Hi Sarah,I agree that is a little confusing - Sinopia refers to the monochrome painting/composition layout done on arriccio coat it helps to visualize composition in its final place and to line up the sections (giornatas) during the actual painting - intonaco applied over the arriccio completely consealing sinopia beneath.
http://www.pinterest.com/pin/115897390384317972/
In pre-renaissance cartoons were not used so sinopia was also the only way to create a composition/study and present the concept to the patron.
What you refer to is "Verdaccio"
Verdaccio - an Italian name for the mixture of mars black and yellow ochre and very small amounts of white and red resulting in an olive tone, brownish soft green. Verdaccio also refers to an under-painting done using verdaccio color. During the Renaissance, verdaccio became an integral part of fresco painting. This color is used for defining tonal values, creating complete monochromatic under-paintings. Architectural details in frescoes are often left in verdaccio without any additional color layers. Michelangelo’s Sistine Chapel is the best example where you can clearly see verdaccio under-painting left as is on most of the architectural details of the composition.
Check here: http://iliafresco.com/medium-of-fresco/multi-layer-fresco-didactic-panel-demo.html
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