Hard water?

Experts advise to pay attention on the look of the film. Uniform color of the RAID or the smallest inclusions it suggests that the reason for the formation of the film became too hard tap water. High content of calcium carbonate in water in conjunction with organic substances contained in tea that cause the appearance of the film. If you add a slice of lemon or a drop of lemon juice, the film will disappear.
There is also an opinion that the cause of the formation of the film is oxidation of the contained iron in water.

Rainbow in a Cup

Thin rainbow film, homogeneous and easily broken while stirring the drink, founded in essential oils and tannins, contained in tea and giving it a unique aroma and tart taste. If the brewed tea for some time will stand untouched, oil and tannins are oxidized – the result of oxidation and becomes iridescent film on the surface of the liquid. The stronger the tea, the more it will be noticeable.

Another reason for the appearance of a brownish film, according to experts – oxidation of mineral and organic compounds, including caffeine and catechins contained in tea, under the influence of oxygen in the air. The composition of this film is very complex – it includes protein compounds, purines, tannins, iron, calcium, and other substances and compounds.

Harmful or helpful?

The opinions of experts about the effects of tea with the film on human health differ. On the one hand, it is a Testament to the high content of beneficial essential oils, with other forms insoluble plaque that is with constant use of strong tea may settle on the mucous membrane of the stomach and intestines that prevent the absorption of nutrients.

Found that the worst impact on human health the use of long-standing tea after brewing black tea should be drink within a few hours, in any case without leaving the brew overnight. The drink loses its useful properties, it increases the content of harmful substances, including toxic guanidine, formed in the oxidation of non-hazardous guanine contained in tea.
High concentrations of guanidine are extremely dangerous, and for the emergence of symptoms of poisoning, it is sufficient to drink several cups of strong welding yesterday.

On the walls of the cups due to film forms bad plaque rinse to get rid of that sometimes does not help even wash in the dishwasher. If the tap water in your area contains a lot of salts of calcium, magnesium and iron, it is better to get a special filter or use tea purified drinking water in bottles. Don't forget that distilled water, as too hard, would be of no benefit to the body.