Cup-shaped porous silica preforms suitable for manufacture of large 24-inch crucibles used in Czochralski crystal-growing furnaces are produced by a unique electrophoretic casting process using a high-purity aqueous silica slip or slurry having a predetermined particle-size distribution, an average particle size of from 6 to 10 microns and a solids content of from 80 to 85 percent by weight. The slurry contains an electrolyte, such as ammonium hydroxide, has a pH of from 7.5 to 8.5, and can be wet milled at a pH of at least 7 in such manner as to provide the micronized silica particles with excellent electrophoretic mobility, thereby providing a superb process for economical mass production of large pure silica preforms using safe voltages, such as 20 to 40 volts. The electrophoretic casting apparatus can be of the type shown in FIGS. 1 and 2 and includes a permeable porous cup-shaped carbon-graphite mold (2) that serves as a positive anode and an internal reticulate or perforated cathode (3) of similar shape having a thin pervious cover means, such as the shroud 25, to prevent local dilution of the slurry at the cathode.