominates and the mixture would still be called water. Invalid matter is every liquid that is not usually designated true water. Such are oil, saliva, wine, tears, milk, sweat, beer, soup, the juice of fruits, and any mixture containing water which men would no longer call water. When it is doubtful whether a liquid could really be called water, it is not permissible to use it for baptism except in case of absolute necessity when no certainly valid matter can be obtained.
