Any advice on mixing hydrocal correctly? Followed every step, came apart perfectly, but stayed kind of damp and soft even after a full day of sittting out. Carefully getting the clay out and it just started cracking and chipping away. Lost the sculpture, materials, time.
Chris Ellerby AdminHi Adam,
I use the "dry lakebed method" Fill a container with cool water and sift in the hydrocal/untracal until the surface of the water turns into a dry cracked lake bed. At that point you start mixing.
Another issue you may be facing is that Hydrocal is a one step plaster, meaning you can't layer it like you can Ultracal.
As the different layers cure, they can expand slightly and crack the plaster between them. You may be able to get away with adding a fiber layer while the Hydrocal is still very "green" or uncured, but you risk disturbing the beauty coat as you do.
For making a mold of your sculpt, I'd recommend Ultracal anyway, it's a harder plaster that will allows you to build up multiple layers of beauty, fiber and finish coats, and holds detail better for multiple runs of your mask or makeup. It also has a more gradual cure rate, so it's a little more forgiving as you're getting a feel for it "setting up". Gary Yee's Stone Mold course has excellent step by step instructions for an Ultracal "stone" mold.
But for both Hydrocal and Ultracal, Chris is correct, the "dry riverbed" is the best starting place for a good water/plaster ratio. The mixed plaster should cover the tip of your gloved index finger like a thin milkshake, with nothing visible through it, and hold a vertical surface without running off entirely.
Cover your sculpt with a plastic bag after molding to hold in the moisture until it's completely cool. This adds strength to the stone by keeping the heat/moisture more consistent through all the layers as they cure.
And if you're using foam latex or silicone as a casting material, you'll need to bake the mold at 125-145 degrees for several hours to cook out the remaining moisture before you cast for best results. After clean out, put the mold in the oven before turning on the heat to make the temperature change more gradual. Leave it there for cool down too. Regular latex can go in once the mold is cleaned out, without baking.