Making rock molds Easy techniques for durable molds/Gregor Moe T
he hardest part of making a rock mold is finding the rocks or things that will make scale size
rocks. I look for rocks that have a nice texture and fine detail that you can highlight when you paint the rocks on the layout. You don’t have to use just rocks; you can use anything that gives you the textures, details and shapes you want. My favorite materials for rock molds are petrified wood and some granite rocks I found in a road cut in southern Missouri. I got my petrified wood many years ago at a pet store that specialized in tropical fish. I have also used pieces of old weathered wood that I found while out walking. Coal and galena, pieces of lead, can make nice castings. There are also some nice ure- thane castings available now that could make good molds. Another place to look for rocks would be at some of the land- scape stores that specialize in rocks. The rocks don’t have to be huge as most of us don’t have the space to mod- el huge rock walls. If you look in na- ture, rock is usually covered up unless it is nearly vertical or regularly washed clean. I have gone overboard in a couple of spots on my layout but most of the rocks I have used are small cast- ings butted to each other to form out crops along roads, rail cuts, and river banks. With any mold over two hands full in size, it is going to be hard to get a loaded mold of semi-liquid plaster from flat on the workbench to a verti- cal surface and hold it solidly to a sur- face until is solidifies. Once you have found your masters for molding, the task of making the mold is very simple. You need the latex molding
compound, a paint
PHOTOS BY THE AUTHOR
The masters for the molds are important. They must have textures and detail that will trans- late well into a rock mold. The masters above are from the top left: slate, petrified wood, petri- fied wood, slate, granite, petrified wood and granite. The first coat of latex (below) shows two lines on the lower right corner that are more opaque and thicker than needed. Try and brush them thinner. The shallow edges of the mold will help when overlapping with other molds.
brush,
cheese cloth and an area to work in. The liquid latex “Mold Builder” brand can be bought in most craft stores and some hobby shops. It costs around $11.00 to $15.00 a jar. The paint brush is one that you can throw away after use as you can never get it completely clean. I wash out the brush with cold water and a steel pet comb to break the bristles free after each use, but by the time I finish a set of molds, it is ready for the trash can. The cheese cloth can be found in some hobby and craft stores, as well fabric stores and the baking aisle in grocery stores. The first step in making the mold is to
clean the surface to be molded of any loose pieces of rock or dirt so they do not
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get imbedded in the mold. The first three layers of the mold are the most im- portant as that is where all the detail is. The first layer should be a thin covering. Try to avoid getting build-ups of latex in the joint and cracks of the rock. The la- tex starts to dry very quickly and if you see build ups of latex in corners and cracks try and brush them out. The build ups can cause weak spots as they sometimes don’t dry completely and can
tear when used for casting. Let each layer dry completely before
adding the next layer. Keep the edges shallow. The edges will tend to grow with each added layer of latex. The shallow edges make it easier to overlap a previous casting when you are cast- ing a rock wall. Deep edges tend to get caught under the casting and brake off pieces of plaster when the mold is re- moved or tear the mold.
FEBRUARY 2014
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