Glue boiling

People often find that their glued pieces shift in the firing.  Sometimes dramatically. This can be from placement or boiling of the glue.

Almost all commonly available glues evaporate well below the “sticky” range of glass. 

The boiling points of some of the popular glues:

PVA (also known as: wood glue, white glue, carpenter's glue, school glue, Elmer's glue in the US, or PVA glue)  

Boiling point: 112°C (234°F)

Super glue (and other cyanoacrylate glues) 

boiling point 54-56°C (129-133°F)

Lacquer (hair spray)

Boiling point: 185 to 189°C (365 to 372°F)

CMC  (carboxymethyl cellulose includes wallpaper paste, vitragel and most fusing glues)

boiling point: 260–270°C (500–518°F)  

melts at 274°C 

Aloe vera gel  

ignition point: ca. 232°C therefore, its boiling point is lower.

This shows the popular glues used in kilnforming are not effective above 300°C and many with much lower boiling points.  This means that glues only hold glass in place when cold. Glues may be useful in getting items with many pieces into the kiln, but they will not hold them in place until the glass becomes sticky.  The glass, in general, needs to be at or above the slumping temperature to begin to stick together.

If glue doesn’t work, what can I do?

Other means than glues are required to support the moveable items until the glass become sticky at around 620°C (about 700°C for float glass).

If placing frit supports is not possible without showing, such as in a tack fuse, you can use mechanical means. Two of these are grinding flat spots on rolling pieces; and placing supports under the balanced items. Other support and damming methods will depend on the nature of the project.  In general, if the pieces will not stay in place without glue before placing in the kiln, the pieces will move in the kiln after the glue evaporates.

This evaporation can be so fast as to be called boiling.  Glue boils off in the kiln whether dry before firing or not. Sometimes there is enough glue or rapid enough heating to cause displacement of the glass by the force of the boiling. Just as in boiling water, the evaporation can be explosive.  The force of the built-up pressure of glue trapped under glass can move small pieces relatively large distances.

How do I avoid boiling the glue?

• Use as little glue as possible.
• Use it at the edges of the pieces.
• Do not place it in the middle of large pieces.
• When you do use glue, advance slowly to at least 300°C allow the glue to evaporate, rather than boil.  I’d suggest a rate of 50°C per hour would be slow enough to avoid the boiling of sparingly applied glue.

Best of all, use no glue.

Quickly fired glue - wet or dry - boils.  Sometimes with enough force to move the glass significant distances.  Avoid gluing as much as possible and use sparingly when needed.