This is the first part of a two-part series on making a silicone replica of a body part. However, before we can make the silicone replica, we'll have to make a plaster replica.

*** WARNING: Because of the body part chosen for casting (in this case, a woman's breast), this Instructable may contain material considered offensive or objectionable to some readers. Viewer discretion is advised. ***

Step 1: Gather Your Materials

To do this step you will need the following:

1) Dermagel - at least 2 pounds (or equivalent dental alginate)
2) Plaster - at least 2 pounds
3) Plastic container for placing body part inside of
4) 2 plastic mixing bowls - preferably with measuring lines (might get ruined)
5) Plastic cups for pouring materials
6) Duct tape
7) Metal whisk
8) Wooden mixing stick
9) Gram scale
10) Box cutter
11) Flathead screwdriver
12) An old towel
13) An assistant (optional, but highly recommended)

Step 2: Select a Body Part

This is the fun part.

The first thing you'll want to do is decide what to replicate. Some things are clearly more challenging than others. For instance, casting a finger is definitely easier than casting one's right breast.

However, I say go with whatever will make you or your robot happy.

Step 3: Prepare Your Container for Casting

Once you have decided what you're going to cast, you have to figure out how to cast it.

If you're casting something that you are capable of sticking into a bucket (i.e. a hand, foot or elbow), then all you need to do is find a container slightly bigger than the part you are trying to cast.

If you're casting something like a boob or a belly button, then you have to find a container that will fit nicely over the entire area of the body part.

For my first cast, I tried using an old yogurt container that was cut in half (below). This container cut off part of the breast. So I tried again by cutting in half a two-gallon water container and that was more than large enough to cast the body part (not shown).

Step 4: Other Prep Work

The dermagel mixes 1.5 water to 1.0 of the alginate by volume.

In other words, there is one and a half times water than there is alginate (to begin with).

Since the water is poured first, now would be time to measure out as much water as you think you will need (keeping in mind that the alginate will take up some additional space in the container).

Once that is done, you are also going to want to give the dermagel container a couple of good shakes to break up some of the bigger clumps.

Step 5: Attach Your Rig

If your rig for casting requires any special attachment to your selected body part, you are going to want to attach it before mixing the dermagel. Place the rig over the body part, making sure that its walls are not coming into contact with anything they shouldn't (i.e. anything that will interfere with the casting process or that will ruin the outcome of your cast), then tape it in place with duct tape.

Make sure that all edges are well-sealed with the tape or your rig will leak, creating not only a terrible mess, but also potentially traumatizing the person being casted (dermagel is cold, and has a thick, slimy consistency). If you're being body casted anywhere but an appendage that can just be shoved into a container, leakage is not pleasant. When in doubt, use more duct tape than less.

Step 6: Mix the Alginate

Mixing the dermagel (dental alginate) is not quite an exact science. Although, it is fair to start with a base mixture of 1.5 parts water to 1 part dermagel by volume.

Once you have that base, mix thoroughly and see what the consistency looks like. Ideally, it'll have the consistency of cake batter. If it does not, add more dermagel until it does.

Mix vigorously until there are no clumps left.

It's important to know that the warmer the water is, the quicker the setting time will be. This could be helpful depending on what you are casting.

Figure that the normal setting time is between 15-30 minutes to be on the safe side.

Step 7: Pour and Wait

Slowly and steadily pour your mixture into the rig.

Once all of it has been poured and/or it is filled to the top, sit and wait. Hold as still as you can.

In about 15 minutes, touch the top of the dermagel and see if it is solid. If it is solid, give it a few more minutes to be on the safe side.

When the time comes, slowly remove the tape and peel the mold away.

(It is important to note that if you plan to attach the rig to a part, dermagel gets heavier and expands as it hardens. If the rig that you are using to cast is big, and requires a large quantity of dermagel, plan to be able to properly support it, or else risk 1: the tape/support system ripping off/breaking and the rig tearing off before the dermagel has time to harden, 2: the rig shifting during the hardening process, and the outcome of the mold ruined. You should be able to get a sense of this when you are pouring the dermagel into the rig. If, during that point, the person being casted is already having difficulty supporting the rig him/herself, or can sense the support system failing, consider an alternate method of casting the part or just quickly reinforce the rig.)

Step 8: Plaster

Since dermagel degrades relatively quickly, you'll probably want to pour the plaster into your mold as soon as possible.

The plaster I used is measured by weight in a ratio of 45 grams of water for every 100 grams of plaster (however, you are going to want to check the specs on your plaster). Once you have measured out your water and your plaster, mix them together until you have a toothpaste-like consistency.

Pour the plaster into the mold. You may need to use a spoon or wooden stick to scrape the plaster out of your mixing bowl into the mold. Try to fill it as best as possible.

When the plaster is in the mold, give it a couple of gentle taps against the ground to raise possible air bubbles.

Lastly, I added a few L-brackets and screws to the back before it dried completely so that I could suspend it from strings if need be while silicone casting in the next step. It is helpful to have at least three points of contact.