Glass is joined to glass by heating both ends until soft, pressing them together and blowing slightly to restore the bore, and it is sealed by heating an end and drawing it closed. Joining glass to metal works only when the two shrink by nearly the same amount on cooling, and lead glass is the type recommended for sealing glass to metal. A finished joint must be annealed, because the fused zone is the most stressed part of the piece.
Part of Glass Blowing and Forming
Making leak-tight joints between glass parts and between glass and other materials.
A Joint Is Only As Good As The Match Between The Two Sides
When two softened ends are pressed together they fuse into one. Where both sides are the same glass, they expand and contract together, and the joint behaves like the rest of the tube. Where they are not, the cooling mismatch becomes stress in the joint, and the joint cracks, perhaps hours or days later. The carrier lists “connecting glass to glass and glass to other materials” and names lead glass for sealing glass to metal.
What You Need
- A lampwork flame and tube, per Lampworking.
- The same glass on both sides, unless the pieces are matched.
- For a metal seal, lead glass; see Glass Types.
- For a non-fused joint, a cork or rubber stopper; see Rubber Products, or wax and pitch cement.
- An annealing oven.
Method: Joints By Kind
Glass to glass, butt joint. Heat both ends, press together, blow slightly, rotate until the joint is the same thickness as the tube.
Closing a tube end. Heat the end and draw it closed.
Glass to metal, fused. Use lead glass, per the carrier, and a metal that suits it. The corpus gives no metal or temperature figures. Test every new combination on a spare joint: heat and cool it several times, then check for cracks with crossed polarising filters; see Stress Testing.
Non-fused connections. A stopper in a neck holds a fitting for lab work, and pitch cement seals metal to glass at low temperature; see Pitch Making. Neither will take sustained heat.
Common Mistakes
- Joining soda-lime to borosilicate. The two expand differently, so the joint is stressed even when it looks sound.
- Not blowing the bore back open. The joint necks and chokes the flow.
- Skipping the oven. Joints are the first place stress shows.
- Using a mismatched metal for a glass-to-metal seal without testing a sample first.
Safety
Hot glass does not look hot
A fresh joint below about 600 °C looks cold and burns. Lay it on a marked rest.
Lead glass and fumes
Lead glass heated in a flame can give off lead-bearing fume. Work in moving outdoor air or under a flue, and do not use lead-glass apparatus for food or drink.
What’s Next
- Learn next: Stress Testing — checking a joint
- Combine with: Laboratory Glassware — assemblies made from joints