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why did my plaster set so suddenly and unevenly?

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Yesterday I attempted to make a plaster slab for reclaiming clay and something happened that I've never seen before -- it very suddenly set very unevenly.

I used No. 1 Pottery Plaster and calculated the plaster/water ratio from Glazy. https://plaster.glazy.org (I used the top ratio, the one labeled "for potters.") The bag of plaster has a lot # indicating it's 4 months old. I used a scale to measure both the plaster and the water and am pretty sure I measured correctly; the plaster did the "mountain" thing once I had added it all. I added the plaster to the water by strewing small handfuls into the bucket, then let it slake for 2 minutes.  By that time the "mountain" was all wet. I stirred very slowly with a gloved hand, being sure to get my fingers into every corner of the bucket. NOTHING happened for at least five minutes.

At that point I saw a couple bubbles and rapped the side of the bucket to get rid of them. Some of the plaster IMMEDIATELY started to set, fast enough that I thought I might not be able to pour it out of the bucket (and wasn't able to get the last few cups out). The parts that were starting to set were big lumps surrounded by really watery liquid (mashed potatoes + milk, basically); it was not homogeneous. I gave it a couple real quick stirs to see if I could break up any of the lumps (sort of), then dumped it into my mold as quickly as I could and sprayed a little rubbing alcohol on top to break a few bubbles, which I always do. The plaster developed a layer about 1/8" thick of water on top as it continued to set, which then absorbed over the next hour or so. I de-molded it after about two hours and the top was soft enough that I could poke my finger in deeply enough to leave a fingerprint. The bottom was rock hard. I left it elevated on some yogurt cups for 24 hours in front of a fan to see if that would help, and the top is maybe very slightly harder, but it sticks to my finger when I touch it. The studio was about 50-55 overnight; I know that's not enough time to fully dry it under those conditions, even with a fan.

The last time I mixed plaster I lived in Phoenix, and I'm now in Colorado. The water was definitely cold as opposed to cool, and Phoenix water is really, really hard. I also saw a tip about lining the bucket with a plastic bag to make cleanup easier, which I tried for the first time.  I don't have a ton of experience working with plaster (maybe on 15-20 occasions?), but the new water source/temperature and the bag-in-the-bucket are the only differences between yesterday and my previous location.

Does anyone know what happened? I'd really appreciate some advice before I try again!


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I did a little research and found that old plaster sets up fast,  a bag that is 4 to 6 months may to old.   And the big hard lumps are from adding to much plaster at one time and not getting well mixed.  The plastic bag might have made it harder to mix,  I use a pretty old bucket and pop off the cured plaster and wash it.   When it gets to the point it won't clean up anymore I find another old bucket.     To make sure my water is really cold I put ice cubes in my water,  when they are melted I am ready to mix.    I never slake my plaster slab plaster,  I haven't had trouble with bubbles.  I set the edge of the bucket in the bottom of the mold and pour it out letting the plaster fill in the corners.  Sometimes I need two buckets of plaster so I find a friend to help mix a second bucket  while I empty the first one.   I made two to four piece molds for a while,  and used the finger flip method on ornate pieces to eliminate bubbles.   I was train dental lab technician and spent the first couple of years making plaster molds off the forms the dentist sent us.  You should be alright on you next slab,  just one of those days.    Denice

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On 3/16/2024 at 2:17 PM, Frogesan said:

NOTHING happened for at least five minutes.

I think maybe you're working too slowly. It should only take a minute max to sift the plaster into the water, another minute at most to gently hand mix. Then lit it sit for a couple of minutes and then power mix.

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It's really important to remember that when mixing larger amounts of plaster that the longer you mix, with a mechanical device, the stronger your plaster will be. A slow mix will result in soft plaster.

When I mix large amounts of plaster I run a small spade thru it to break up any lumps BEFORE I add it to the water. (That way it "wets" quickly.)

I get the plaster in the water sooner than later than I spend my time mixing with a "jiffy mixer" type of device. 

Old plaster - precovid 4-6 months was considered "getting old" but post covid I'm lucky if I get plaster that is less than 6 months old. 


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  • 1 month later...

I just realized that I never came back to reply, and it's super irritating when people do that...

Anyways, I tried working faster and stirring more vigorously, and the plaster set up as usual. Success!

I didn't know that faster stirring made for stronger plaster, so I'll get out the jiffy mixer next time. (I learned to mix plaster in a community college class, so we might have been stirring by hand because it was more practical for a group of people than taking turns with a jiffy mixer.)

Thanks, all, I really appreciate the advice.

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