ocean_nerd Posted February 20, 2022 Report Share Posted February 20, 2022 I'm new to making molds for slip casting, but I did everything according to all of the instructions I could find online. I measred out 7763 g's (17 lbs) of potters plaster, and 6 quarts of water. I mixed it with a mixing attachment on a drill. I was going to vibrate my mold once I poured the plaster to get the bubbles out. But before I could even pour, the consistency was like jello, and then as it poured, I realized it was setting instantly. I have no idea what I did wrong. Why did it set so quickly?? Link to comment Share on other sites More sharing options...
Dick White Posted February 21, 2022 Report Share Posted February 21, 2022 Did you mix it while you were adding the dry plaster to the water, or did you pour the water over the dry plaster and mix from there? The proper technique is to have the measured amount of water in a large mixing container and gently sift the premeasured amount of dry plaster onto the top of the water and let it sink. By the end of the premeasured dry plaster, it will likely be sitting in an underwater "mountain" that nearly reaches the surface of the water. Let it sit there to slake for a few short minutes, and then mix it. It will be creamy, and then you can pour your mold. If you mix it too soon or too much, it will curdle and begin to set instantly. Another possible fault is mixing in a container previously used to mix plaster but was not completely and thoroughly cleaned. The previously set plaster remains will instantly set off the chemical reaction of setting in the new plaster. For this reason, I tend to mix the plaster in a bucket into which I have inserted a plastic trash bag. It will be a bit trickier to pour the plaster out, but then I can just throw away the bag. Link to comment Share on other sites More sharing options...
Hulk Posted February 21, 2022 Report Share Posted February 21, 2022 Been a while since mixing and pouring plaster; I believe water temperature can make a difference as well - cooler should be slower. Link to comment Share on other sites More sharing options...
neilestrick Posted February 21, 2022 Report Share Posted February 21, 2022 For #1 Pottery Plaster, the ratio is 10 parts plaster to 7 parts water, by weight. I find it's best to actually weigh out the water, not use a volume measurement, because it's more accurate. You were close with the 6 quarts, though, so I think the problem is with your mixing technique. First, make sure you're not using warm or hot water. Second, like Dick said, sift the plaster into the water, very gently mix it if needed so everything is totally wetted, then mix for only a short time with the drill mixer. Link to comment Share on other sites More sharing options...
ocean_nerd Posted February 21, 2022 Author Report Share Posted February 21, 2022 I have an update! I did use cold water, so that wasn't the problem. The issue is probably that I added the water to the plaster, and mixed it as I added the water. I also probably over-mixed it or mixed it too fast with the mixer. I tried it again, this time with 4 quarts of water and 11.4 lbs (5176 g) with the water measured out (though I didn't weigh the water), and slowly sifted the plaster into the water. I was worried about it setting too fast again... so i didn't let it sit for long.. probably less than a minute, then gently mixed it by hand. And it worked! Thanks for the help! Link to comment Share on other sites More sharing options...
Mark C. Posted February 21, 2022 Report Share Posted February 21, 2022 welcome to the school of hard knocks learning my old school.Learned a lot from my mistakes welcome to the forum Link to comment Share on other sites More sharing options...
Jeff Longtin Posted February 21, 2022 Report Share Posted February 21, 2022 Glad you achieved success. If you do plan on pouring more molds, in the future, I would strongly suggest getting a digital kitchen timer so you accurately time your mixing process. That way you can compare the mix times from bag to bag. Ideally your supplier will provide you with bags of plaster less than 6 months old. But if your supplier has provided you, with older bags, a timer will help you anticipate the mix times so you can plan out your pouring accordingly. Link to comment Share on other sites More sharing options...
k-kay Posted March 7, 2022 Report Share Posted March 7, 2022 Hi there coming in late, yep all those suggestions I am sure fixed your issue. My first experience with mold making was #1 plaster would not set-up. So thought it no good. When you use to standard plaster, since I been using that kind to make wax waste molds, your on that train of thought. I did the ratio correctly just did not realize it need at least 70 rpms of velocity to active it. Called my local supplier and he told me my arm could not stir that fast. lol Link to comment Share on other sites More sharing options...
Harriet15 Posted May 8, 2022 Report Share Posted May 8, 2022 I usually mix my plaster using a little less than the recommended ratio so it gives me enough time to get rid of the air bubbles. It feels more controllable and I have a better plaster mix while still retaining the strength. Link to comment Share on other sites More sharing options...
Mark C. Posted May 9, 2022 Report Share Posted May 9, 2022 Yes add the plaster to the water-(weigh out both ingredients 1st) use a power drill mixer. I also tap the bucket on the ground to raise up any air bubbles just before pouring the plaster If you want to sieve the plaster do that 1st as its a slow process- then add plaster to water slowing and mix with drill Link to comment Share on other sites More sharing options...
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