H Palmer Posted May 9, 2023 Report Share Posted May 9, 2023 I think that I have usually made my slip casting moulds with plaster of paris which seems to work well. Last week i made a load with Potters Plaster, it seems quite different so i just want to check in with you all about wether it is that different, or maybe I got a bad bag.. I used the same method that I always have; I put water in the bucket and added plaster until there was a little mound showing above the water, and then i added about a cup more. Then I left it alone for 5 mins... then i started to stir it and put in a hand to mix it.. it stayed cold but thickened somewhat so I poured it into the moulds. I tapped the moulds up and down to get the bubbles to the surface. After a while it got hot, eventually hardening. when i de-molded , the surface of the plaster seems quite rough and there are loads of tiny holes from bubbles. I never had this result with paster of paris and it seemed to heat much more quickly so does anyone out there recognise what I am saying and can advise... many thanks.. henrietta Quote Link to comment Share on other sites More sharing options...
Dick White Posted May 9, 2023 Report Share Posted May 9, 2023 The island/mound will occur when the proper amount of dry plaster has been added to the water in an appropriate size and shape bucket. However, an island/mound may also occur with the incorrect amount of plaster and /or an inappropriate size or shape bucket, and so the mere appearance of this infamous island/mound should not be regarded as the proper mixing ratio. The only accurate method to get a strong plaster mold is to add the exact weight of plaster to the exact quantity of water as specified by the plaster manufacturer. The ratio of plaster to water is different for plaster of paris vs. pottery #1 plaster. neilestrick, Kelly in AK, Hulk and 1 other 4 Quote Link to comment Share on other sites More sharing options...
Jeff Longtin Posted May 10, 2023 Report Share Posted May 10, 2023 Hello Henrietta, Welcome to the Forum. My suggestion is that you should not wait 5 minutes for the plaster to "get wet". Two minutes is all I leave it for. (Both USG and Georgia Pacific recommend the same.) You really just want the particles to get moistened. The key to plaster strength is the mixing. Shorten the wet time from 5 minutes to 2 and make up the difference in mixing. That way you can pour while the mix is still fluid but you also have plenty of mix time. Good luck! H Palmer 1 Quote Link to comment Share on other sites More sharing options...
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