Water gilding is used to achieve a mirror finish on glass, also called a bright finish. Only loose leaf is used, not transfer leaf. It shares some similarities with water gilding on bole (clay) used for picture frames and antique furniture.
The area to be gilded is wetted fully with the water size made from gelatine dissolved in hot water. The leaf is picked up with a gilders tip and placed onto the wet glass. As soon as part of the leaf touches the size, the whole leaf is sucked onto the glass. It appears to almost jump onto it. The leaf often suffers some damage when it jumps onto the glass. The trick is to bring the leaf smoothly and confidently to the glass, parallel to it. If it isn't parallel or is waving around it usually breaks up or goes on wrinkled. Sometimes wrinkling can be reduced or eliminated by flowing more size onto the glass above the leaf. It will smooth out but be careful, too much water size and the leaf will float away down the glass. If it does start to float down, hold it in place with the gilders tip or a finger for a second until it stops moving. Once it floats down it can be difficult to push it back up as any time you touch the leaf it seems to break. Sometimes you can blow it up the glass and back into place.
The gild is initially satin in appearance but becomes bright as it dries. It can then be lightly rubbed with cotton wool to smooth it down and any holes and damaged areas are patched with a second gild. It is also given a second wash over with size. This brings the lustre up and makes the gild stronger. Some people recommend scalding the gild with very hot water. I've only found this to weaken the gild and doesn't improve the lustre any more than a cold wash with size.
Once the gild is dry it can be engraved with the design in reverse in the manner of early glass gilding. For lettering, the required design is marked out and then painted in reverse on the gold using a suitable oil paint. This is allowed to dry and the excess gold, which is left unpainted, can be cleaned off by gentle rubbing with water and a fine abrasive (whiting) on a soft cloth or sponge.
The gold can be used in whole leaves or cut down for smaller lettering and lines. Half size is much easier to handle too. You can also use a gilders cushion and knife to cut the leaf but I gild directly from the book. To cut the leaf you simply fold back the page to expose the required size strip of gold. Run a dry fingernail over the leaf using the folded paper as a guide. Don't press too hard or it will mark the leaf underneath. Pick up the piece with the tip and with a bit of a wiggle it should tear along the crease. You can cut smaller strip of 1/4 width for lines. I have cut it as fine as 1/6 but it isn't really practical because the edge sometimes tears a bit jagged and you need the extra width to allow for this.