Why Does Hard-Water Tea Form a Film on Its Surface?

After a cup of tea stands for a while, a transparent, shiny and sometimes faintly iridescent film may appear on the surface. Stir it gently and it can fracture like broken ice. This is neither wax released from the leaves nor floating oil. Research shows that the film forms at the boundary between tea and air, involving tea polyphenols together with calcium, magnesium, bicarbonate and other ions in the water.

Hard water contains more dissolved calcium and magnesium. During brewing, polyphenols can bind with these ions. At the surface, exposure to oxygen allows some polyphenols to oxidise further, while calcium carbonate and other salts help organise an extremely thin interfacial layer. Tea brewed with very low-ion purified water generally produces no collectable film; harder water usually makes the film mechanically stronger. Its oily sheen comes from the way the thin layer reflects light, not from fat in the cup.

Lemon juice often weakens the film by lowering the pH and changing the availability of calcium ions. Tea variety, the exact mixture of ions, and additions such as milk or salt also alter the result, so the film is not a simple water-hardness meter. More precisely, it records an interaction among organic molecules in tea, mineral ions in water and the air–liquid interface.

https://pmc.ncbi.nlm.nih.gov/articles/PMC10411494/