Kyūsu (or kyuusu), is a general term for Japanese teapots, but commonly refers to a specific form of pot with a side handle and internal screen to filter leaf at the base of the spout. Technically, side-handled kyūsu are referred to as "yokode kyūsu," and rear-handled kyūsu are known as "ushirode kyūsu." However, the former style seems to be much more common than the latter, and thus the general term kyūsu is conventionally understood to refer to yokode kyūsu.
Why Buy a Kyūsu?
Kyūsu teapots were designed with sencha and other Japanese teas in mind. Their shape and filter designs allow for maximum expansion of tea leaves, which is absolutely crucial for Japanese greens. They are also sized appropriately, most being large enough to accommodate one or two tea-drinkers (though there are some larger models for serving more people).
Although kyūsu vary in capacity, 10 oz (c. 300 ml) or less is common.
The internal screen may be made of clay (known as "sasame" filters), but is also commonly and less expensively composed of stainless steel. Stainless steel filters that cover the spout alone are referred to as "ita-ami" filters, while those that inscribe the inside of the teapot are referred to as "obi-ami" filters.
Some purists maintain that sasame filters are best, as they believe metal filters to influence the taste of the tea. These purists also believe a 100% clay pot to be more aesthetically pleasing. However, many do not believe the metal to significantly affect the taste or aesthetics, and prefer metal filters because of their cost and filtering ability.
Generally, sasame filters have larger pores than their stainless steel counterparts. Small particles of tea leaf may pass freely through sasame filters, but they are normally blocked by fine-mesh steel screens. Some sencha enthusiasts enjoy these particles, so they tend to gravitate towards sasame screens, while those who prefer a clear/unclouded brew prefer the steel ita-ami or obi-ami screen style.
The pottery may be glazed or unglazed.
If the kyūsu is glazed, it may be used with a wider range of teas without concern of one tea's essence and flavor influencing another. However, unglazed kyūsu are valued by some as pots dedicated to tea of a particular variety, with the intention that the pottery will take on some of the qualities of that tea.
Care and Maintenance
Some owners perform little to no special maintenance on their kyūsu, and intentionally allow residue from tea brewing to accumulate inside the pot.
However, routine maintenance is not uncommon. The most common cleaning methods involve boiling water and/or a baking soda soak.
The use of soap is often discouraged, but is even encouraged in other cases. If in doubt, it may be advisable to avoid the use of soap or detergents until after seeking sound advice. Undoing the results of soap and detergent use in cases for which they are not appropriate could present a significant challenge.