Journal of Eugène Delacroix, 25 January 1857
It would be wrong to suppose that this method is more difficult than painting in oils because it needs to be done in a single operation. The fresco painter demands less of himself materially speaking, and knows that the beholder will not require from him any of those niceties which in oil painting can be obtained only by using an elaborate technique. He arranges that his preparatory work shall make the final stage as brief as possible. How can he possibly bring the least unity into a work that is put together like a mosaic - and even worse than that, seeing that each portion differs in tone while he is painting it - in other words, into work which he puts together by portions lying adjacent to one another, without being able to harmonise what he is working on today with what he did the day before (unless he makes an exact note of the general effect before beginning the work)? This is the function of the cartoon or drawing, in which the painter makes a preliminary study of the lines and effects, and even of the very colour he wishes to render. What is more, we must not take too literally all that they say of the marvelous facility with which the fresco painters overcame their difficulties. There is scarcely a fragment of fresco where the painter was sufficiently satisfied with his work to dispense retouching. There are many retouchings on the most celebrated works, and after all, what does it matter whether a work has been executed with facility or not? What does matter is that it should produce all the effect that we have a right to expect. But to the detriment of fresco, it must be admitted that such retouchings, added later in a kind of tempera, and sometimes even in oil, are apt in course of time to stand out from the rest of the work and tend to cause a lack of soundness. As years go by fresco becomes increasingly dull and faded until after a century or more, it is hard to tell what it may have been, and what are the changes which time has produced. These changes are the reverse of those that damage oil paintings. In oil paintings, blackness and the effect of darkness is caused by the carbonisation of the oil, but even more by the dirt of varnishes. On the other hand, fresco, which has lime as its base, is subject to appreciable attenuation of the colours because of humidity in the atmosphere or in the places where it is applied. Every painter who has worked in fresco has observed the whitish skin, like a grey film, that forms from day to day over the surface of the colours in their different jars. The effect is more pronounced on a large surface of the same tint and ultimately appears on the painting itself, forming a veil over it as it were, and tending in course of time to throw it out of harmony; for since this attenuation occurs chiefly in colours containing a large proportion of lime, it follows that those which contain less remain more brilliant, and by their relative crudeness bring about an effect that was not the intention of the painter.