Use chalk pastels to create custom colors of silicone "Oogoo." This is my contribution to the long tradition of Oogoo tutorials on Instructables. Oogoo is the name given to a quick-drying, easy-to-mold material made from silicone caulk and corn starch. The cornstarch makes the silicone caulk dry more quickly so it's easier to work with, and can give it a slightly more pleasant, almost powdery finished surface.

History and Other Resources:

Mikey77 introduced us to Oogoo back in 2010 and his Instructable still has the best of basic techniques and ideas after nearly 2 million views.
You can create and fix lots of stuff with Oogoo, from grippy robot wheels to a toothbrush holder.
You can press it into a mold or sculpt it like clay - either cutesy or alien.
How about making a wearable Glowing Spine specialFX prop, or conductive Robot Skin?

Well, you get the idea... it's a simple, fun and useful material.

Step 1: Making Oogoo

The basic technique is easy - mix corn-starch into 100% pure silicone caulk. The exact proportions depend on how you want to work with the Oogoo.

Small amounts of corn-starch keeps the silicone a sticky, gelatinous putty for a long time
Large amounts of corn-starch very quickly turns the silicone stiff enough to work by hand

By varying the amount of cornstarch (and time) you can create a material that behaves like acrylic texturing gel (minimum cornstarch) to polyclay (medium amount) to pliable rubber (lots of cornstarch). As it dries, the Oogoo also changes over time and follows a similar change in consistency.

My solution is to mix a medium amount of corn-starch into the silicone and wait patiently until the Oogoo achieves the desired workability. In fact, I often work in stages - when the mixture is still gooey and putty-like, I can spread it into thin sheets, or put strings or strips into cracks and crevices. Then I come back to add details when the Oogoo begins to solidify. If I need to join parts or ends, I try to do that just after the mixture begins to cure.

I tend to use a piece of glass and a metal or plastic spatula to mix Oogoo. I'm familiar with this technique from other materials and it feels the most natural to me. Other people suggest mixing it inside a plastic sandwich bag, on wax paper, tiles or metal. Other people mix it in cups using sticks or electrically powered augers, while other people use gloved hands for the up close and personal feel.

Step 2: Other Oogoo Colorants

Paints and Ogoo
I've seen people suggest both oil paints and acrylic paints as tinting agents. They both work for me, but both change the consistency and working-time of the resulting material and introduce more materials and chemical reactions to deal with. I've also successfully used watercolor paints, and I want to try liquid food coloring.

My only addition to this technique is to use the best quality paint (professional artist grade.) You simply can't get really rich colors with cheap craft paint, but with good paint you can get really deep, rich colors.

However, there are cool metallic and glow-in-the dark craft paints that are fun to play with and give the same results as the expensive versions.

Mica Powder Etc.

I have tried mica powder and iridescent powder pigments. They both worked well for me, giving the Oogoo a nice sparkly look. And they triggered a memory from a class on quick concept rendering. We were taught to scrape off chalk pastels onto a mixing palette so that we could mix custom colors of powder. So I tried that in Ogoo, and it worked so well that I wanted to share it.

(I also want to try the glow-in-the-dark, heat-reactive and UV-color-changing pigments some day.)

Step 3: Using Chalk Pastels As Coloring Agent

What are Chalk Pastels

Chalk pastels are basically pigments that have been pressed into sticks with chalk as a binding agent. They come in all colors and many grades of quality. There are cheap versions that range from kid's sidewalk-chalk to beginner level artist varieties. There are also expensive professional grade sets meant for creating gallery quality art.

Like most artist's materials, you get more pigment in professional quality products. This means you can get deeper, richer colors using less material - but they are also more expensive. If you just want a light tint to your Oogoo, then even children's toy chalk will work. But if you want the deepest, darkest blues and reds then you will probably need to get some good sticks.

The oil based and wax based pastels do not work with Oogoo the same way as the chalk based. Feel free to play with them if you already own some. When I tried, I got mottled effects, which might be just what you're looking for.

Basic Use

Scrape some powder off the pastel stick onto your mixing surface. A very light touch from the blade of your craft-knife is all that's needed. You do not need to saw or crumble it, just a light scrape will produce a fine powder that's easy to mix with the cornstarch.

Mix the pastel powder with the cornstarch and silicone caulk. You can do this all at once, or add the materials one at a time in any order. You can scrape pinches of different color pigments into a mixing pile (just like with paints on your palette) to create a custom color. Or you can mix directly into the Oogoo. Just remember, the Oogoo is constantly drying, sometimes very quickly, so make sure you have enough working time remaining to get a good mix.

The chalk pastel powder does seem to have the same kind of drying effect as the cornstarch, but I don't recommend using pure pastel as your drying agent. Use the cornstarch as your main drying agent. It's much cheaper than pastels and it doesn't seem to reduce the intensity of the coloring very much. However, if you are willing to wait for a long time for the caulk to dry, you can get a semi-translucent effect by adding a tiny bit of pastel but no corn starch.

Step 4: Advanced Techniques

Mixing Colors
The same rules of paint and color mixing apply to this process.

You can create a light blue by using very small amounts of a dark blue. But you can't create a darker blue by adding more light blue pigment.
The cornstarch does act like a white paint to lighten the color somewhat. The more cornstarch you use the more pronounced the lightening effect becomes.
You can mix colors to create other colors. So you might only need a few basic colors to start with.
Yellow and blue do not always make green. Make sure you know how the pigments interact.
There are "strong" and "weak" colors. Yellow is notoriously weak. Yellow fades with just the addition of cornstarch, and a pinch too much of a strong blue or red can consume yellow.
Once you have too much of a strong color it's almost impossible to recover. Don't keep adding more yellow. Instead, take a small pinch of the ruined mix and add it to a pure pile of the weak color.
Start with the weakest color when mixing, so start with yellow and add the blue or red very slowly

I cannot blend or create gradients with Oogoo the way I blend acrylics or oils, or even polyclay. Please experiment and share your techniques if you succeed. The closest I've ever gotten is a king of stringy, marbled blend - but I can't do a smooth gradient by mixing Oogoo. But there are other techniques that give reasonably good blends and gradients

Pat and Paint

When the Oogoo is still slightly wet, you can brush or sprinkle some pastel powder onto the surface. Some of it will stick to the silicone. If you press or brush it into the surface, even more pigment will stick to the Oogoo. If you think of the Oogoo like "texture gel" for acrylic painting you will get a feeling for how it handles.

You can get nice subtle marbling and cloud/fog effects with this technique. Frankly, I can't tell the difference between this technique and simply mixing two colors without finishing the blending. I do get a little more control of where the colors go, but the end result looks the same.

You can get a decent gradient effect by "dry brushing" the pigment powder onto the surface before pressing or rolling it into the material. I always get clumps, bumps and splotches - but I call it a feature that proves it's hand made.

I also tried and failed to get detailed paintings onto Oogoo because I can't predict how much pigment will stick to the surface. I got closer by sculpting and pressing strings of colors into the main body. If I were more patient maybe I could "tattoo" the pigment in with a toothpick.

Stencils and Embossing

Freehand painting, brushing and dusting is interesting, but a stencil helps if you want a more predictable result, like a pattern or logo or other symbol. Just put the stencil onto the surface of the Oogoo and liberally sprinkle the openings with pigment powder, then lightly press the powder into the Oogoo. Blow off the remaining powder before removing the stencil.

You can also brush the pigment onto the raised part of an embossed surface. If you have an opening in the embosser tool (like a mesh) you can use the embosser like a stencil. Leave the embosser on the Oogoo, then brush the pigment onto the exposed Oogoo before removing the embossing form.

The durability of the coloring created with patting, painting and stenciling depends on how deeply the pigment gets worked into the Oogoo.

Conclusion

After writing this guide, I realized that this is really just like working with thick acrylic gel medium or polyclay. So I think the best advice I can give you is: Study the classic Instructables on Oogoo and study the techniques for acrylic texture gel medium and polyclay.

Oh, and of course - have fun and experiment - go make something cool !!!