Clay as a positive mold for a hydrocal sculpture? in Studio Operations and Making Work Posted October 12, 2018 · Report reply 4 hours ago, Rae Reich said: But definitely build your clay form thickly and with inflexible exterior support for it. The plaster in such large quantities (even with center-void form in place) will be very heavy and will try to escape through any weak spot. Don't know how intricate the sculpture will be, but simple forms are best to start with. How tall/wide will your piece be? Rae, The piece will be approximately 13" high, 15" long, and 9" thick.