Normally, you don't mix oil-based and water-based paint. However, to get the cool effect of the fiery sphere of the Eye of Sauron, this is exactly what you've got to do. Acrylic craft paint is spritzed with oil-based spray paint to create streaks without a fuss.
Step 1: Supplies
Acrylic craft paints: Red and Black
Metallic Gold spray paint
Black oil-based model paint or black nail polish
Gold oil-based model paint or gold nail polish
Cardboard scraps (to protect your work area)
Clear spray paint (optional)
Small paint brush
Heavy cup or other container
Glass marbles, pennies, etc. to hold down the stick of the drying stand you will create
Pencil or Pen
Step 2: Preparation
Working outside is best because you are using spray paint. Your area should be well-ventilated.
Create a Drying Stand.
This can be anything that can be placed into the ornament and then secured over a surface suitable for drip-drying. I placed a pen in a cup of glass stones. Try it out to make sure it works with the ornament. The paint will add some weight to the ornament so make sure it can hold more before the stick flops out of the cup. Note that the pen is tilted at an angle that, when I test-fitted the ornament, allowed it to hang horizontally.
Place this on top of your scrap cardboard.
Getting Ready to Paint
Look at the Eye of Sauron. You will be making the two-part pupil. The inside is black and the outer border is gold. To get the best approximation of the eye, you will be creating a black, red, and gold streaked side of the ornament. The paint will be streaking away from the center point of this side.
At this point you might want to draw yourself an approximation for the pupil to use as a reference later on. Note the blue tape which I used to gauge the size. (I made one ornament with the tape on it to peel off and reveal the gold base but this was a bad idea. Just freehand paint it when all is done.)
Step 3: Painting the Fire
1. Mix approximately 2 TBSP red acrylic craft paint and 2 TBSP water together and set aside.
2. Mix approximately 2 TBSP black acrylic craft paint and 2 TBSP water and set aside.
(You may want other colors for more depth but these are what I used.)
3. Shake your spray paint and remove the cap.
Place your ornament on the hanger so the top hole is on the side, as shown. Make sure the cardboard is below it to prevent drips from ruining your surface.
Drip the mixture black paint and water onto the top of the sphere's side, as shown. Use a paint brush to coax the drips evenly around the sides. Drip the paint strategically. The rest of the paint layers tend to follow his layer so make sure the area is well-covered.
Repeat with the red paint and water mixture. Drip a little more black over the red.
Keeping the ornament in this position, Spray immediately with gold spray paint, directing the spray in the very center and spraying in very small bursts. You don't need much. It will immediately create streaks in the black and red paints and then drip down the sides as well.
Depending on how well this worked, you may be satisfied. I wasn't, however, and dripped another layer of black and red paint, just where it was needed. It swirled with the gold spray paint. You may need to fiddle with it: a spurt of gold here, some red or black there in alternating layers. Once you are satisfied, walk away and let it dry COMPLETELY. The paints will behave poorly to each other as long as they are wet so be sure it's totally dry before moving on.
Step 4: Detailing the Pupil
In the off-season, you can store this on an ornament hanging stand or on a "One Ring" sphere stand (Instructable to come). Note that it looks kind of nice without the hanger and placed on the top branch of a small tree as it's topper. Maybe for next year I'll decorate a small tree in cheap plastic gold rings (often used in wedding favors and cheap to buy in bulk).