AY Honors/Plaster Craft/Answer Key

1. What is the principal ingredient of plaster of Paris?

Plaster of Paris is based on calcium sulfate which is derived from gypsum.

2. Give the steps in pouring a plaster item and preparing it for painting.

Preparing the mold

Plastic molds generally cannot support themselves once they are filled with wet plaster, so you must prepare a support base to set them on. This support base can be as simple as a sealable plastic bag filled with uncooked rice or sand. Squeeze as much air out of the bag as you can, and then lay the bag flat on the work surface. Place the mold on top of it and press it into the support, wiggling it around until the support conforms to the shape of the mold. Try to get the mold as level as possible. Once the mold is in place and well supported, coat the mold with mold spray (see requirement 3).

Mixing the plaster

The most fool proof way to mix dry plaster powder with water is to weigh them. Use seven parts plaster to 10 parts water by weight. If you know how much your plaster weighs, multiply that weight by 1.43 to get the required weight of the water. It's a lot easier to do this using the metric system, because a milliliter of water weighs one gram. So if you know how many grams of water you need, that's also the number of milliliters of water you need. Let's run through an example:

If you have one kilogram of plaster (that's 1000 grams), you will need [math]\displaystyle{ 1000 \times 1.43 = 1430 }[/math] milliliters (1.43 liters) of water.

Pour cold water into a plastic mixing bowl, and then add the plaster powder to it, stirring as you go. Warm water will set faster - maybe too fast! Never add water to plaster - always add the plaster to the water.

SAFETY TIPS
The plaster will heat up as it reacts chemically with the water, so never attempt to pour plaster over a body part in an attempt to cast it. Plaster is also somewhat caustic, so it can cause a chemical burn as well. Wear gloves. Wear eye protection Wear a dust mask Do not wear contact lenses Keep powdered and wet plaster away from children and pets

Stir the plaster and water until it is well mixed. Allow it to sit for no more than two minutes to allow the plaster to fully absorb the water and quit bubbling. If desired, this is the time to add pigment for color. Add the pigment and mix it in with a power drill (using a paint mixing attachment). You can also use a potato masher, and for small amounts, you can just use a stick. If using a power drill, do not over do it, as blending it will hasten the setting time. Take it easy!

Pouring the plaster

Pour the wet plaster into one corner of the mold and allow the plaster to flow across to the other side. When the mold is full, tap it a few times to help work out any air bubbles. If you need a hanger, place it in the plaster at the top (pay attention!). You can buy hangers or you can use a paper clip. Make sure the hanger is well embedded in the plaster.

Releasing the mold

Once the plaster heats up, you can remove it from the mold. Do this by gently pulling the edges of the mold away from the cast piece. Work your way around until the mold has been separated all around (it's OK to let the mold snap back into place). Do not overflex the mold or you may crease it. Turn the mold upside down over the support base (the bag of rice) and gently shake it until the piece pops out. It should land gently on the support base. Set the piece aside to allow it to finish curing. For fastest curing, set the piece on a grate so that air may circulate over all surfaces.

3. Know how to remove air bubbles from a poured item.

Air bubbles are caused by the surface tension of the water mixed into the plaster. This surface tension can be eliminated by spraying a mold treatment such as Airid onto the mold before pouring the plaster. The mold treatment will reduce the surface tension and prevent bubbles from forming.

You can also mix up a small amount of new plaster and apply it onto any holes in the finished piece with an artist's brush. Let it dry for an hour before painting.

4. Know how the setup time can be increased or decreased for plaster.

You can speed up the setup time by using hot water, or slow it down by using cold water.

5. What precautions should be taken when cleaning the mixing and pouring equipment?

Under no circumstances should you rinse plaster down the sink! Remember that plaster hardens when it comes into contact with water - it will set even when fully immersed. If you pour it down the drain, you are asking for a very expensive plumbing repair.

The easiest way to clean plaster out of the plastic mixing bowl is to just let it set. Then flex the bowl and the plaster will pop out. Make sure you put the plaster in the trash - not in the sink!

To get plaster off a potato masher or paint blender, strike it smartly with a hammer. The plaster should flake right off.

Wipe your molds out with a damp paper towel.

References

• http://www.onestopcandle.com/plaster/basicplasterinstr.php
• http://www.milliesplastercraft.com/BasicInstructions.asp