Moreover, quantities and proportions are always quite inexact: "take a good portion of madder and put it in a certain amount of water; add a bit of vinegar and a lot of tartar." The same imprecision applies to the length of time one should boil, decoct, or soak the cloth, which is rarely indicated or else highly implausible. A text from the late thirteenth century, for example, explains that to produce green paint, copper shavings should be soaked in vinegar for either three days or nine months. As is often the case in the Middle Ages, the ritual is more important than the result, and numbers are more symbols than actual quantities. For medieval culture, three days or nine months represented more or less the same thing in that both expressed a period of waiting and (re)generation—Christ's resurrection occurred on the third day, a child arrives after nine months. It is typical of medieval thinking to superimpose such symbolically significant periods on other spheres of life and activity.
Blue—The History Of A Color, Michel Pastoureau 2000.
But I like the thirteenth century way of proportions and time; they're very similar to the directions for cooking in Larousse Gastronomique, 1938. One doesn't follow the recipe blindly, one creates as one goes—it doesn't always work—and that's what makes it so special when it does.