There is provided a friction body which is adapted to rub a thermochromic image or handwriting formed on a paper surface by use of thermochromic ink to thermally discolor the thermochromic image or handwriting by frictional heat resultant from rubbing, a thermal conductivity of a friction portion of the friction body is set to be within 0.05 W/(m.Math.K) to 50 W/(m.Math.K).

Provided is a ballpoint pen refill including: an ink storage tube made of a synthetic resin and storing an ink thereinside; a ball holder made of a synthetic resin and attached to a distal end of the ink storage tube; a ball pressing rod mounted inside the ball holder; and a writing ball stored in a distal end of the ball holder and having a rear end side held by the ball pressing rod, in which the ball pressing rod includes: a fixing portion fixed to the ball holder; a movable portion slidable in a front-rear direction with respect to the ball holder; a compression portion connecting the fixing portion and the movable portion; and a rearward movement restricting portion protruding from a distal end of the fixing portion and provided with a gap from a rear end of the compression portion.

A writing tip assembly for dispensing a free-flowing low-viscosity ink from a writing instrument may include a micro-spring, a writing ball configured to transfer ink to a substrate, and a second ball between the writing ball and the micro-spring. The second ball is configured to transfer ink to the writing ball. A writing instrument for dispensing a free-flowing low-viscosity ink includes an ink reservoir, the writing tip assembly, and a feeder fluidically coupling the ink reservoir to the writing tip assembly.

A writing tip assembly for dispensing a free-flowing low-viscosity ink from a writing instrument may include a micro-spring, a writing ball configured to transfer ink to a substrate, and a second ball between the writing ball and the micro-spring. The second ball is configured to transfer ink to the writing ball. A writing instrument for dispensing a free-flowing low-viscosity ink includes an ink reservoir, the writing tip assembly, and a feeder fluidically coupling the ink reservoir to the writing tip assembly.

A reversibly thermochromic aqueous ink composition having excellent writability and few problems such as aggregation of a colorant, an increase in viscosity of the composition, or a decrease in handwriting density, and a refill or an aqueous ballpoint pen using the reversibly thermochromic aqueous ink composition. The reversibly thermochromic composition is a reversibly thermochromic aqueous ink composition containing a reversibly thermochromic microcapsule pigment encapsulated in a microcapsule formed from an organic resin, an N-vinyl-2-pyrrolidone polymer, glycerin, and water, in which when contents (% by mass) of the reversibly thermochromic microcapsule pigment, the N-vinyl-2-pyrrolidone polymer, and the glycerin with respect to a total mass of the ink composition are P.sub.MC, P.sub.PVP, and P.sub.G, respectively, 0.3P.sub.MC/(P.sub.PVP+P.sub.G)4, and 0.2P.sub.PVP/P.sub.G5, and a refill or an aqueous ballpoint pen incorporating the reversibly thermochromic aqueous ink composition.