Collages are typically very flat. The minuscule thickness of paper is enough to give collages a unique quality, appearing to exist between two dimensions and three dimensions. You could call it 2.5 dimensional. There is a similar quality to low relief sculpture, where the carvings reveal a partially flattened version of what it is depicting. I've decided to use the idea in collage of layering materials in a plane, but have added a casting medium to give the collage more thickness.

In this Instructable, I will explain my process of layering and casting so you can try it too. This is not a strict process whatsoever; feel free to use as much artistic license as you want!

Step 1: Build Your Mold:

Supplies:

foam core/presentation board
X-acto or box cutter
metal strait edge ruler
cutting mat
pencil
packing tape
hot glue gun with glue sticks

Begin!

First, cut foam core into 5 pieces of the following dimensions:

4 x 7"
2 x 3.5" (x2)
2 x 6.5" (x2)

Refer to the picture for the next 4 steps where we will be making guide lines. (This is specific to making a 1.5x3x6" box.)
On the 4x7" piece, using a pencil and ruler, draw strait lines running parallel to each edge 0.5" from the edges of the foam core
On one 2x3.5" piece, draw lines perpendicular to the edges 0.5" from the edge only on 2 edges, one short, one long.
Do the same on the next 2x3.5" piece, however make sure the markings match the picture, because they will be marked on different edges. (These pieces will be staggered to make gluing easier, have straiter walls and make a water-tight seal.)
For the 2x6.5" pieces, repeat the last two steps

Step 2: Assemble the Mold

Use the masking tape to tape over the foam core that will appear on the inside of the mold. It is best to make sure the tape stays above all the guides to cover them and wraps around the bottom of each piece to seal it so that the hot glue seals the sides completely.
Start heating your hot glue gun.
In the first picture, the pieces of foam core are set up, for ease of assembly, as if you were to tilt up the outside pieces. This will leave the guide lines you made appear on the inside of the mold, so you know when to stop layering materials.
Notice that the sides are staggered.
Hot glue the sides of the box so that the inner faces (the ones with guide lines) line up with the guides on the base (the 4x7" foam core).
When the sides are held together well, go once more with the hot glue gun over each outside seam making sure the mold is completely water tight.

This will be where you layer materials and pour.

Step 3: Prepare Materials

For my example, I have chosen mostly flat or sticklike materials to be able to make flat layers within the mold. I have used basswood sticks, cut paper, metal tacks, small metal tubes, wire mesh, perforated paper, plaster of Paris and sliced hot glue sticks. You can use practically any material you want though.

Make sure they are already in manageable pieces that will fit in the mold and that you can arrange quickly as you pour the layers of casting material which is glycerine. You can find this at most craft stores. It is used for making bar soaps, so it will stay fairly soft after it sets. However it will keep its shape and it is non-toxic.

Step 4: Melt Glycerine and Pour First Layer

Read entire step before proceeding

**make sure before melting the glycerine you have your mold and materials nearby and ready to begin layering**

Place your first layer of materials in the mold.
Melt glycerine

To melt glycerine you will need...

a hot plate or stove
a small sauce pan with about an inch and a half of water in it
a bowl (NOT glass) that can be boiled in the water (tall enough to hold onto while in the hot water)
a spoon
glycerine

Scoop some glycerine into the bowl.
Heat the water over the stove until almost boiling.
Once water is hot (does not have to boil to melt the glycerine), hold the bowl with glycerine in the hot water, not letting it touch the sides or bottom of the sauce pan.
Continue to slowly stir the glycerine until it is melted.

The glycerine will not stay liquid for very long, so plan ahead if you need to place materials quickly in the melted glycerine.

Pour glycerine over materials in the mold.
Tilt to make sure the glycerine spreads evenly to over the whole bottom surface.
Place next layer of materials.
Continue to scoop more glycerine into the bowl, heat it, pour it evenly and place materials. I decided place globs of plaster in the mold as well. If you desire this effect, follow the directions on the plaster packaging and scoop a little into the mold as you go. (I went light on the water to make the plaster more controllable.)
Let the glycerine set for 20 minutes (or more if it still seems to give).

Step 5: Remove the Mold

When removing the mold, remember that glycerine will stay soft, so try to handle the mold rather than the contents of it.

Simply rip away the foam core, starting from the top corner. The hot glue should just pop off.
Pull the foam core away from the pour rather that pulling the pour away from the foam core or else you will see some denting.
You are finished! Now you have a 3 dimensional collage.

Step 6: Make Your Own!

The instructions for this project are very loose. It is meant simply to be a new way to make something. It doesn't "do" anything. But it has its purpose for thinking about materials and objects as something new!

I have made another iteration of this same idea, and would like to share it. I narrowed down my materials to wire mesh and wooden applicator sticks. I also began layering in voids to begin to achieve some spatial quality within the block.