Double-walled glass
Two glass shells with a gap between them, sealed at the rim. It is the vacuum flask idea in a cup you can see through, and when four teapots were measured side by side the double-walled one won by a distance. It is also the most likely thing on your shelf to burn you, for exactly the reason it works.
- Capacity
- 150 ml cup to 600 ml pot
- Material
- hand-blown borosilicate glass
- Origin
- modern, on Dewar's 1892 principle
- Best for
- holding temperature through a long steep, and seeing the leaf
What it is
Hand-blown borosilicate, usually, with the space between the walls either evacuated or simply sealed air. Made as cups, mugs and teapots. Borosilicate is the laboratory glass: silica with boron trioxide added, far more resistant to thermal shock than ordinary soda-lime glass, which is what lets a thin-walled vessel take boiling water without cracking.
What it does to your tea
It keeps the temperature up, better than anything else in normal use. Francl poured 500 g of water at 100 degrees Celsius into four pots. The double-walled glass one held 93 degrees on contact and 89 after four minutes; cast iron managed 80 and then 76. Pre-warmed, the double-walled pot started at 95 and ended at 91, comfortably inside the range for a black tea for the whole infusion. The reason is that the gap conducts almost nothing, and conduction is how tea mainly loses heat.
What it suits
Teas brewed hot and steeped long, which are exactly the ones a thin single-walled vessel fails. It also suits watching the leaf, which is why it is a common choice for silver needle and blooming teas.
How to use one
Pour carefully and wait before drinking. The outer wall stays cool, so touch gives you no warning at all about what is inside, and that is a real hazard rather than a caution label. Otherwise use it as you would any glass vessel.
How to choose one
Borosilicate, and check the seal at the rim where the two walls meet: that join is where they fail, and a failed seal fills the cavity with water and fogs it permanently. Hand-blown pieces vary in wall thickness, which is cosmetic rather than functional.
Care and cleaning
Hand wash. A dishwasher's heat cycle stresses the seal between the walls, and a cavity that has taken on water cannot be dried out.
What people get wrong
Trusting your hand. Everything else on your shelf tells you how hot the tea is by how hot it feels, and this one does not.
What it is made of
Sources
- a double-walled glass pot held 500 g of water at 93 °C on contact and 89 °C after four minutes, against 80 and 76 °C for cast iron Francl, M. (2024). Steeped: The Chemistry of Tea, chapter 7. Royal Society of Chemistry