When it comes to copper leaf adhesive, it is up to you whether or not you use oil or water based size. Gilding on certain surfaces, such as glass, may be best gilt with water based size. Water based size also provides longer tack times. Having said that, oil based sizes are typically regarded as the easiest and more beginner friendly size to use. Just be careful to follow the directions on the label of whichever copper leaf adhesive you use to ensure best results.