From an article in Electrical World, January 22, 1898, discussing failures of insulators on some lines comments in part:
The reason for their poor insulating properties was easily made apparent by breaking up one of the insulators and examining the porcelain, which was found by the application of red ink to an unglazed surface to be very porous. In nearly every case also, flaws caused by imperfect kneading of the clay were found, poor lots of insulators being often filled with these imperfections, while only one or two flaws would be found in the imperfect ones from a lot which gave a high percentage of satisfactory insulators.
. . . .
Many other sample lots were found to have this defect also, it being often readily discernible upon an application of red ink to an unglazed surface, the red ink showing clearly all cracks and porous parts of the porcelain.