Sometimes, it is easier to mosaic on mesh first, rather than directly onto the substrate.
I buy my mesh from a tile shop. Then, once I have my design idea drawn onto the mesh, I apply a watered down  glue to the back of the tile and stick it down on the mesh. Except it’s not that easy. All sorts of things can go wrong. The glue might block the holes of the mesh, and the mesh gets stuck to your work surface. If the weather is wet, the glue never seems to dry. If the completed mesh squares get wet or are kept in a humid environment, the glue stops working and the pieces fall off. All of these things have happened to me and were accompanied by lots of swearing and worrying about the commission being a big flop.
Then I would go to sleep, and in the morning I would think I had the solution and try again. 
I believe this project has brought me to tears more than any other. Collaborating with a renowned artist meant following their direction closely. I significantly underestimated my quote, yet I still found joy in the process and felt privileged to be part of it.
 

