You can make a flexible, putty-like craft material from 100% silicone caulk and cornstarch, a mixture often called Oogoo. An informal recipe suggests starting with roughly equal volumes, but this is not a standardized casting compound: working time and results vary. It may suit a small, simple craft mold or object, but not food contact, skin contact, medical use, or work that needs dependable detail or performance.
What this mixture can—and cannot—do
Oogoo is caulk mixed with cornstarch and kneaded into a moldable putty. You can press it into or around a simple master to form a flexible shape. It is a low-cost experiment, not a product with controlled specifications for precision, durability, or repeat use.
There are two different jobs people may mean by “casting silicone”: making a silicone mold to reproduce another object, or casting a silicone part by pouring material into a mold. This caulk mixture is an informal craft material; it should not be assumed to perform like a dedicated mold-making rubber or a purpose-made casting material.
What you need
- 100% silicone caulk. Check the label; the cited BOSTI-SIL 25, for example, is sold for construction and industrial sealing, not as a craft casting product. Bostik BOSTI-SIL 25 product information.
- Cornstarch.
- A disposable mixing surface and tools, plus gloves if you want to avoid direct contact with the uncured caulk.
- A simple master or form to shape the putty around or into.
How to make a small trial batch
- Prepare a ventilated workspace. Read the caulk label and follow its precautions. One-part acetoxy-cure silicone sealants can release acetic acid as they cure; the odor during curing is one reason ventilation matters.
- Measure a small amount of cornstarch. The informal Oogoo recipe suggests roughly equal volumes of caulk and cornstarch as a starting point. This is guidance from a DIY recipe, not a guaranteed ratio or cure specification. Make: How-To: Make cheap castable silicone from caulk; OOGOO Silicone Casting or Molding.
- Add caulk to the starch and knead. Use disposable tools or protected hands to combine the ingredients into a putty. Work promptly enough to shape it, but expect timing and handling to vary; the recipe does not establish a reliable working time.
- Shape the putty. Press it into a simple form or around a master. A small trial lets you see whether the mixture can be shaped as needed before you commit more material or a larger project.
- Let it cure, then assess the result. Follow the caulk maker’s instructions and allow the material to cure; do not assume the cure time printed for a bead of straight sealant applies to a thick mass mixed with starch.
Why the result is unpredictable
The DIY recipe reports that more starch tends to make the mixture set faster and less starch slower, while also warning that setting can be unpredictable. Treat this as a rough handling observation, not a dependable way to control cure time. Make: also questions the recipe’s proposed explanation for how starch affects the mixture, so the mechanism should not be treated as settled.
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#1 Best Overall
- 100% Waterproof and Weatherproof: This 100% silicone formula offers protection from water damage, providing a reliable seal in areas exposed to harsh weather
- Excellent Flexibility and Strong Adhesion: Provides a flexible, durable seal for both exterior and interior projects
- Same-Day Rain-Ready: Ready for rain exposure in as little as 12 hours
- Compatible with Most Materials: Adheres to most metals and woods; glass, drywall, plaster, granite, cultured marble, ceramic and porcelain tile; natural stone, cement board, composites, and more
- Color : Clear
The Oogoo tutorial characterizes its material as somewhat weaker than commercial two-part silicone and says it offers a narrower range of working times, firmness, and colors. Those are informal comparisons, not controlled tests. The available sources do not establish the mixture’s precision, service life, or repeat-use performance.
When to choose mold-making silicone instead
For detailed reproduction, repeatable results, or a material with stated performance properties, use a product expressly sold for mold making and follow its own mixing and curing directions. Dow describes SILASTIC RTV-3483 with RTV-3083 curing agent as a mold-making rubber intended for detailed reproduction, with properties including flowability, low hardness, tear resistance, and elasticity. Dow SILASTIC RTV-3483 product information.
Rank #2
- 100% Silicone Caulk Protection: This silicone caulk clear provides ultimate waterproof protection, ensuring a reliable seal against water damage. Ideal for areas with consistent exposure, keeping your surfaces safeguarded and pristine under any weather condition.
- Strong and Flexible: Delivers a silicone sealant waterproof clear that remains permanently flexible, adapting to surface movements for a lasting, durable hold. Perfect for dynamic environments like bathrooms and kitchens where adaptability and resilience are paramount.
- Quick 30 Minute Water Ready Performance: Experience the convenience of a silicone caulk that becomes water resistant in just 30 minutes, making it perfect for time sensitive projects. No more waiting hours, dependable, quick sealing efficiency with clear silicone caulk.
- Shrink and Crack Proof: Experience superior durability with clear silicone caulk that withstands temperature fluctuations. Its flexibility guarantees your surfaces stay intact without shrinking or cracking, ensuring consistent protection and a flawless finish over time.
- Low Odor and Safe Indoors: Enjoy stress free application with this silicone sealant featuring a low odor, neutral cure formula. Great for indoor use without overwhelming fumes, enhancing comfort and safety while ensuring effective waterproof clear protection for your spaces.
The right material also depends on whether you need a silicone mold to reproduce a rigid object or a silicone part cast into a mold; those are distinct workflows with different material and design considerations. Formlabs: Silicone mold making and silicone part production.
| Need | More suitable direction |
|---|---|
| Simple, low-stakes craft experiment | A small caulk-and-cornstarch trial may be worth trying, with variable results expected. |
| Fine detail, defined properties, or repeatable mold production | A dedicated mold-making silicone with published product specifications and instructions. |
| Food, skin, medical, or safety-critical use | Only a material expressly intended and certified for that use; ordinary hardware-store caulk is not made safe for these applications merely because it cures to silicone rubber. |
Safety and use limits
Use the caulk label and manufacturer precautions, including ventilation. Bostik describes BOSTI-SIL 25 as a one-component RTV sealant that cures with atmospheric moisture and releases a small amount of acetic acid. Its product page says a 3/16-inch bead normally cures in 24 hours, potentially taking up to 48 hours in cooler or drier conditions; that guidance is specific to that sealant and bead size, not a thick caulk-and-starch casting. Bostik BOSTI-SIL 25 product information.
Rank #3
- 100% Silicone Caulk Protection: This silicone caulk white provides ultimate waterproof protection, ensuring a reliable seal against water damage. Ideal for areas with consistent exposure, keeping your surfaces safeguarded and pristine under any weather condition.
- Strong and Flexible: Delivers a silicone sealant waterproof white that remains permanently flexible, adapting to surface movements for a lasting, durable hold. Perfect for dynamic environments like bathrooms and kitchens where adaptability and resilience are paramount.
- Quick 30 Minute Water Ready Performance: Experience the convenience of a silicone caulk that becomes water resistant in just 30 minutes, making it perfect for time sensitive projects. No more waiting hours, dependable, quick sealing efficiency with white silicone caulk.
- Shrink and Crack Proof: Experience superior durability with white silicone caulk that withstands temperature fluctuations. Its flexibility guarantees your surfaces stay intact without shrinking or cracking, ensuring consistent protection and a flawless finish over time.
- Low Odor and Safe Indoors: Enjoy stress free application with this silicone sealant featuring a low odor, neutral cure formula. Great for indoor use without overwhelming fumes, enhancing comfort and safety while ensuring effective waterproof clear protection for your spaces.
Do not use hardware-store caulk mixtures for food molds, body contact, medical purposes, or safety-critical parts. Choose a product expressly intended for the end use and follow its directions. A vinegar-like odor during cure is not evidence that the finished material is food-safe or skin-safe. Selleys Silicone Putty Recipe.
Quick Recap
Best Value
- 100% Waterproof and Weatherproof: 100% silicone formula offers protection against water damage, providing a reliable seal in areas prone to harsh weather
- 10-Year Product Protection: Cured sealant is resistant to stain causing mold and mildew growth
- Permanently Flexible with Strong Adhesion: Sealant offers 40% more flexibility than Class 25 sealants and 5X stronger adhesion than our Silicone 1 sealant
- Durable: Rain-ready in as little as 30 minutes, and resistant to shrinking and cracking due to temperature fluctuations
- Compatible with Most Materials: Adheres to most metals and woods; aluminum, composites, brick, stone, stucco, masonry, cement board, glass, drywall, ceramic and porcelain tile, and more
Rank #4
- Kitchen & Bath Ready – Premium white silicone caulk formulated for kitchens and bathrooms, delivering a durable, long-lasting seal in wet, humid environments
- 100% Waterproof Seal – Perfect for showers, tubs, sinks, tile, and backsplashes, keeping water out where it matters most
- 30-Minute Water-Ready – Can be exposed to water in as little as 30 minutes; full cure in 24 hours for reliable results
- Flexible and Durable – High-flex silicone caulk prevents shrinking, cracking, or breaking down; perfect for indoor and outdoor projects
- Long-Lasting Mold-Resistant – Formulated to resist mold and mildew after curing; easy to clean with soap and water
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