It's got to be at least 30 years since I first cast something in plaster so it won't come as any big surprise that I've tried lots of different types. If I were planning something really big I'd seek out a 25kg bag of CASTING PLASTER (not the stuff for putting on walls) from a builders merchant. Normally however, I'll reach for a bag of Keramin.
Keramin is a high quality casting plaster with ceramic powder added to make it stronger and is made by Glorex of Switzerland under the name "Hobby Time". It has a working time of 10 minutes and a setting time of 30 minutes.
To be honest one of the main reasons I use it is that I can get it from Hobby's at the same time I'm buying other model making stuff. I also like the fact that it comes in 1kg or 5kg bags. Remember that plaster has a shelf life and as it ages its working time shortens. When it's really past it, the blooming stuff will start to set in your mixing bowl before you've even finished pouring it. Thus it's better to buy the quantity you need when you need it rather than buying a huge bag and trying to store it. As I write this I've just been doing some casting with Hirst Arts' moulds and I reckon that a 1kg bag, two at the most, will probably be enough for most projects.
Hobby's also sell Plaster of Paris (the 'usual' hobbyist's choice) but Keramin is cheaper and stronger too. In fact the first time I tried Keramin (a few years ago now) I cast a bar in Keramin and another in PoP for the purpose of doing a comparison and the Keramin was a lot tougher. Having said that, it's easy enough to work with it when it's set. In the image to the right I'm modifying a Keramin cast (from a Hirst Arts' Mould) by rubbing it on a sheet of 60 grit abrasive paper. I'm not pressing hard, just drawing it across the surface and letting the paper do the work.
The instructions (yes, I read instructions) on the pack suggest a 3:1 ratio of powder to water however I generally prefer to mix 5:2 for small moulds like Hirst Arts so it's a bit runnier.
I like to add a couple of drops of rinsing agent to the mix. By 'rinsing agent' I mean the stuff you buy at the supermarket and use when you're washing dishes (or rather when rinsing them) to prevent water marks. The rinsing agent decreases the surface tension of the water, making the plaster flow better and helping any small air bubble to rise out of it. I've not noticed any adverse effects of doing this.
Check out our article on working with plaster for tips on how to mix plaster.
Keramin is a good casting plaster at a good price. It's not the only casting material on the market and if your usual source doesn't have Keramin then it may be easier to try whatever they do have. However if they have Keramin, or you're placing an order with Hobby's, then I recommend that you give it a try.
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