How did preparatory sketches guide the placement of tesserae in wall mosaics?

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Bird mosaic — mosaic Mosaic floor fragment from a synagogue or church, cut stone with mortar from Israel, late 5th–6th century ce; in the Jewish Museum, New York City.

In the creation of wall mosaics, preparatory sketches served as essential guides for the placement of tesserae. Artists often executed detailed drawings directly onto the final layer of the setting bed, frequently including indications of the main colors to be used.1

These sketches functioned as a roadmap for the disposition of the cubes, ensuring the artist maintained the intended composition while working in small, daily sections of mortar. In some instances, evidence suggests a multi-stage process: artists would first create rough preliminary sketches on the wall surface or on layers beneath the setting bed to establish the overall decorative scheme, followed by a more refined, detailed sketch on the final mortar layer to guide the actual laying of the tesserae. This practice mirrors techniques found in wall painting, allowing the craftsman to visualize the final design before committing the individual pieces to the mortar.1