Glass
Inert, transparent, and much better at holding heat than its reputation suggests. Glass is the material most often dismissed on thermal grounds, and when four teapots were measured side by side the glass one beat the cast iron one comfortably.
°C, on contact and after 4 minutes
- Cast iron1200 gcold vessel80 to 76pre-warmed90 to 85
- Glass400 gcold vessel90 to 82pre-warmed90 to 82
- Double-walled glasscold vessel93 to 89pre-warmed95 to 91
Francl (2024), chapter 7. The glass pot sits to the right of the cast iron one at every point, cold or pre-warmed. Pre-warming barely moves the glass pot because it had little heat to absorb to begin with, and it moves the cast iron pot a great deal because it had a lot.
What it is
An amorphous solid: unlike a ceramic, it has no crystal structure at all. Ordinary drinking glass is soda-lime, silica with sodium carbonate and lime added to lower the forming temperature, and it is vulnerable to thermal shock. Borosilicate adds boron trioxide instead, which is why it is the laboratory glass and the right one for teaware. Both are impermeable and chemically inert, and both have roughly the same thermal conductivity as ceramics.
What it does to your tea
Chemically nothing, like porcelain. Thermally, less than you are told. Francl poured 500 g of water at 100 degrees Celsius into four pots and measured: the 400 g glass pot dropped the water to 90 degrees on contact and 82 after four minutes, while the 1200 g cast iron pot dropped it to 80 and then 76. The glass pot won because it weighed a third as much, and a cold vessel takes heat out of water in proportion to its own mass and specific heat. What glass adds beyond that is sight: you can see the liquor's colour and watch the leaf open, which is genuinely useful for judging strength.
What it suits
Anything, and specifically anything worth looking at: silver needle, blooming teas, a large-leaf oolong unfurling. Also cold brewing, where thermal behaviour is irrelevant and transparency is not.
Care and cleaning
Dishwasher-safe as a rule, though hand washing is kinder to thin hand-blown pieces. Tannin film comes off with bicarbonate of soda. Double-walled pieces should be hand washed, since the seal between the walls is the failure point.
What people get wrong
The claim that glass does not retain heat and is therefore unsuitable for oolong, black and dark tea. It is stated flatly in the tea literature and it does not survive measurement. The variable is the vessel's mass and whether you pre-warmed it, not the name of the material, and a light glass pot steals less heat from your water than a heavy iron one does. If you want the best thermal performance available, the answer is also glass: the double-walled kind held 89 degrees at four minutes, the best of everything tested.
Made of this
Sources
- 500 g of water at 100 °C fell to 80 then 76 °C in a 1200 g cast iron pot and to 90 then 82 °C in a 400 g glass pot; a double-walled glass pot held 93 then 89 °C Francl, M. (2024). Steeped: The Chemistry of Tea, chapter 7. Royal Society of Chemistry