Home Forums Explore Media Sculpture Plaster mold not drying!

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  • #983712

    I tried making molds of clay objects from plaster of paris; four small ones poured into paper cups dried fine; the two I did in pie tins are not dry after 24 hours. One of them is even still sort of squishy. Even the clay pieces I set in these tins as my originals haven’t dried any.

    Now, my studio is about 60 degrees in the day and 50 at night. Is this the problem? I used 2 parts plaster to 1 part water for the little ones that dried okay, but used MORE plaster in the two pie tin batches. Is that the problem?

    Boy, messy stuff and when it’s ready to set up, it is ready NOW, thank you! I’ll have to keep practicing before I use this on a “real” project, but any suggestions would be appreciated!

    Thanks!

    #1018570
    James Taylor
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        The more you mess with plaster the more you learn the material as you are right in experimenting first.

        I have never taken measurements for mixing plaster – all though I am a measurement nut. The amount of water is the volume needed to do the job with a little left over. Shaking the plaster into the water in the bucket until it heaps above the level of the water pretty good. This is the rule to learn from that has been passed down through the years.

        Always add your plaster to the water. Never add water to plater, even a little will spoil the lot.

        Mixing plaster at 40 deg. f and above is not uncommon

        enjoy
        JT

        #1018567

        Thanks for letting me know the low temp shouldn’t be the problem. Last night I brought the sloppiest of the tins into the house but, although a bit drier this morning, it is far from hard.

        Yeah, I heard about using 1 inch of water in the bucket bottom and shaking it in til it doesn’t sink. That is the method I used for one of the pie tin molds–which it why I’m sure I used more than 2 parts plaster to 1 part water and also I don’t know how much.

        I can see I’ll have to start taking numbered notes with each batch til I figure this out. In the meantime any other ideas on why this isn’t drying would be great. Can plaster get too “old”? This was a box I bought (but kept dry!) a couple years ago.

        I understand there is a “casting plaster” that has some PVC or the like in it. Does it harden more obediently than plaster of paris? So many questions…

        #1018568

        Wracking my brain last night I finally realized that when I’d mixed that one batch (the one that is still – 3 days later now – still mushy) it had started to thicken too soon (I was still spooning over indents of my original) so I’d added more water, breaking up the thickening clumps with my fingers to get back to a more pourable state. Maybe I destroyed the chemical reaction that had already started in the process of adding more water at that stage?

        Now I’ve found a new source of both paster of paris (fresher supply I think) and hydrocal. I’ll pick some up today and start again–but this time I’ll work really fast! (Wow – sculpting is sure easier than molding!)

        #1018571
        James Taylor
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            [i]Originally posted by James Taylor [/i]
            [B]Always add your plaster to the water. Never add water to plaster, even a little will spoil the lot.[/B]

            Congratulations :clap: Have fun

            JT

            #1018566

            Sparky…the advice that James gave you is good and essentially the method I learned.
            I usually do not measure, just put in enough water for the amount of mix I think will be enough PLUS some overage, then sift through my fingers the plaster till it fills most of the water with a little peak standing above. Then let it SIT for a few minutes. Plaster & water is a chemical process. I never stir the mix but GENTLY squish the mix with my hand till it’s creamy. (yes, it’s messy. I keep a bucket of water handy to rinse it off. Never pour this down the drain. You knew that.) Then let it sit a minute more and pour in the mold.

            Yes, plaster can get too old and become ‘dead’. Old time plasterers used ‘dead’ plaster to patch pin holes or bubble holes in their finished plaster. Don’t ask me why…guess it just ‘works’.

            As to the pie tins drying slowly, as you discovered, you altered the mix by adding water. Plaster is cheap…just throw out a bad mix. Another factor that may be part of the problem is that plaster works best in another absorbent mold. i.e. a plaster mold. The reason is that an absorbent mold draws the moisture out of the new plaster and aids in the curing process.

            Hope this is of some benifit to you.
            Max

            #1018569

            Thank you all!

            Yeah, I’d goofed by adding more water after my initial mix, so I messed that up! Plus I think the plaster was old. And I’d measured by volume, not weight (someone told me about that – the box didn’t explain THAT!). Those goofs were (I believe) my culprits.

            Now … I bought fresh plaster and pulled out a scale and carefully weighed out 1-1/2 parts plaster to 1 part water. The resulting mixture was MUCH thicker from the get-go. And I sprinkled it in over the water and, yup, it made that nice peak of dry mix at the point when nearly all the weighed amount of plaster went in. So I could just use that eyeball method now that I have seen how the right amount really looks. And …

            … success! I made two simple molds, each from separate batches (so I could get the feel for it twice). They turned out fine!

            Thanks for the info on not sitrring to keep out air bubbles–I had lots of them in my first successful batch, less the next when I figured that out. The use of the pie tins worked fine. I coated them slightly with PAM first. I also used green soap tincture on the original (one original was of plasticine, the other regular clay) and that worked fine too. All these odd assortments of materials to work with, heavens! My studio looks like a combination kitchen and apothecary!

            My next big step is a 2-part mold of an already fired mask (too many undercuts to do in one piece). So I’m gearing up my bravery cells for that one next week. I plan to use plastine to fill in potential undercuts. I figure plastine might be easier to remove later from the original? Then I’ll stick a “dam” of clay up the middle and green soap it all up. Then make my plaster and dribble, then spoon or spatula it on. When that side dries I’ll remove the dam, soap up the side of the plaster where it will meet the other half mold plus the remaining half of the mask and do it again. Sound about right? Any hints on this will, of course, be appreciated.

            Thanks again!

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