casein is a powder with a cheeselike color, made from cheese or from the cream of milk. It has no taste, does not smell and can be used with water, alcohol and mixed with lime. if mixed with lime it can be used as glue on canvas or on fresco.

Perhaps the artist you came across uses a casein/lime mixture, adds pigments and maybe some sand and paints this on canvas to make it look like a fresco (but of course it is not a real fresco!