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.

6. When is a sealant applied to a plaster item and why?

Acrylic sealant is sprayed onto a piece after the paint has dried. This will preserve the color and strengthen the surface of the piece. You can select matte sealants or gloss sealants. Matte sealants will seal the item without having much of an effect on the way it looks. Glossy sealants will brighten the colors and make the piece more shiny.

Be sure to check that the sealant you choose is compatible with the paint you have used. Some combinations can cause the colors to run. If in doubt, paint and seal a piece of scrap first, or apply them to the back or bottom of the piece to test for compatibility.

7. What type of paint is best to use on plaster craft items?

Acrylic paint is the most commonly used type of paint in plaster craft. It is water soluble and cleans up easily.

8. Paint three items that will include the following designs and techniques or equivalent:
a. Floral to show shading
b. Fruit to show highlighting
c. Animal to show fine line or detail
d. Religious motto to show letter highlighting

Start by applying a basecoat. Allow it to dry and the begin painting the lightest colors first. If you make a mistake, let the paint dry for five minutes and then paint the desired color over the undesired one.

Shading

For shading, paint the lighter color on to the piece. Then mix the darker color on you pallet and paint it on an adjacent area. Rinse and dry the brush, and then use it to blend the light and darker areas together.

Highlighting

For highlighting a fruit, paint the entire fruit the color of your choice. While the paint is still wet, put a small amount of white paint on a clean brush and paint a small circle on the area you wish to highlight. Work the circle into the wet paint surrounding the highlighted area, blending the paint as you go.

Fine lines

For fine lines, use a brush with a very fine tip. Add a small amount of paint to the brush and draw the tip over the area using a steady hand. Experiment with resting the base of your palm on a table as you paint (do your experiments on something other than your piece!) When you think you've got the hang of it, try it on your piece.

Lettering

For raised lettering, paint the background first. Let it dry, and then carefully paint the top surface of the letters. For painting free-form letters on a flat surface, practice on some scrap first until you get the hang of it. Plan our what you will letter, being careful to center it on the piece.

9. Mold and paint two additional items of different designs.

The easiest items to cast are open-top molds. It is best to not try 3D or two-part molds until you have a little more experience, though these can certainly be used to fulfill this requirement. The reference links below offer hundreds of molds that you can purchase and reuse. You should also be able find molds at your local craft stores.

References

• http://www.onestopcandle.com/plaster/basicplasterinstr.php
• http://www.milliesplastercraft.com/BasicInstructions.asp
• http://spiritcrafts.stores.yahoo.net/ You can find all the molds you need on this one site.

http://store.plastercrafters.com