The volume of mixing water required is roughly equal to three-quarters (75%) of the volume
of the piece you want to pour. I generally calculate the volume in cubic inches
then multiply by 12.3 to obtain the number of cubic centimeters of
mixing water required. (NOTE: 1 in³ = 16.4 cm³). You can then divide by 30 to get
the ounces of mixing water for the volume you're filling.
Once you have your mixing water measured out and everything is ready to go, simply start sprinkling dry plaster into the water and keep adding until you wind up with the surface looking something like desiccated soil (see top image). This will give you a reasonably fluid plaster that is relatively strong after setting. If you need something more fluid (or less if you need stronger plaster), you can adjust the amount you add to the mixing water. Keeping track of the actual amount you use (volumetrically is the quick & easy way), can prove useful if you need to accurately reproduce the consistency in the future.
NEVER pour plaster down a drain. Keep a bucket of clean water handy and use that for
rinsing and cleaning tools.
- I prefer to measure virtually everything volumetrically. For most things, a measuring cup or graduated cylinder is handy. If greater precision is needed, one can even resort to using a large capacity syringe (you can remove the needle). Large syringes can generally be found at your local farm supply.
- Cheaps plastic cups (e.g. 3oz, 5oz, 8oz, etc) can also come in quite handy. You can buy these in bulk at big-box stores.
- Be sure to coat solid surfaces with some type of mold-release. I prefer petroleum jelly when pouring plaster.
- Keep some moist clay around for plugging gaps in your formwork or for "tacking" pieces in place. Sometimes C-clamps are useful and sometimes using wads of sticky clay are all you need to keep things positioned properly long enough for the plaster to set after pouring.