How do I glue glass, and which adhesive stays invisible?

Glass is one of the easier surfaces to bond chemically — it is hard, non-porous and, once clean, extremely receptive. What makes glass repairs difficult is everything else: the break faces are irregular and cannot be abraded without losing the fit, the material is brittle so any stress concentrates instead of spreading, and every flaw in the bond line is visible through the glass.

Safety

What you'll need

• UV-curing glass adhesive and a UV lamp, or clear two-part epoxy
• isopropyl alcohol and lint-free cloth
• cut-resistant gloves and eye protection
• low-tack tape for clamping
• a cocktail stick or fine applicator
• a blade for trimming cured excess

Step by step

1. Decide whether the item should be repaired at all.Anything that holds hot liquid, is under load, forms part of a door or a shelf, or would injure someone if it failed is not a repair candidate. Glass gives no warning before it goes.
2. Dry-fit the pieces first and work out the order.With several fragments, the order of assembly decides whether the last piece will go in at all. Lay them out, number the faces with a marker on the outside, and rehearse the whole assembly before any adhesive is opened.
3. Degrease both break faces thoroughly.Isopropyl alcohol on a lint-free cloth or a cotton bud, then let it flash off. Skin oil from handling the fragments is the usual reason a glass bond looks cloudy or lets go. Handle by the outside faces afterwards.
4. Choose the adhesive by whether the joint must be invisible.UV-curing glass adhesive has a refractive index close to glass and produces a nearly invisible line, but it needs light to reach the joint, so it only works on clear glass. Clear epoxy is stronger and more forgiving but leaves a faint visible line. Superglue is fast, brittle and often leaves a white bloom around the joint.
5. Use as little adhesive as will wet the whole break.Apply along one face only and let capillary action pull it through as the pieces close. Excess squeezes out onto the surface where it is much harder to remove from glass than it is to avoid.
6. Close the joint and hold with tape rather than a clamp.Low-tack tape across the joint in several directions gives even, gentle pressure. A mechanical clamp on glass concentrates force at a point and cracks it, which is how a repairable break becomes an unrepairable one.
7. Cure it properly before touching it.UV adhesive needs the lamp held close for the stated time and only cures where light reaches. Epoxy needs its full cure at room temperature, and a slow epoxy is stronger than a fast one. Do not test the joint early.
8. Trim the excess with a blade held flat.Cured adhesive on glass shaves off cleanly with a razor blade laid almost flat to the surface, which is why glass is forgiving about squeeze-out if you wait until it has hardened. Do not use a solvent, which can run into the joint.

Tips

• Glass to metal — a lamp base, a hinge, a mirror clip — is an epoxy job, and the metal side needs abrading and degreasing while the glass side needs only degreasing.
• Silicone sealant is the right adhesive for bonding glass in a wet, moving or structural-panel situation such as an aquarium or a shower screen, because it stays flexible. It is not the answer for a break.
• A repaired glass item should be hand-washed in cool water. Dishwasher heat and detergent will find the bond line eventually whatever the adhesive.
• Frosted, textured or coloured glass blocks UV, so UV-curing adhesives will not work through it. Use epoxy instead.
• Keep every fragment, including the small ones. A missing sliver leaves a gap that has to be filled, and a filled gap is always visible.

Common mistakes

• Abrading the break faces — It removes the interlocking detail that makes the pieces register perfectly, and the joint then never closes properly. Glass needs cleaning, not keying.
• Using too much adhesive — The joint cannot close fully, the line becomes visible, and the excess on the surface is much harder to deal with than a thin one would have been.
• Clamping glass in a G-clamp — Point loading a brittle material is how you turn one break into three. Tape distributes the pressure.
• Repairing a drinking glass or a cooking vessel — No household adhesive is food-safe in a broken bond line, and heat cycles will open it. These are not repairable items.

Questions people ask