In Colour by Victoria Finlay, I read about Aidan Hart, a New Zealand icon painter. His process of layering glue, chalk and linen on wood was fascinating. He is quoted as saying that Icons "introduce you to reality. They show you not what you see, but what is real."
Pictured is a sketch of a 13th century icon. I'm tempted to try layering glue, chalk and linen on wood too.