Make a Latex Rock Mold

Making your own molds for rock casting opens up possibilities beyond those available using commercial rock molds or castings. Once you have your own mold, you can cast as many rocks as needed, cutting them and rearranging into varied patterns to disguise their origins. Making a latex mold is simple and inexpensive, and the materials are readily available. Although a latex mold will not last as long as a silicone or urethane mold nor make as many castings, this is usually not an issue for an individual modeler.

For this project I used some rock castings from "Ultimate, Make-A-Mold Master Castings" rock castings by Ultimate Scenery Center (www.ultimatescenery.com). These are highly detailed, urethane foam castings that the manufacturer actually intends to be used as masters for the purchaser to use to make his own molds.

I started with #123 "Pennsylvania Shale", a large (13" x 9 ") casting exhibiting both stratified and 'blocky' rock in the same casting. Here's a photo:



Here's a detail picture:



Click on any photo to get a full-size view.

Here's a photo just before starting the process. I used 'Mold Builder' latex mold compound that I bought at Michael's.



The surface that I am building this mold upon is a 18" x 18" ceramic floor tile obtained from Home Depot. I chose the smoothest 18" x 18" tile that I could find, although this was a floor tile and not a glazed wall tile like I build smaller molds on. To make the tile's matte surface smoother, I coated it with clear gloss spray from a spray can. A smooth surface will prevent the latex from adhering and facilitate removal of the finished mold.

The latex is brushed on. The first coat of latex must be applied thin and without bubbles - unless you want bubbles to be a permanent part of the mold and of each subsequent casting; so watch your brush strokes. I extended the latex beyond the sides of the master casting to form a lip on the finished mold about 3/8" wide.

And here's the first coat of latex applied.



Allow each coat to dry before applying subsequent coats. If you are impatient, the cure of the latex can be accelerated with hot air from a hair dryer. Additional coats are applied in the same way. Resist the temptation to apply the latex in thick coats. Thick coats tend to pull away from the master and they tend to dry with voids in the rubber.

Here's a pic of the mold after a couple of coats of latex have been applied and dried.



Apply four coats of latex before the next step.

Apply a coat of latex and wait for it to get 'tacky' then layer the entire thing with gauze. Forced the gauge into all of the cracks and crevices.

After the gauze is applied, add another coat of latex. Get the gauze saturated with this topcoat of latex. Here's a picture of the mold after adding this overcoat of latex:



After the latex/gauze/latex coat as dried; at least four additional coats are added before the mold is done; more coats can be added for additional strength.

Here's what the finished mold looks like ready to be peeled from the master casting:



And here's a video of removing the mold from the master casting: