Mixing Plaster and Hydrocal: The Beginning

molds1Mixing Plaster and Hydrocal 101

While allowing plaster and hydrocal to set up on your hands or other body parts can cause serious burns, mixing plaster by hand is preferable for sculpture. My un-burned hands testify to hundreds of buckets mixed safely. Here are the basic steps: 1. Fill a clean, dry plastic container with lukewarm water. The volume of water should equal about half the final volume of plaster you need. Remember that a little extra is better than not enough. 2. Using an old metal flour sifter, scoop some dry plaster and sift gently into the water. Do this until there is a small, dry “island” of plaster rising above the water. The island should not sink or become waterlogged quickly. You might be surprised at how much dry plaster this requires. 3. Using your (yes!) hand, slowly mix the plaster through the water with a scooping motion, so you don’t introduce air bubbles into the mix. Feel around for lumps and gently squish these between your fingers until the plaster has a smooth and creamy consistency. It should feel like thick soup. When the plaster is completely mixed and lump-free, wipe your hands off on a nearby towel.

Remember: Plaster disposed of in a sink is likely to set up somewhere in the pipes and necessitate an expensive plumber’s visit. Dispose of all waste plaster in a bag or waste bucket, and wipe up hands and spills with paper towels or waste paper.

4. If you are pouring the plaster into a flexible mold, or into a box mold for later carving, you can pour it now. When the plaster has filled the mold (overfilling a bit is better than not quite filling all the way), tap the sides of the mold gently to release any air bubbles and cause them to rise to the surface. You can scrape these off, spray them with a little water to make them burst, or just ignore the situation and sand later if bubbles cause any objectionable dimpling in the dried plaster.

Remember: plaster expands somewhat as it sets up and then shrinks a bit as it fully cures. Plaster can become quite hot when curing and for this reason, don’t leave large patches of it on your skin as it sets up.

Mixing Plaster and Hydrocal 110

5. Gently pick your hand up out of the liquid plaster and (this is a trick I learned from Thom Haxo, my first sculpture teacher) look at the back of your hand as the plaster flows back into the container. Does the plaster run right off the hairs on the back of your hand, or does it hold together enough to cover them? This is a key indicator of the plaster’s readiness to adhere to a surface, which is extremely important for some applications. If you really don’t like to mix things with your hands (and if you don’t, why are you in this business?), you could use the same test with a wooden spoon—watch to see if the plaster adheres to the grain and imperfections in the surface (good), or runs off leaving plaster-less rivulets behind (bad—wait another several minutes before using).

Are you using the plaster to cover a wire, foam, or paper maché frame? Are you about to brush or pour it on a vertical surface as part of the first layer of a plaster mold? If the answer to either question is yes, then you need to wait an extra amount of time for this adherence phase of the setup to begin, or else the plaster will run off the surface in useless puddles, seep through the corners of your train layout, and other undesirable things.

Curing time: There is a different between “setup time” and true curing time. Plaster that has not cured will feel cold to the touch, even though its shape has been set. If you want the plaster to be thoroughly cured before de-molding (for figurines, etc.), wait until it feels warmish and dry. This can take up to 24 hours or more for a thick surface. On the other hand, there are many situations, like making a plaster waste mold, when you do not want the plaster to dry out before using or de-molding. In this case, wait two or three hours (maybe longer on a really damp or humid day), and then test the surface by scratching gently with a fingernail or metal pin. If the plaster can be scratched without squishing into crumbles, you’re good to go.


5 responses to “Mixing Plaster and Hydrocal: The Beginning

  1. THANK YOU! Ten hours ago, I mixed and poured my first batch of Hydrocal (after watching many YouTube videos!). But I needed to know how long a full cure would take. Eventually, my surprisingly unproductive internet search turned up your excellent summary.
    I now know that I need to wait until the exposed surface of my Hydrocal mold feels dry and warm before removing the sides of my containment box. (The plaster, though hard, now feels damp and cold, just as you describe.)
    I truly appreciate your sharing your expertise.

    • I don’t mean to mix things up (so to speak) but even if the surface still feels a bit damp and chilly it may be de-moldable if you are in a hurry. Scratch the surface a bit with your fingernail and see if you leave a clean scratch on the surface, or one that looks somewhat crumbly or wet. If the scratch looks clean, relatively, it’s probably set up enough to de-mold carefully. Hydrocal can be a bit of a trial and error process as it does not really behave like plaster, even though it looks like plaster. Good luck with your cast.

      • I really appreciate the follow-up guidance! Fortunately, I’m not in a hurry, and waiting a full 24 (or even 48) hours is fine. Do you have any experience with putting a Hydrocal “construction” in a low oven (say, 120 degrees) to finish the cure?

      • Hydrocal sets up by forming longer molecules independently of ambient heat as far as I know. Heating it will dry it out more but not effect a real cure.

      • Thanks yet again, Carolyn! I truly appreciate it.

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