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!
Thanks!