The position is different when it comes to large and protracted works of art. Then the last layers will consist of undiluted fatty tempera colors with, if required, the admixture of a wax mastic to make them flow better. Thus, you begin with a lean tempera diluted with water. Over this place a coat of undiluted tempera. Continue with fatty tempera, first thinned with turpentine, then undiluted.
Lastly, use super-fatted undiluted tempera colors. This not only obviates the need for a varnish to seal and protect your surface, but also gives a depth of color as rich as that of pure oil painting. The colors harden without any perceptible lightening in shade.
The technical significance of fat on lean is quite clear: the ground should dry or, it may be, oxidize as quickly as possible, so that if delayed-action oxidation occurs it will cause no increase in volume which might make the upper layer split or give rise to early crack formation. This is not really as elaborate as it may sound once you are used to the fat on lean principle in all painting; you will then find that you conceive of the construction of a picture in the appropriate artistic terms.
Note that even though while painting in lean tempera the result looks like glue painting, and in fatty tempera more like oil paint, the tempera color always results in a slightly brighter and chalkier tone. This does not altogether hold true when the upper layer is super-fatted, however. Tempera color laid on a shiny surface, like casein painting, can be brushed and rubbed. Naturally, the less fat it contains, the weaker it will be. When applying a protective top layer which will preserve the colors' luster and depth, then it is pleasanter and more in keeping with the materials to apply the sheerest coat of wax rather than a greasy resinous varnish.
Within reason, there is unlimited scope for the impasto application of exceptionally fatty tempera, and certainly more so than for pure oil painting (with the exception of palette knife painting). Thick applications of tempera color always remain spongy and porous, owing to the inner structure of the binder; and the result is a picture at once transparent and, when exposed to the air, able to dry right out and oxidize through and through, and become exceptionally hard. Tempera hardens to such an extent that
it takes considerable force to remove a paint layer from its ground, while a layer of oxidized oil paint remains softer and more sensitive after oxidation.
A tempera painting attains its greatest degree of hardness and permanence, however, when from the ground up its construction follows its own special rules. This is a method known as a mixed or alternating technique, and is only relevant to studio conditions. As against working in alia prima oils, this tempera technique starts from a completely different visual concept, a creation in the true sense of the word, in which the model or impression is received but not copied from nature, and renipins a more or less distant memory.