After more than nineteen years in the concrete stone mold business, some of our customers and DIY stone, paver and tile owners who use our molds still have problems with air bubbles or insect holes in concrete products. cast that they manufacture. Almost 100 percent of the time, it’s because they don’t follow the free, simple instructions provided with every purchase. So here are the instructions and techniques again in the form of a DIY home improvement article.
Is a release agent really necessary when casting concrete? – We always suggest using some type of release agent, even when using rubber molds. The release agent will help extend the life of your concrete, cement, or plaster molds. Using a mold release will also make removing the stone, tiles, pavers, or other items you are duplicating from the mold easier and faster.
Make a homemade liquid mold release to use with Plaster of Paris, plaster for trim, and other plasters. When working with plaster, a simple mix of a tablespoon of liquid dish detergent in a cup of water is a good release agent. Keep in mind that different plasters offer different shrinkage properties. The less a cast shrinks, the more difficult it can be to remove the cast from a plastic or other semi-rigid mold. Be sure to read the instructions and properties on the package of all plasters before choosing one for your project. We also recommend using a soapy mold release when working with rubber and plaster molds. Produces a cleaner release and smoother finished part, as well as helping to prevent plaster from sticking to rubber molds, saving cleaning time and effort.
How to Make a Release Release for Concrete and Cement: There are several commercially prepared release agents for concrete on the market. We currently offer our customers two types: a water-based and an oil-based release agent. We market it as a convenience to our clients. A light vegetable oil works just as well as a commercial blend. Commercial versions may contain wax suspension or other agents to facilitate use or offer other benefits.
Regular vegetable oil is fine, and a “light” vegetable oil is even better. But don’t worry about it. The problem with air bubbles in concrete castings is rarely the type of release agent used, usually the amount that is left before pouring. It’s usually too much release, or too much vibration is used to dislodge air adhering to the surface of the mold, or should I say between the mold and the freshly poured concrete. Have you heard the old saying about “too much of a good thing”? That is usually the problem. With too much vibration, you can introduce air bubbles into the mix. And excessive vibration, especially with only half the batch of concrete poured first into the mold, can certainly cause excess air.
How much oil should be left to cover the mold? – For best results, be sure to clean most of the oil from your molds before filling them with concrete. You should be especially aware of low mold crevices where oil can collect. You should barely be able to see that there is a coating on the mold. Then fill your mold about half to three-quarters full with your concrete mix. On a flat, level surface, lift each side of the pan about a half inch or so and drop it onto the tabletop or other surface you are using. If it is a larger mold, say over 12 “in diameter, lift it higher so the center also benefits from the vibration. Do this about four to five times on all four sides. Then fill the mold to depth. finish you want. This time, lift all four corners one at a time four or five times, letting them drop. Now shake the entire mold a couple of times in each direction to make sure the concrete is level. Cover with plastic sheeting and wait for it to harden. This technique allows you to dislodge any air bubbles from what will be the surface of your stone.
Why is there air in my concrete mix? – Yes, having a certain amount of air in your mix is really desirable … especially if the stone is for an exterior application. This is called air entrainment. Allow space for moisture within concrete to expand and contract in an icy environment. This is called freeze-thaw. When you use our mixing admixtures, there is actually an air entraining agent in the admixture to put air into the concrete. However, normally, air entrainment in concrete and cement should only be around 5%. That air really has little to do with the “bubbles” or what in the industry are called “bug holes” that appear on the surface of your stone casting.
Think how happy your arms will be now that they don’t have to vibrate that heavy concrete so much!