In the Geometric manual, ‘Patterns, a fundamental principle’ I write of working out the measurements for your mosaic in terms of the tesserae size and not inches of centimetres. This way you look at the mosaic in terms of what it is made of and not just trying to make it fit into the space you want it to.
On the last Advanced course I was very fortunate to have as my student, Stephen Fitts, a tiler of 40 years experience. We discussed many of the theories of how the Roman mosaicists may have worked and one thing that he brought up was the point that you can mark out an entire floor without using any form of measuring tape or ruler. Nothing that is marked with standard measurements in inches or centimetres.
Briefly, how it is done is like this. Take the measurement of one tesserae as your most basic unit. In the band with 3 colours and a line of black bordering each side you have another unit of measurement which is 5 tesserae, and so on for all the shapes in your mosaic.
Instead of you working what these are in terms of centimetres/inches you just mark these out on a length of wood, almost like a staff. They can be of varying lengths but if you’re working in the same tesserae size then all the measurements you mark on there remain constant.
Image you’re marking out something like the 2 strand guilloche, below;
On your staff you have marks which would be the width of this pattern (the area top to bottom), the space between each centre point, (the white tesserae in the middle) and also the width of 5 tesserae. You can mark out the length of the border and width of the whole border and also using a piece of string and charcoal or a large set of dividers with a spike at each end to score the lines you can draw a circle to mark out the loops.
You would have any number of staffs, some with general markings plus some which are marked up for specific geometric patterns.
This is a large subject to cover but over the next month I will go over it in more detail and I’ll also show how a large mosaic (4m x 8m) can be accurately marked out without using any of the measuring instruments which we have available today.