Because my mosaic was to be portable in a picture frame, several materials and processes were changed.  The original mosaic is attached to a brick wall as a substrate.  My substrate is a piece of lightweight Wedi board which is strong and stable.  I could not do all 3 layers of mortar which would have cracked when my mosaic was moved in a picture frame.  Mosaics weren't meant to be portable in the Byzantine period.  I used modern thinset mortar for a 1st layer with no additives and let dry.  On top of that, I painted an under painting with a gouache.  Since I don't have workers to help me,  I put tinted thinset in frosting bags to squeeze out to work on a small area at a time.  In that period, the mortar would have been white because of the lime,  I chose to tint mine gray to simulate the aged look.  For my tesserae/tile,  I choose a light weight glass which could be cut with modern nippers instead of the hammer and hardie.  I was looking for ways to reduce the weight.  When it comes to the gold tesserae, I have not been able to replicate the gold inside 2 layers of clear glass.  Instead I used a gold backed glass tile.  This is the largest mosaic I've ever made: 2'x4'.  To turn my mosaic into an artwork that is portable,  I had to use some modern processes and materials stated above.  The most difficult aspect, besides the size, were the faces.  If I were to do this again, I would start with the faces.  I spent hours trying to balance the look of a Byzantine face with big eyes and elongated chin with the modern training I've ad with facial proportions. I ended up attaching tesserae/tile and taking pictures trying to figure out the proportions over and over again.