| 摘要 |
PROBLEM TO BE SOLVED: To obtain a pen set enabling double check surely upon checking the entry items in a document or the like through double checking by a method wherein the color of a mark upon a first checking can be changed upon a second checking so as not to allow any omission of the checking. SOLUTION: Blue aqueous ink A containing a pH indicator of bromthymol blue and sodium carbonate is prepared to obtain a pen A by filling the ink. Aqueous ink B containing sulfamic acid is prepared and another pen B is obtained by filling the same. A blue mark is written by the pen A and, thereafter, rewriting is applied on the blue mark by the pen B whereby the color of the mark is changed into yellow. |
