Detail of "Noli me tangere" (touch me not), with imitation marble in "Roman" stucco

The importance of the sequence of execution in mural painting and specifically in the a fresco method is well known. Fresco, indeed, has characteristics and specific requirements that other forms of painting do not have: it requires a fresh coat of plaster which is usable only up to a particular stage of dryness, so that you cannot plaster a greater area than you expect to paint that day. This area - the giornata, or 'day' - varies according not only to the complexity of the image to be painted and the skill of the painter, but also to the season and the local conditions. You cannot alter or retouch a fresco except by painting over it (the 'a secco' method) or by cutting out the dry portion of plaster and starting again. The technique demands that you work from the top downward - and usually, though only for the sake of convenience, from left to right. Its use is incompatible with a number of pigments (especially azurite), which are damaged by the main ingredient, lime: consequently, these can only be used in conjunction with a binding medium, which is mixed with the pigment and will cause it to adhere to the dry plaster. Metal foil - gold leaf, or tin either gilded or covered with silversmith's varnish (meccato), can also be applied only to the dry plaster, on top of one or more intermediate layers. In addition, before the design can be drawn and the pigments applied to the final layer of plaster, a whole series of preparatory operations must be carried out: snapping the cords (establishing a centre line or a grid by means of a taut cord dipped in paint and snapped against the surface), applying the rough plaster undercoat or arriccio, incising lines, drawing the design with sinopia red. It is thus clear how unreliable it can be to calculate the time taken to execute a fresco simply from the number of giornate - especially if we do not know how many workers were involved. To get a comprehensive idea of the time needed to produce a fresco, you also have to take into account all the preliminary work, which in the fourteenth century included the preparation of much of the materials and of drawings to be transferred to the wall. Even if we consider only the application of the water-based pigments on the freshly laid plaster, all that can confidently be said is that it is possible to paint more than a single giornata in one day, if small areas of plaster are used, but that it is not usually possible to paint less than one giornata, because of the risk of imperfect carbonation of the area painted later.

Innovations
That Giotto's commitment to a reform of the Byzantine tradition was programmatic and total is demonstrated by the way in which he confronted the methodology of painting. It was a field in which innovation was difficult, because the only methods known were those handed down in the workshops and because in a technologically complex process like mural painting it was difficult to experiment with new materials that had not already been tried out, over a long period, in the workshop. We must remember that, at that time - and indeed long afterwards - the only way of judging the success of new materials or procedures was to examine their performance over time. Individual experimentation on a scientific basis did not begin until at least the early fifteenth century. And yet in the Arena Chapel a technique was devised to give the fictive marble panels of the dado and similar elements in the narrative panels the appearance of real marble. The effect produced by this technique is similar to that achieved later by so-called 'Roman stucco' or 'polished plaster' and, by all accounts, to that of ancient Roman encaustic murals, but there appear to have been no medieval precedents for it. Even without this particular instance of technical mastery, it is clear that the Arena cycle introduced a new way of executing mural decoration, both in terms of organisation and in specific technical procedures.
It is well known that already in the great Assisi cycle, from the Isaac scenes onward, the technique of painting a fresco based on giornate had replaced the a secco method (or at any rate the characteristic use of large areas or colour painted a secco) with its attendant structure based on pontate or scaffolding levels. This new method involved the rejection of the traditional, rigid graphic schemes developed over the centuries and codified in Byzantine manuals on painting, with the result that each new image required a new drawing. The master had the task of inventing the images and drawing them, while for the actual painting he often used assistants, of varying degrees of specialisation depending on their individual experience. There is insufficient evidence to prove that by Giotto' s time there were methods and instruments for transferring drawings on to the wall (like the later use of cartoons); but certainly there is no point in the cycle where the presence of the master cannot be felt (even if his hand is not always apparent). This suggests a rationally organised hierarchy within the workshop. Furthermore, a totally new system of painting was devised which permitted novel effects of softness and blending of colours - a system codified a century later in a treatise by the Paduan artist Cennino Cennini. Through the use of these highly sophisticated and complex techniques, most of which were hitherto unknown, Giotto was able to create in Padua one of the greatest masterpieces of European art, with a sureness of touch (in the entire cycles only a few pentimenti have been found) and a rapidity that are indeed extraordinary.