Ink composition for reversibly thermochromic stamp and stamp

A stamp being provided with a stamp material having continuous pores, wherein:

• the stamp material has an ink impregnating surface with pores allowing impregnation of an ink, and an ink non-impregnating surface rejecting impregnation of an ink;
• the stamp material is impregnated with an ink composition comprising:
• a reversibly thermochromic microencapsulated pigment encapsulating a reversibly thermochromic composition comprising:
• Component (A):
• an electron-donating color-developing organic compound,
• Component (B):
• an electron-accepting compound, and
• Component (C):
• a reaction medium causing reversibly an electron transfer reaction between the Component (A) and the Component (B) in a specific temperature range;
• water, and
• a thickener,
• wherein:
• an ink composition viscosity measured at 25° C. with a BL-type viscometer at 6 rpm is from 3,500 to 7,000 mPa·s, and a thixotropy expressed by (ink composition viscosity measured at 6 rpm)/(ink composition viscosity measured at 60 rpm) is from 1.2 to 2.0;
• P expressed by the following formula is from 90 to 100,000:
• P=Sm2/D2
• wherein Sm stands for an average distance of depressions and projections of the ink impregnating surface of the stamp material and is 30 μm or less, and D stands for an average particle size of the microencapsulated pigment and is from 0.1 to 10 μm;
• and
• a surface roughness Ra of the ink non-impregnating surface is 2 μm or less.