Carbonless paper for use in letter printers

Claim 1

A carbonless paper used as a pressuresensitive copying paper after having been subjected to printing by a letter printer, which comprises at least one of (a) an upper sheet coated at one side with synthetic high molecular wall membrane microcapsules containing a leuco dye solution, (b) a lower sheet coated with one side with an inorganic solid acid and (c) an intermediate sheet coated at one side with synthetic high molecular wall membrane microcapsules containing a leuco dye solution and at the other side with an inorganic solid acid, said inorganic solid and being semi-synthetic solid acid produced by acid-treating a clay mineral having a layer structure of regular tetrahedrons of silica so that the SiO 2 content becomes 82 to 96.5% on dry basis (drying condition: 105° C. for 3 hr), contacting the acid-treated clay mineral in an aqueous medium with a magnesium and/or aluminum compound which is at least partly soluble in the medium (if this soluble magnesium and/or aluminum compound is not a hydroxide, it is neutralized with an alkali or acid so that the compound is converted to a hydroxide) to introduce magnesium and/or aluminum into the acid-treated clay mineral, and, if necessary, drying the resulting clay mineral.