Working on a display piece. This is showing how Roman mosaicists could have worked to set a pattern without drawing out the pattern to follow. The tesserae sequence is 2- 3 -2 -3 -2..... and you can see these marks on the batons at each side (these are removed after) and the staff across the top.
Why might they work like this? In my opinion, because there are errors in the spacing in many patterns which would not be there if they had been using sinopia or etched lines. Also why go to the time needed to draw out a border pattern which may be 20m length when you can work without?
The use of batons for the edging is proven in a paper by Dr Will Wootton, 'Ancient Mosaic Techniques and modern conservation: An archaeologist's perspective'.
Watch out for more updates on this work.