And why is this blog post actually about plaster? Well first of all, pendants in many ways are like tiny little tiles. This book by Frank Giorgini is one of my favourites and so many of the techniques he describes are perfectly suited to making beads. Much of what I learned about plaster comes from this book. Way back when, most of the people working in clay that I knew, were potters. Nobody was working with plaster except for making sprig molds. The internet was just young and there really wasn't a lot of information out there. This book is great! You can get it on Amazon here or just search the web and you'll find other sellers.
Not all plasters are suitable for making molds for beads. Plaster of Paris, which most of us know is really not that great for bead molds. It's a bit soft and won't hold up over time. The most commonly used plaster for our purposes is Potter's Plaster. The second is HydroCal, HydroStone, or some other proprietary name. Potters Plaster is great in that it is absorbent and releases your pendants quite quickly. Hydrocal, which is my favourite, is very very strong and capture details really well.
See that broken plaster mold up there? Whatever plaster you are using you need to pay particular attention to the plaster to water ratio. That broken mold was weak because I was hasty and didn't measure my ratios properly. For small things, like bead molds, weight measurements are best. I know you can do volume measurements, but if you are only mixing up one molds worth of plaster for a pendant, the volume measurements don't give you much room for error. I use a digital gram scale and that works well for me in terms of precision. I usually mix up twice as much plaster as I think I'll need.
So here is a list of my tips for working with plaster:
1. Plaster has a shelf life and if you are storing it, put it in an air proof container. It takes on water and if you are in a humid climate your plaster will go off and weaken over time. Even in an arid climate, like where I live, it will go off.
2. If you can, buy your plaster from a supplier that has a large turnover so that you are always getting 'fresh' stock. Most plaster comes in 50 pound bags. Look for someone who repackages in smaller quantities or share a 50 pound bag with another clay person. Although, out here in my semi-arid climate, my plaster has lasted well over a year.
3. If your plaster sets up with a lot of water on top, you probably have too much water in the ratio. Your mold may be weak and may break just like mine up there in the photo. Press another model out of your mold and recast it just to be sure that all of your hard work making the model doesn't go to waste.
4. Never, ever mix plaster and cast molds where you are working with clay! Tiny little bits of plaster will get into your clay ad cause nasty, unsightly blow outs and pits when you fire your beads. Believe me, I know form experience :-)
5. Tap, tap, tap. I lightly tap the table I'm casting my mold on once I pour the plaster. I pour plaster from the corner of my mold and let it flow over my model. Then I tap the edges of the mold form. It helps to get any bubbles on the surface of your model to rise to the surface.
This is not in anyway comprehensive. Plaster mold casting can be somewhat daunting but is an exciting way to put your carefully made pendants into production. I'd love to hear any questions you might have. And I'd love to hear how any of you out there make molds. Let's learn from each other!