Metallic containers should never be used since when they are heated, some of the metal's properties are imparted to the herbs, altering the composition of the herbal mixture.
Some combinations are potentially poisonous, especially when aluminum cookware is used.
Likewise, tap water or chlorinated water is not recommended. Use purified or distilled water rather than bottled water, unless you know the source of the water in the bottle. Here in southern California, some deep wells in Orange County were found to contain carcinogens. The first thing to be cleared up is the difference between an herbal tea and an infusion. A tea, to the lay person, conjures up images of prepackaged, individually wrapped tea bags.