The homogeneous design method, which is used in experimental design and optimization development, disperses the experimental points homogeneously in the entire experimental area. Because the common solvents in laboratories can all resolve the ball-point ink more or less, i.e., the solvents that can be selected and the combination among them are very large. The homogeneous design method was used to determine the best single-solvent or mixed-solvent. The sample material consisted of strokes of WC ball-point ink in blue, black, and red and written on the same paper 1 year and 4 years ago. After examining the solubility of single solvents, the homogeneous design test method was used to examine the solubility of mixed-solvents. A pair of fresh and old samples were tested for their solubility in each solvent. The solvents were classified according to the resolved amount and the ratio of the amount of dye between the fresh and old strokes that were used as parameters of solubility for the solvents. Solvents found to show little difference in solubility between fresh and old ink samples were excluded. The study found that the solubility of the ball-point ink was related to the color and the composition of the ink, aside from solvent. Further, the difference of dissolved amount of fresh and old ink in a solvent was related to the solubility of the solvent. The difference in soluble amount between fresh and old ink was due to the difference in their respective solubility rates. Six kinds of solvents were selected as common solvents for various colors and various ink compositions: chlorobenzene, dichloromethane, chloroform-tetrahydrofuran (1:1), benzylalcohol-tetrachloromethane (1:1), chloroform-N, N-dimethylformamide-tetrochloromethane-cyclohexane (1:1:1:1), dichloromethane-isopropyl alcohol-n-heptaneN, and N-dimethylformamide (1:1:1:1). Their common characteristics are weaker polarity, average soluble rate, and average soluble amount for the ink. 1 table and 2 references