Karen Kukla Posted January 16, 2023 Report Share Posted January 16, 2023 Morning all! Brand new to mold making. Tried my first mold yesterday, mixed 1.42 lbs #1 pottery plaster to each pound fresh water. Sifted into water, once all in, gave it two minutes to soak and then gently hand mixed. The mixture did not begin to warm up at any time. Poured in mold, wouldn't set, waited a day and the top inch was water, it settled out and the bottom hardened. Clearly unusable. What did I do wrong??? Quote Link to comment Share on other sites More sharing options...
Min Posted January 16, 2023 Report Share Posted January 16, 2023 (edited) Hi Karen and welcome to the forum! How old was the plaster you used and how was it stored? edit: link from USG on weighing and mixing plaster here. Edited January 16, 2023 by Min added link from USG mixing instructions Quote Link to comment Share on other sites More sharing options...
Mark C. Posted January 16, 2023 Report Share Posted January 16, 2023 (edited) USG makers of #1 pottery plaster (white) say 70 parts water to 100 parts plaster . I have my schedule (water to plaster) out in studio now and will look later. Your mix sounds off a bit when using plaster you want dry new plaster-it sound like since you added the words sifted your plaster I assume it had some lumps in it? if so then its old and will behave not as well as new plaster. also you let it soak and in that time you shoild use a power mixer to get all the smalls wet. I mix in a bucket with a small cordless drill with a jiffy mixer at the ready with a fresh bucket of water to clean jiffy mixer-2 minutes of power mixing then drop bucket a short distance on floor to allow air bubbles to rise. Then pour. The water temp is also a key deal. Colder water gives you a bit more working time. We have some real plaster experts here that also may help as time goes by welcome to the forum https://www.usg.com/content/usgcom/en/products/industrial/ceramics/no-1-pottery-plaster.html Edited January 16, 2023 by Mark C. Hyn Patty 1 Quote Link to comment Share on other sites More sharing options...
Karen Kukla Posted January 16, 2023 Author Report Share Posted January 16, 2023 Thank you both! I got the plaster from Amazon and it was sealed in a plastic bag but definitely had lumps. Directions on it said lbs water = pounds plaster/1.42 so if that's wrong, please give me the right one?? :). And so I should use the drill?? Ok! How long do I let it soak before the drill? Thank you so much in advance! Quote Link to comment Share on other sites More sharing options...
Jeff Longtin Posted January 16, 2023 Report Share Posted January 16, 2023 Hello Karen. Your plaster mix is fine. (It equates to 7:10) Your mix was simply too short. You allowed for a "soak" period, which was good, but then you need to mix for 2 minutes more. Then allow the plaster to sit for another minute if it seems watery. (This last sitting period allows for the air bubbles to rise up.) If the plaster seems to be thickening, after the 2 minute mix, you can pour, but it's best if you can wait that extra minute. Mark's method of dropping the bucket is a fine method but I prefer to swirl the bucket instead. As you do so you can actually see the bubbles rise up and pop. USG recommends "mechanical mixing" but I hold a jiffy mixer blade in my hand and mix for 2 minutes with it. My molds have lasted years so I see no need to change. Quote Link to comment Share on other sites More sharing options...
Karen Kukla Posted January 16, 2023 Author Report Share Posted January 16, 2023 I did mix with my hand gently for probably three minutes. The expiration date on the bag says July...now I am really confused! Quote Link to comment Share on other sites More sharing options...
Karen Kukla Posted January 16, 2023 Author Report Share Posted January 16, 2023 Just bought these, will mix with drill for two minutes next time. Quote Link to comment Share on other sites More sharing options...
Karen Kukla Posted January 16, 2023 Author Report Share Posted January 16, 2023 Is it supposed to start getting warm? As it didn't change temperature at all. Quote Link to comment Share on other sites More sharing options...
Mark C. Posted January 16, 2023 Report Share Posted January 16, 2023 Just a thought was this #1 pottery plaster-My guess is thats what you said by #1. Also since you live in the humid souther parts you need to keep air off(sealed plastic bag) it unless it a new unopened bag. Plaster will absorb any moisture in the air right away . Plaster usually gets warm setting off. My experience is when its old and has any lumps it does weird things like not setting well (or warm) Was your bag unopened -around here we can only get #1 pottery Plaster ina 50# bag as well. Quote Link to comment Share on other sites More sharing options...
Karen Kukla Posted January 16, 2023 Author Report Share Posted January 16, 2023 It was two separate 10 lb unopened bags of #1 pottery plaster from Amazon Quote Link to comment Share on other sites More sharing options...
Min Posted January 16, 2023 Report Share Posted January 16, 2023 Does it feel gritty? 3 hours ago, Karen Kukla said: The expiration date on the bag says July...now I am really confused! Can you find out who put the date on the bags? Old plaster won't set, makes me wonder how valid the date on the bag is. Have you contacted the seller? Roberta12 1 Quote Link to comment Share on other sites More sharing options...
Karen Kukla Posted January 16, 2023 Author Report Share Posted January 16, 2023 No, I haven't but someone told me my gentle hand mixing might be the culprit...not mixer well enough. I will use what's left and run a trial with drill agitation for two minutes. Wish me luck!!!! Quote Link to comment Share on other sites More sharing options...
Mark C. Posted January 16, 2023 Report Share Posted January 16, 2023 Ok you got me there on an Amazon order for #1 pottery plaster. I suspect the age of bag or how it was stored. It could have been rebagged a year or two ago. let us know on what the rest does after mixing Quote Link to comment Share on other sites More sharing options...
Karen Kukla Posted January 17, 2023 Author Report Share Posted January 17, 2023 Thank you so very much!!! Quote Link to comment Share on other sites More sharing options...
neilestrick Posted January 17, 2023 Report Share Posted January 17, 2023 If you're buying a 10 pound bag, the seller is re-bagging it from a 50 pound bag. Hard to know how that bigger bag was stored, or if it was left open for a long time, or if it was really old. I recommend buying your plaster from wherever you buy your clay. I think they're more likely to take good care of it. Power mixing will definitely help. Quote Link to comment Share on other sites More sharing options...
Jeff Longtin Posted January 17, 2023 Report Share Posted January 17, 2023 A few more thoughts: Expiration Date: typically, 50 lbs. plaster bags have a date on them but its the date they were bagged. NOT an expiration date. July is on the older side but not terrible. January would be terrible. Ideally you get plaster 2-4 months after bagging but with "supply chain issues" these days I have run into old plaster. Older plaster 6-12 months after bagging, will tend to be lumpy and will harden more quickly. The fact that you have plaster that has been re-bagged allows for two conclusions: 1) its really old plaster and it will be challenging, or 2) it was put into a plastic bag soon after it came to the seller and its better protected in the plastic bag than it was in the 50 paper bag. My hunch is that it was better protected in the plastic bag than the paper bag but its hard to say until you work with it some more. (Really fresh plaster may not set up for 6-8 minutes.) Mixing - a good analogy is to mix plaster as hard as you mix/beat scrambled eggs. You need to mix eggs aggressively to get the whites and yolks to combine, the same is true for plaster. Something more than your hands would aid in the process. The mixing blade you picture is referred to as a "blending" device and I use it to mix 10 lbs batches, of Ceramical, at my day job. Its not bad but it does tend to throw the plaster around quite a bit. What amount of plaster are you mixing? Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.