Rock Castings
Article by: Bob Farquhar

General :
Real rocks are used to make latex molds which are then used to make plaster rock faces for mountains, rock outcroppings, etc.. Once a scenery base is completed, the latex rock molds are filled with plaster and applied to the scenery base. After colouring with stains or paints, the plaster rock will look like the real thing.

Real Rock :
Select rocks from nature that have surfaces similar to the rock effects you want on your scenery. For jagged type rock faces, a rock called "Feldspar" is especially good. Jagged type rock can also be found at construction sites where blasting is used. This yields a rock face with sharp edges because nature hasn't had time yet to weather wear the features. Once you have the rocks, you must clean them. This is where your neighbours will be thoroughly convinced that you are certifiably insane when they see you out in the driveway washing rocks !

Latex Mold Material :
Lewiscraft sells a product called LiquidTex which is a latex based mold material used to make Halloween masks, etc.. Once your rocks are clean, the LiquidTex is painted directly on the rock face. To ensure the latex material gets into every nook and cranny, spray the rock with water before applying the first coat. The latex mixes with the water and will flow into every rock feature. After each coat dries, simply paint on another layer of LiquidTex. The number of coats depends on the mold stiffness you want. Some like an extremely flexible mold - this would require 6 or 7 coats. A flexible mold is good for wrapping around scenery features but, there's a down side - the mold will wear quicker and holes will begin to develop. However, even if a hole does develop from wear, simply touch up the hole with some LiquidTex and you're back in business. A stiffer mold will need at least 8 coats and then some supporting material such as pantyhose or fiberglass screen which is painted in with a LiquidTex coat. Another coat or two of LiquidTex and your mold is finished. One down side to a stiffer mold - you won't be able to wrap the mold around scenery features thereby restricting the mold applications.

Mold Plaster :
The recommended plaster for rock molds is Hydrocal. Mix the plaster as you normally would and fill the molds laying on a flat surface. If needed, prop up sections of the mold that do not lay flat so the plaster doesn't run out of the mold.

When To Apply Molds :
Dry Molds - The dry mold process basically waits until the plaster in the mold is dry. The plaster rock is removed from the mold and glued to the scenery. The down side to this process are the gaps that are left around the mold. Plaster will have to be applied around the mold to blend the rock in with the rest of the scenery. The up side is that timing is not an issue. Just wait until the mold dries and then glue the rock in place.

Wet Molds - Leave the mold with the plaster for approximately 4 minutes. This time will vary depending on how the plaster was mixed. A 50/50 mixture of plaster to water will take about 4 minutes. A mixture of 60% plaster to 40% water will only take about 2 1/2 to 3 minutes to set up. The 50/50 mixture will allow more working time to blend in the wet plaster to the scenery base and existing rock molds. If the plaster is left in the mold too long before applying, the plaster will start to crack as soon as you move the mold. If this happens, it is too late to apply the mold. Chalk this up to experience and start again.

Applying and Blending Molds :
Dry Molds - The dried plaster rock is simply glued on to the scenery base. Plaster or other scenery filler type material, such as Sculptamold, is used to fill in the gaps left around the plaster rock and used to help blend the rock to the surrounding surfaces.

Wet Molds - Before pouring the plaster in the mold, you should have a good idea exactly where the mold will be positioned. When the plaster is ready, beginsetting the plaster to the base by lightly pressing the mold against the scenery base starting at the centre of the mold. Work the plaster from the centre towards the outside of the mold until it oozes out from the edges. The plaster that oozes from the edges will be used to blend the molded rock to the surrounding surfaces. A butter knife is a good tool to blend the mold to the surrounding surfaces. Use whatever works best for you. Basically, carve the oozed plaster to existing shapes next to the molded rock. If the rock is next to flat surfaces, work the oozed plaster from the mold edge gradually into the flat surface. The plaster rock should appear to emerge from the flat surface. If the molded rock is next to another rock, use the knife to carve the oozed plaster similar to the rock face it touches. The plaster rock should appear to be one continuous rock face with the other rock molds.

Creating Rock Shadows :
This step can be applied whether the plaster rock is dry or still wet. A mixture of India Ink and water is sprayed onto the completed molds. A spray is necessary to ensure the black ink gets into all the nooks and crannies because this is where the shadows appear. As the ink is applied, you will notice the mold edge areas that were blended into the surrounding surfaces will tint differently to the rest of the mold. In fact, it will look downright terrible. This is normal and will be covered later with the stains and paints.

Plaster Rock Stains :
Water based paints and acrylics are used as stains to colour the plaster rocks. Using stains work best because tinting and hues can be altered easily during the colouring process. There are 3 acrylics that should be used because they best represent earth tones - raw sienna, burnt sienna and raw umber. Purchase these acrylics in the smallest tubes you can because a little goes a long way. Mix these colours with water to use as stains. The rest of the colouring depends on whether you want to use gray tones or brown tones for your rocks. Pick a base colour - for example, water based gray. Mix a large batch with water. Using another container, mix half of this first batch with some white paint thereby lightening the gray colours. Using other containers, use this lightened gray second batch and mix with some more white paint to lighten the gray even more. Also mix with some black to darken the gray tones. About 5 different stains should be used for best results. All in all, you should have about 8 stains using the 3 acrylic colours and 5 gray stains.

Colouring Process :
Using the 8 stains, begin by dabbing hap-hazardly each of the 3 acrylic stains. Then start dabbing the gray stains inter-mixing from each of the different containers. There is no sequence to this. The colours will start to flow into each other resulting in a colour combination you want. If it doesn't happen the way you want, keep dabbing colours...At the worst case, let everything dry and start again. You can start again and keep trying until you get the colour combinations you want. This must be experienced - it is not a magical formula... One trick to point out...keep using the water spray to remove or, alter the colours you have applied. If all else fails, wait until it dries and start again. The shadows have already been applied with the ink process...so colouring can be adjusted to what you feel is right.

End Result :
This is an experience process. Until you try plaster rocks and colouring them, you can never understand how they are done. Try it...you'll like it...The worst case scenario will only result in lost time...

Click here for pictures of Bob's layout where you can see some of his great rock work.




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This page was last updated on May, 1999.

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