This is a hand-made wood-fired stoneware mug with a one-finger handle.
It was fired for around 20 hours in our train-style wood kiln to cone 12+ (2375 F or higher). It is glazed on the inside with temoku (a traditional-style iron-rich glaze). The outside has a shino slip and a circular black brushstroke, but it is mostly glazed by the natural ash and salts from the wood-firing process.
This particular piece is on Etsy (or was?...). If you would like one of these, you can visit my Etsy, or you can contact me to order one (or more).