by Lawrence Payne, 6th December 2012 ·
This is a prime example of why you question everything about what you do. I've always worked to 'Leave a gap of 1 - 3mm around the tesserae'. This is an ideal but not strictly correct in practice. Let's go through it from the beginning;
Why leave gaps? Simply put, the less gaps, the more work cutting you have, the more time it takes. This is a common thing I see in mosaics that have been made using very regular tesserae, i.e. each side is 10mm x 10mm. For some reason there is a tendency when using the square cut tesserae for people to jam them right up against each other. One reason for this could be that if you have gaps then it makes it all the more apparent the regular shape of the tesserae.
So you leave gaps, 1 - 3mm around each piece. In practice though you don't have to be too concerned about being exact in applying this rule. If you know The Rules then you know you have to avoid 'channeling', this is where the tesserae match the position of the one underneath. This creates vertical lines of grout.
Just as you don't want these vertical lines nor do you want horizontal lines! So in straight lines allow some tesserae to touch at thee top and bottom.