Stephen E. and Christopher J. Kersey of kerseyglass.com say there are a number of variables that may be encountered in glue chipping. The first and perhaps the most important is the condition of the glass. To prepare the glass properly, make sure that the glass is clean and roughened by a process called “frosting” (sand blasting) or etching with acid. Roughening the surface gives the applied glue a place to get a firm hold on the glass as it hardens and shrinks, allowing it to take bits of the glass away when the glue is removed. It is important that the glass be level on the work surface, and this may require shimming the glass. A level platform is necessary to keep the applied glue from moving from one area to another and becoming too thin in places, not giving the glue enough strength to remove the glass.
Glue chipping is achieved through the use of an animal-hide glue, which comes in powder form and is mixed with water in any container the user desires as long as it has no leaks. A typical formula calls for two parts of water to one part of glue. Adding the powdered glue to the water in the container allows the glue to sift downward and mix with the water on its way down. This helps eliminate clumping. After sitting for at least 30 minutes, the mixture forms into a rubbery material.
The glue has to be heated to be usable. Heating the material turns it back to a liquid form for application. This can be done on a stovetop by any double-boiler method (a container of the mixture set over another container of heated water so there is no direct contact with the heat source). A medium heat setting will suffice. Stir the glue mixture occasionally to prevent lumps.
A turkey basting tube is the applicator of choice at kerseyglass.com. The narrow opening allows for precise application and the length permits easy access to the glue container. (Tip: replace the basting tube in the container when done to prevent splashing of the glue.) If chipping a design, mask the portion of the glass that is not being etched.
Two processes allow for different effects in chipping. One is the feather method, in which a fan blows air directly on the glass, causing a “blow dry” effect. When the glass is dry, place it in a heated room to finish. One or two fans should blow air over the surface of the glass. The second drying method is the “snail” chip. Here, no direct air is blown over the glass; rather, the glue is left to dry on its own. This may take a little longer, but it works just as well.
AngelGilding.com warns that when the dried glue chips, it can fly off the glass under high pressure and contain small, sometimes almost invisible pieces of glass. Therefore, wear safety glasses when chipping. These fragments are also attractive and dangerous to pets, so keep your work contained to the chipping area.