Glass or crystal may have a surface residue from the manufacturing process. This must be cleaned thoroughly using solvents (i.e. alcohol). If it isn't cleaned thoroughly, the ink may puddle or pool, causing a mottled look. Print a white pass first (two passes if you need more opaqueness), then print the full color twice. Please note that the surfaces of crystal and glass do not all have the same texture, so some may print better by first printing a clear ink pass before printing the color passes.