Things You'll Need
Plaster of Paris
Most costume and party supply stores sell thin, plastic masks that can be decorated and used to make custom costumes. Though they are inexpensive, these masks cannot support heavy ornamentation or frequent use like a hard, plastic mask. You can cast your own hard mask using liquid plastic and a stock mask mold. Or you can build a completely original prop by designing a mask mold from scratch; although a longer process, this method offers the advantage of custom detailing and fit.
Work in a well-ventilated area. Protect your work surface with a tarp or drop cloth. Wear gloves and a breathing mask.
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Select a mask to use as a template.
Tape or hot glue the mask template to the glass pane so that the front of the mask faces upward.
Paint a thin, even layer of liquid latex over the mask. Place the mask in front of a fan to dry for an hour. Repeat to build up 15 to 20 layers of latex. Do not add a new layer until the previous layer is dry.
Mix plaster of Paris and water according to the manufacturer's directions.
Pour plaster into a shallow plastic container large enough to accommodate the mask's height, width and depth with 1 to 2 inches clearance between the mask and the walls of the container.
Insert the mask into the mold latex-side down.
Wait for the plaster to set; drying can take 15 minutes to 12 hours depending on the humidity, type of plaster and size of the mask.
Remove the mask template, leaving the latex mold within the plaster master mold.
Estimate the volume of the mask. For a rectangular mask, such as a tiki mask you can find the volume by multiplying the length, width and depth. For irregular shapes, fill the original mask template, not the mold, with clay. Remove the clay and press it into a regular shape such as a rectangle and then use the standard volume formula to estimate the size of the mask.
Spray the mold with mold release.
Mix enough liquid plastic to fill the mask based on the volume. Liquid plastic comes in two parts which must be mixed in a particular proportion such as 1:1 by volume or 3:1 by weight. Measure out and mix according to the manufacturer's directions.
Pour the mixture slowly into the deepest part of the mold. Allow the plastic to naturally flow and fill the rest of the mold.
Wait for the plastic cast to set. Invert the mold and remove the cast.
Press clay into the mask template to fill the interior creating a solid block.
Roll a slab of clay 1 to 2 inches larger than the mask. Place the mask onto the slab and press it down so that the clay on the back of the mask attaches to the clay slab.
Place the clay and mask into a bottomless rectangular basin or mold box. Fill any gaps between the slab and the wall of the box with clay.
Press a short dowel or pen into the clay next to the mask such that it touches the mask and the wall of the container; this will make a cavity in the mold so that you can pour in the liquid plastic later.
Make several indents in the clay in the corners of the container to leave a hole. This hole is called a mold key and will help keep the mold halves oriented later.
Pour liquid silicone into the mold box covering the mask and clay. Start pouring slowly in the corner of the mold box letting the silicone overflow naturally into the rest of the mold to avoid air bubbles.
Wait 24 hours for the silicone to cure.
Carefully invert the container. Remove the clay without removing the mask or silicone half-mold.
Spray the mold with mold release.
Fill the mold box with silicone to make the second half of the mold.
Cure for 24 hours.
Hold the halves together so that the keys match. Place a plastic sheet on either side of the mold to support it. Rubberband or tape the mold and plastic together.
Pour liquid plastic into the mold through the hole formed by the dowel.
Wait for it to cure following the timetable provided by the manufacturer. Remove the rubber bands and release the cast.
Make a mask template out of clay or paper mache, or add details to an existing mask before making your mold.
Insert a wall hanger or hook into the back of your mask during casting.